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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.









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