Henry Cavendish

It would not be easy to mention two men between whom there was a

greater contrast, both in respect of their characters and lives, than

that which existed between Benjamin Franklin and the Honourable Henry

Cavendish. The former of humble birth, but of great public spirit,

possessed social qualities which were on a par with his scientific

attainments, and toward the close of his life was more renowned as a

statesman than as a philosopher; the latter, a member of one of the

most noble families of England, and possessed of wealth far exceeding

his own capacity for the enjoyment of it, was known to very few, was

intimate with no one, and devoted himself to scientific pursuits

rather for the sake of the satisfaction which his results afforded to

himself than from any hope that they might be useful to mankind, or

from any desire to secure a reputation by making them known, and

passed a long life, the most uneventful that can be imagined.



Though the records of his family may be traced to the Norman

Conquest, the famous Elizabeth Hardwicke, the foundress of two ducal

families and the builder of Hardwicke Hall and of Chatsworth as it was

before the erection of the present mansion, was the most remarkable

person in the genealogy. Her second son, William, was raised to the

peerage by James I., thus becoming Baron Cavendish, and was

subsequently created first Earl of Devonshire by the same monarch. His

great-grandson, the fourth earl, was created first Duke of Devonshire

by William III., to whom he had rendered valuable services. He was

succeeded by his eldest son in 1707, and the third son of the second

duke was Lord Charles Cavendish, the father of Henry and Frederick, of

whom Henry was the elder, having been born at Nice, October 13, 1731.

His mother died when he was two years old, and very little indeed is

known respecting his early life. In 1742 he entered Dr. Newcome's

school at Hackney, where he remained until he entered Peterhouse, in

1749. He remained at Cambridge until February, 1753, when he left the

university without taking his degree, objecting, most probably, to the

religious tests which were then required of all graduates. In this

respect his brother Frederick followed his example. On leaving

Cambridge Cavendish appears to have resided with his father in

Marlborough Street, and to have occasionally assisted him in his

scientific experiments, but the investigations of the son soon

eclipsed those of the father. It is said that the rooms allotted to

Henry Cavendish "were a set of stables, fitted up for his

accommodation," and here he carried out many of his experiments,

including all those electrical investigations in which he forestalled

so much of the work of the present century.



During his father's life, or, at any rate, till within a few years of

its close, Henry Cavendish appears to have enjoyed a very narrow

income. He frequently dined at the Royal Society Club, and on these

occasions would come provided with the five shillings to be paid for

the dinner, but no more. Upon his father's death, which took place in

1783, when Henry was more than fifty years of age, his circumstances

were very much changed, but it seems that the greater part of his

wealth was left him by an uncle who had been an Indian officer, and

this legacy may have come into his possession before his father's

death. He appears to have been very liberal when it was suggested to

him that his assistance would be of service, but it never occurred to

him to offer a contribution towards any scientific or public

undertaking, and though at the time of his death he is said to have

had more money in the funds than any other person in the country,

besides a balance of L50,000 on his current account at his bank, and

various other property, he bequeathed none to scientific societies or

similar institutions. Throughout the latter part of his life he seems

to have been quite careless about money, and to have been satisfied if

he could only avoid the trouble of attending to his own financial

affairs. Hence he would allow enormous sums to accumulate at his

banker's, and on one occasion, being present at a christening, and

hearing that it was customary for guests to give something to the

nurse, he drew from his pocket a handful of guineas, and handed them

to her without counting them. After his father's death, Cavendish

resided in his own house on Clapham Common. Here a few rooms at the

top of the house were made habitable; the rest were filled with

apparatus of all descriptions, among which the most numerous examples

were thermometers of every kind. He seldom entertained visitors, but

when, on rare occasions, a guest had to be entertained, the repast

invariably consisted of a leg of mutton. His extreme shyness caused

him to dislike all kinds of company, and he had a special aversion to

being addressed by a stranger. On one occasion, at a reception given

by Sir Joseph Banks, Dr. Ingenhousz introduced to him a distinguished

Austrian philosopher, who professed that his main object in coming to

England was to obtain a sight of so distinguished a man. Cavendish

listened with his gaze fixed on the floor; then, observing a gap in

the crowd, he made a rush to the door, nor did he pause till he had

reached his carriage. His aversion to women was still greater; his

orders for the day he would write out and leave at a stated time on

the hall-table, where his house-keeper, at another stated time, would

find them. Servants were allowed access to the portion of the house

which he occupied only at fixed times when he was away; and having

once met a servant on the stairs, a back staircase was immediately

erected. His regular walk was down Nightingale Lane to Wandsworth

Common, and home by another route. On one occasion, as he was crossing

a stile, he saw that he was watched, and thenceforth he took his walks

in the evening, but never along the same road. There were only two

occasions on which it is recorded that scientific men were admitted to

Cavendish's laboratory. The first was in 1775, when Hunter, Priestley,

Romayne, Lane, and Nairne were invited to see the experiments with the

artificial torpedo. The second was when his experiment on the

formation of nitric acid by electric sparks in air had been

unsuccessfully attempted by Van Marum, Lavoisier, and Monge, and he

"thought it right to take some measures to authenticate the truth of

it."



Besides his house at Clapham, Cavendish occupied (by his instruments)

a house in Bloomsbury, near the British Museum, while a "mansion" in

Dean Street, Soho, was set apart as a library. To this library a

number of persons were admitted, who could take out the books on

depositing a receipt for them. Cavendish was perfectly methodical in

all his actions, and whenever he borrowed one of his own books he duly

left the receipt in its place. The only relief to his solitary life

was afforded by the meetings of the Royal Society, of which he was

elected a Fellow in 1760; by the occasional receptions at the

residence of Sir Joseph Banks, P.R.S.; and by his not infrequent

dinners with the Royal Society Club at the Crown and Anchor; and he

may sometimes have joined the social gatherings of another club which

met at the Cat and Bagpipes, in Downing Street. It was to his visits

to the Royal Society Club that we are indebted for the only portrait

that exists of him. Alexander, the draughtsman to the China Embassy,

was bent upon procuring a portrait of Cavendish, and induced a friend

to invite him to the club dinner, "where he could easily succeed, by

taking his seat near the end of the table, from whence he could sketch

the peculiar great-coat of a greyish-green colour, and the remarkable

three-cornered hat, invariably worn by Cavendish, and obtain,

unobserved, such an outline of the face as, when inserted between the

hat and coat, would make, he was quite sure, a full-length portrait

that no one could mistake. It was so contrived, and every one who saw

it recognized it at once." Another incident is recorded of the Royal

Society Club which, perhaps, reflects as much credit upon Cavendish as

upon the Society. "One evening we observed a very pretty girl looking

out from an upper window on the opposite side of the street, watching

the philosophers at dinner. She attracted notice, and one by one we

got up and mustered round the window to admire the fair one.

Cavendish, who thought we were looking at the moon, hustled up to us

in his odd way, and when he saw the real object of our study, turned

away with intense disgust, and grunted out, 'Pshaw!'"



In the spring and autumn of 1785, 1786, 1787, and 1793, Cavendish made

tours through most of the southern, midland, and western counties, and

reached as far north as Whitby. The most memorable of these journeys

was that undertaken in 1785, since during its course he visited James

Watt at the Soho Works, and manifested great interest in Watt's

inventions. This was only two years after the great controversy as to

the discovery of the composition of water, but the meeting of the

philosophers was of the most friendly character. On all these journeys

considerable attention was paid to the geology of the country.



Allusion has already been made to the two committees of the Royal

Society to which the questions of the lightning-conductors at

Purfleet, and of points versus knobs for the terminals of

conductors, were referred. Cavendish served on each of these

committees, and supported Franklin's view against the recommendation

of Mr. Wilson. On the first committee he probably came into personal

communication with Franklin himself.



Cavendish's life consisted almost entirely of his philosophical

experiments. In other respects it was nearly without incident. He

appears to have been so constituted that he must subject everything to

accurate measurement. He rarely made experiments which were not

quantitative; and he may be regarded as the founder of "quantitative

philosophy." The labour which he expended over some of his

measurements must have been very great, and the accuracy of many of

his results is marvellous considering the appliances he had at

disposal. When he had satisfied himself with the result of an

experiment, he wrote out a full account and preserved it, but very

seldom gave it to the public, and when he did publish accounts of any

of his investigations it was usually a long time after the experiments

had been completed. One of the consequences of his reluctance to

publish anything was the long controversy on the discovery of the

composition of water, which was revived many years afterwards by

Arago's eloge on James Watt; but a much more serious result was the

loss to the world for so many years of discoveries and measurements

which had to be made over again by Faraday, Kohlrausch, and others.

The papers he published appeared in the Philosophical Transactions of

the Royal Society, to which he began to communicate them in 1766. On

March 25, 1803, he was elected one of the eight Foreign Associates of

the Institute of France. His eloge was pronounced by Cuvier, in

1812, who said, "His demeanour and the modest tone of his writings

procured him the uncommon distinction of never having his repose

disturbed either by jealousy or by criticism." Dr. Wilson says, "He

was almost passionless. All that needed for its apprehension more than

the pure intellect, or required the exercise of fancy, imagination,

affection, or faith, was distasteful to Cavendish. An intellectual

head thinking, a pair of wonderfully acute eyes observing, and a pair

of very skilful hands experimenting or recording, are all that I

realize in reading his memorials." He appeared to have no eye for

beauty; he cared nothing for natural scenery, and his apparatus,

provided it were efficient, might be clumsy in appearance and of the

cheapest materials; but he was extremely particular about accuracy of

construction in all essential details. He reminds us of one of our

foremost men of science, who, when his attention was directed to the

beautiful lantern tower of a cathedral, behind which the full moon was

shining, remarked, "I see form and colour, but I don't know what you

mean by beauty."



The accounts of Cavendish's death differ to some extent in their

details, but otherwise are very similar. It appears that he requested

his servant, "as he had something particular to engage his thoughts,

and did not wish to be disturbed by any one," to leave him and not to

return until a certain hour. When the servant came back, at the time

appointed, he found his master dead. This was on February 24, 1810,

after an illness of only two or three days.



It is mainly on account of his researches in electricity that the

biography of Cavendish finds a place in this volume. These

investigations took place between the years 1760 and 1783, and, as

already stated, were all conducted in the stables attached to his

father's house in Marlborough Street. It was by these experiments that

electricity was first brought within the domain of measurement, and

many of the numerical results obtained far exceeded in accuracy those

of any other observer until the instruments of Sir W. Thomson rendered

many electrical measurements a comparatively easy matter. The near

agreement of Cavendish's results with those of the best modern

electricians has made them a perpetual monument to the genius of their

author. It was at the request of Sir W. Thomson, Mr. Charles

Tomlinson, and others, that Cavendish's electrical researches might be

given to the public, that the Duke of Devonshire, in 1874, entrusted

the manuscripts to the care of the late Professor Clerk Maxwell. They

had previously been in the hands of Sir William Snow Harris, who

reported upon them, but after his death, in 1867, the report could not

be found. The papers, with an introduction and a number of very

valuable notes by the editor, were published by the Cambridge

University Press, just before the death of Clerk Maxwell, in 1879. Sir

W. Thomson quotes the following illustration of the accuracy of

Cavendish's work:--"I find already that the capacity of a disc was

determined experimentally by Cavendish as 1/1.57 of that of a sphere

of the same radius. Now we have capacity of disc = (2/[pi])a =

a/1.571!"



Cavendish adopted Franklin's theory of electricity, treating it as an

incompressible fluid pervading all bodies, and admitting of

displacement only in a closed circuit, unless, indeed, the disturbance

might extend to infinity. This fluid he supposed, with Franklin, to be

self-repulsive, but to attract matter, while matter devoid of

electricity, and therefore in the highest possible condition of

negative electrification, he supposed, with AEpinus, to be, like

electricity, self-repulsive. One of Cavendish's earliest experiments

was the determination of the precise law according to which electrical

action varies with the distance between the charges. Franklin had

shown that there was no sensible amount of electricity on the interior

of a deep hollow vessel, however its exterior surface might be

charged. Cavendish mounted a sphere of 12.1 inches in diameter, so

that it could be completely enclosed (except where its insulating

support passed through) within two hemispheres of 13.3 inches

diameter, which were carried by hinged frames, and could thus be

allowed to close completely over the sphere, or opened and removed

altogether from its neighbourhood. A piece of wire passed through one

of the hemispheres so as to touch the inner sphere, but could be

removed at pleasure by means of a silk string. The hemispheres being

closed with the globe within them, and the wire inserted so as to make

communication between the inner and outer spheres, the whole apparatus

was electrified by a wire from a charged Leyden jar. This wire was

then removed by means of a silken string and "the same motion of the

hand which drew away the wire by which the hemispheres were

electrified, immediately after that was done, drew out the wire which

made the communication between the hemispheres and the inner globe,

and, immediately after that was drawn out, separated the hemispheres

from each other," and applied the electrometer to the inner globe. "It

was also contrived so that the electricity of the hemispheres and of

the wire by which they were electrified was discharged as soon as they

were separated from each other.... The inner globe and hemispheres

were also both coated with tinfoil to make them the more perfect

conductors of electricity." The electrometer consisted of a pair of



pith-balls; but, though the experiment was several times repeated,

they shewed no signs of electrification. From this it was clear that,

as there could have been no communication between the globe and

hemispheres when the connecting wire was withdrawn, there must have

been no electrification on the globe while the hemispheres, though

themselves highly charged, surrounded it. To test the delicacy of the

experiment, a charge was given to the globe less than one-sixtieth of

that previously given to the hemispheres, and this was readily

detected by the electrometer. From the result Cavendish inferred that

there is no reason to think the inner globe to be at all charged

during the experiment. "Hence it follows that the electric attraction

and repulsion must be inversely as the square of the distance, and

that, when a globe is positively electrified, the redundant fluid in

it is lodged entirely on its surface." This conclusion Cavendish

showed to be a mathematical consequence of the absence of

electrification from the inner sphere; for, were the law otherwise,

the inner sphere must be electrified positively or negatively,

according as the inverse power were higher or lower than the second,

and that the accuracy of the experiment showed the law must lie

between the 2-1/50 and the 1-49/50 power of the distance. With his

torsion-balance, Coulomb obtained the same law, but Cavendish's method

is much easier to carry out, and admits of much greater accuracy than

that of Coulomb. Cavendish's experiment was repeated by Dr.

MacAlister, under the superintendence of Clerk Maxwell, in the

Cavendish Laboratory, the absence of electrification being tested by

Thomson's quadrant electrometer, and it was shown that the deviation

from the law of inverse squares could not exceed one in 72,000.



The distinction between electrical charge or quantity of

electricity and "degree of electrification" was first clearly made

by Cavendish. The latter phrase was subsequently replaced by

intensity, but electric intensity is now used in another sense.

Cavendish's phrase, degree of electrification, corresponds precisely

with our notion of electric potential, and is measured by the work

done on a unit of electricity by the electric forces in removing it

from the point in question to the earth or to infinity. Along with

this notion Cavendish introduced the further conception of the amount

of electricity required to raise a conductor to a given degree of

electrification, that is, the capacity of the conductor. In modern

language, the capacity of a conductor is defined as "the number of

units of electricity required to raise it to unit potential;" and this

definition is in precise accordance with the notion of Cavendish, who

may be regarded as the founder of the mathematical theory of

electricity. Finding that the capacities of similar conductors are

proportional to their linear dimensions, he adopted a sphere of one

inch diameter as the unit of capacity, and when he speaks of a

capacity of so many "inches of electricity," he means a capacity so

many times that of his one-inch sphere, or equal to that of a sphere

whose diameter is so many inches. The modern unit of capacity in the

electro-static system is that of a sphere of one centimetre radius,

and the capacity of any sphere is numerically equal to its radius

expressed in centimetres. Cavendish determined the capacities of

nearly all the pieces of apparatus he employed. For this purpose he

prepared plates of glass, coated on each side with circles of tinfoil,

and arranged in three sets of three, each plate of a set having the

same capacity, but each set having three times the capacity of the

preceding. There was also a tenth plate, having a capacity equal to

the whole of the largest set. The capacity of the ten plates was thus

sixty-six times that of one of the smallest set. With these as

standards of comparison, he measured the capacities of his other

apparatus, and, when possible, modified his conductors so as to make

them equal to one of his standards. His large Leyden battery he found

to have a capacity of about 321,000 "inches of electricity," so that

it was equivalent to a sphere more than five miles in diameter. One of

his instruments employed in the measurement of capacities was a "trial

plate," consisting of a sheet of metal, with a second sheet which

could be made to slide upon it and to lie entirely on the top of the

larger plate, or to rest with any portion of its area extending over

the edge of the former. This was a conductor whose capacity could be

varied at will within certain limits. Finding the capacity of two

plates of tinfoil on glass much greater than his calculations led him

to expect, Cavendish compared them with two equal plates having air

between, and found their capacity very much to exceed that of the air

condenser. The same was the case, though in a less degree, with

condensers having shellac or bee's-wax for their dielectrics, and thus

Cavendish not only discovered the property to which Faraday afterwards

gave the name of "specific inductive capacity," but determined its

measure in these dielectrics. He also discovered that the apparent

capacity of a Leyden jar increases at first for some time after it has

been charged--a phenomenon connected with the so-called residual

charge of the Leyden jar. Another feature on which he laid some

stress, and which was brought to his notice by the comparison of his

coated panes, was the creeping of electricity over the surface of the

glass beyond the edge of the tinfoil, which had the same effect on the

capacity as an increase in the dimensions of the tinfoil. The

electricity appeared to spread to a distance of 0.07 inch all round

the tinfoil on glass plates whose thickness was 0.21 inch, and 0.09

inch in the case of plates 0.08 inch thick.



His paper on the torpedo was read before the Royal Society in 1776.

The experiments were undertaken in order to determine whether the

phenomena observed by Mr. John Walsh in connection with the torpedo

could be so far imitated by electricity as to justify the conclusion

that the shock of the torpedo is an electric discharge. For this

purpose Cavendish constructed a wooden torpedo with electrical organs,

consisting of a pewter plate on each side, covered with leather. The

plates were connected with a charged Leyden battery, by means of wires

carried in glass tubes, and thus the battery was discharged through

the water in which the torpedo was immersed, and which was rendered of

about the same degree of saltness as the sea. Cavendish compared the

shock given through the water with that given by the model fish in

air, and found the difference much greater than in the case of the

real torpedo, but, by increasing the capacity of the battery and

diminishing the potential to which it was charged, this discrepancy

was diminished, and it was found to be very much less in the case of a

second model having a leather, instead of a wooden, body, so that the

body of the fish itself offered less resistance to the discharge. One

of the chief difficulties lay in the fact that no one had succeeded in

obtaining a visible spark from the discharge of the torpedo, which

will not pass through the smallest thickness of air. Cavendish

accounted for this by supposing the quantity of electricity discharged

to be very great, and its potential very small, and showed that the

more the charge was increased and the potential diminished in his

model, the more closely did it imitate the behaviour of the torpedo.



But the main interest in this paper lies in the indications which it

gives that Cavendish was aware of the laws which regulate the flow of

electricity through multiple conductors, and in the comparisons of

electrical resistance which are introduced. It had been formerly

believed that electricity would always select the shortest or best

path, and that the whole of the discharge would take place along that

route. Franklin seems to have assumed this in the passage quoted[4]

respecting the discharge of the lightning down the uninsulated

conductor instead of through the building. The truth, however, is

that, when a number of paths are open to an electric current, it will

divide itself between them in the inverse ratios of their resistances,

or directly as their conductivities, so that, however great the

resistance of one of the conductors, some portion, though it may be a

very small fraction, of the discharge will take place through it. But

this law does not hold in the case of insulators like the air, through

which electricity passes only by disruptive discharges, and which

completely prevent its passage unless the electro-motive force is

sufficient to break through their substance. In the case of the

lightning-conductor, however, its resistance is generally so small in

comparison with that of the building it is used to protect, that

Franklin's conclusion is practically correct.



[Footnote 4: Page 96.]



In his paper on the torpedo Cavendish stated that some experiments had

shown that iron wire conducted 400,000,000 times better than rain or

distilled water, sea-water 100 times, and saturated solution of

sea-salt about 720 times, better than rain-water. Maxwell pointed out

that this comparison of iron wire with sea-water would agree almost

precisely with the measurements of Matthiesen and Kohlrausch at 11 deg.C.

The records of the experiments which led to these results were found

among Cavendish's unpublished papers, and the experiments also showed

that the conductivity of saline solutions was very nearly proportional

to the percentage of salt contained, when this was not very large--a

result also obtained long afterwards by Kohlrausch. In making these

measurements Cavendish was his own galvanometer. The solutions were

contained in glass tubes more than three feet long, and a wire

inserted to different distances into the solution; thus the discharge

could be made to pass through any length of the liquid column less

than that of the tube itself. From the Leyden battery of forty-nine

jars, six jars of nearly equal capacity were selected and charged

together, and the charge of one jar only was employed for each shock.

The discharge passed through the column of liquid contained in the

tube, from a wire inserted at the further end, until it reached the

sliding wire, when nearly the whole current betook itself to the wire

on account of its smaller resistance, and thence passed through the

galvanometer, which was Cavendish himself. Two tubes were generally

compared together, and the jars discharged alternately through the

tubes, and the tube which gave the greatest shock was assumed to

possess the least resistance. The wires were then adjusted till the

shocks were nearly equal, and positions determined which made the

first tube possess a greater and then a less resistance than the

second. From these positions the length of the column of liquid was

estimated which would make the resistances of the two tubes exactly

equal. But the result which has the greatest theoretical interest was

obtained by discharging the Leyden jars through wide and narrow tubes

containing the same solutions. By these experiments Cavendish found

that the resistances of the conductors were independent of the

strengths of the currents flowing in them; that is to say, he

established Ohm's law for electrolytes in a form which carried with it

its full explanation. This was in January, 1781. Ohm's law was first

formally stated in 1827. The physical fact which is expressed by it is

that the ratio of the electro-motive force to the current produced is

the same for the same conductor, otherwise under the same physical

conditions, however great or small that electro-motive force may be.



Cavendish devoted considerable attention to the subject of heat,

especially thermometry. In many of his investigations on latent and

specific heat he worked on the same lines as Black, and at about the

same time; but it is difficult to determine the exact date of some of

Cavendish's work, as he frequently did not publish it for a long time

after its completion, and most of Black's results were made public

only to his lecture audience. Cavendish, however, improved upon Black

in his mode of stating some of his results. The heat, for instance,

which is absorbed by a body in passing from the solid to the liquid,

or from the liquid to the gaseous, condition, Black called "latent

heat," and supposed it to become latent within the substance, ready to

reveal itself when the body returned to its original condition. This

heat Cavendish spoke of as being destroyed or generated, and this

is in accordance with what we now know respecting the nature of heat,

for when a body passes from the solid to the liquid, or from the

liquid or solid to the gaseous, condition, a certain amount of work

has to be done, and a corresponding amount of heat is used up in the

doing of it. When the body returns to its original condition, the heat

is restored, as when a heavy body falls to the ground, or a bent

spring returns to its original form. Cavendish's determination of the

so-called latent heat of steam was very slightly in error.



About 1760 very extraordinary beliefs were current respecting the

excessive degree of cold and the rapid variations of temperature which

take place in the Arctic regions. Braun, of St. Petersburg, had

observed that mercury, in solidifying in the tube of a thermometer,

descended through more than four hundred degrees, and it was assumed

that the melting point of mercury was about 400 deg. below Fahrenheit's

zero. It then became necessary to suppose that, while the mercury in a

thermometer was freezing, there was a variation of temperature to this

extent, and thus these wild reports became current. Cavendish and

Black independently explained the anomaly, and each suggested the same

method of determining the freezing point of mercury. Cavendish,

however, had a piece of apparatus prepared which he sent to Governor

Hutchins, at Albany Fort, Hudson's Bay. It consisted of an outer

vessel, in which the mercury was allowed to freeze, but not throughout

the whole of its mass, and the bulb of the thermometer was kept

immersed in the liquid metal in the interior. In this way the mercury

in the thermometer was cooled down to the melting point without

commencing to solidify, and the temperature was found to be between

39 deg. and 40 deg. below Fahrenheit's zero.



As a chemist, Cavendish is renowned for his eudiometric analysis,

whereby he determined the percentage of oxygen in air with an amount

of accuracy that would be creditable to a chemist of to-day, and for

his discovery of the composition of water; but to the world generally

he is perhaps best known by the famous "Cavendish experiment" for

determining the mass, and hence the mean density, of the earth. The

apparatus was originally suggested by the Rev. John Michell, but was

first employed by Cavendish, who thereby determined the mean density

of the earth to be 5.45. At the request of the Astronomical Society,

the investigation was afterwards taken up by Mr. Francis Baily, who,

after much labour, discovered that the principal sources of error were

due to radiation of heat, and consequent variation of temperature of

parts of the apparatus during the experiment. To minimize the

radiation and absorption, he gilded the principal portions of the

apparatus and the interior of the case in which it was contained, and

his results then became consistent. Cavendish had himself suggested

the cause of the discrepancy, but the gilding was proposed by

Principal Forbes. As a mean of many hundreds of experiments, Mr. Baily

deduced for the mean density of the earth 5.6604. Cavendish's

apparatus was a delicate torsion-balance, whereby two leaden balls

were supported upon the extremities of a wooden rod, which was

suspended by a thin wire. These balls were about two inches in

diameter, and the experiment consisted in determining the deflection

of the wooden arm by the attraction of two large solid spheres of lead

brought very near the balls, and so situated that the attraction of

each tended to twist the rod horizontally in the same direction. The

force required to produce the observed deflection was calculated from

the time of swing of the rod and balls when left to themselves. The

force exerted upon either ball by a known spherical mass of metal,

with its centre at a known distance, being thus determined, it was

easy to calculate what mass, having its centre at the centre of the

earth, would be required to attract one of the balls with the force

with which the earth was known to attract it.



Dr. Wilson sums up Cavendish's view of life in these words:--



His theory of the universe seems to have been that it consisted

solely of a multitude of objects which could be weighed,

numbered, and measured; and the vocation to which he considered

himself called was to weigh, number, and measure as many of

these objects as his allotted three score years and ten would

permit. This conviction biased all his doings--alike his great

scientific enterprises and the petty details of his daily life.

[Greek: Panta metro, kai arithmo, kai stathmo], was his motto;

and in the microcosm of his own nature he tried to reflect and

repeat the subjection to inflexible rule and the necessitated

harmony which are the appointed conditions of the macrocosm of

God's universe.