Cuap. IIL., § 1.] 
amounts to only a very minute fraction of a grain. 
How could such quantities be accurately estimated, so 
| as not only to leave no doubt of the phenomenon of 
gravity thus acting on the small scale, but to deduce 
its amount, and hence to weigh the globe ? 
Mitchell imagined for this purpose the balance of tor- 
sion, which was re-invented by Coulomb(who probably 
first executed it) forthe purpose of measuring electrical 
forces. Mitchell’s apparatus came first into the hands 
of Wollaston, then of Cavendish, who made the experi- 
ment. He used a very light rod of deal, six feet long, 
suspended by a fine silver or copper wire, forty inches 
long, within a wooden case to defend it from currents 
of air. At each end of the lever was hung a ball of 
lead, two inches in diameter, and by a simple con- 
trivance a pair of leaden spheres, weighing, together, 
348 pounds, could be brought simultaneously into 
the neighbourhood of the balls (but outside the case), 
on opposite sides, so that their attractions might con- 
cur to swing the suspended lever out of the position 
of repose which it had previously taken up, under 
the action of the slight twisting force of the silver 
wire. A new position of rest was thus established, 
the small balls being pulled as much one way by the 
attraction of the spheres as they were urged in the 
opposite direction by the torsion of the wire. The 
position of repose being observed from a distance by 
a telescope (to avoid disturbance from the heat of 
the observer's body), the great spheres were then 
changed in position so as to act upon the opposite 
sides of the small balls, from what they formerly did. 
The deflection and new stable position would be as 
much on the other side of the zero, and the are de- 
scribed would be an accurate measure of the double 
deflection. The force of torsion for 1° of deflection 
is known by the time of oscillation of the lever 
and balls when free, and as the forces are exactly as 
the angles, the force corresponding to any displace- 
ment becomes known. Cavendish conducted the ex- 
periment with his usual patience, judgment, and suc- 
cess; he found the joint attraction of the small balls 
and large spheres to be about ,.4,5 of a grain, their 
centres being 8°85 inches apart, and he thence com- 
puted the density of the Earth to be 5-48 times that 
of water. Cavendish’s paper is as usual a model of 
precision, lucidity, and conciseness. The attraction 
of the fixed parts of the apparatus is calculated and 
allowed for. 
(158.) It would be difficult to mention in the whole range 
Baily’s re- of physics a mére beautiful and more important ex- 
= gs periment. It has been repeated since by Reich of 
Freiberg and Baily of London. The former obtained 
544, the latter 5°66 for the Earth’s specific gravity ; 
Result. 
ASTRONOMY.—MASKELYNE—DELAMBRE. 
835 
this last result being worthy of much confidence from 
the extraordinary care taken to avoid errors and to 
obtain independent values of the quantities sought." 
Maskelyne continued his zeal for the promotion of 
astronomy to the last. He superintended the publi- 
cation of 45 annual volumes of the Nautical Alma- 
nae. He left the whole of the observatory work in 
perfect order, and the greater part printed. He had 
the well-earned satisfaction of finding his observa- 
tions in request in every civilized country, the bases 
of the most useful tables, and the tests of the most 
advanced theories. He cultivated the friendly cor- 
respondence of astronomers in every country. Not 
given to change, he preserved the instruments and 
chief methods of the immortal Bradley ; but suffi- 
ciently alive to the necessity of progress in the 
sciences, he introduced many simple but practical im- 
provements in the art of observation. Even in his 
last years, satisfied that the celebrated quadrant of 
Bird was no longer the best instrument for its pur- 
pose, and was besides sensibly deteriorated by use, 
he adopted the circle instead (then recently come into 
notice, though first used more than a century before 
by Rémer), and directed the construction of that by 
Troughton, though it was not placed at Greenwich 
until after his death, which occurred in 1811, in the 
79th year of his age. His biographer Delambre 
mentions, that a considerable number of his posthu- 
mous memoirs were put into the hands of Professor 
Vince for publication. They have not, however, ap- 
peared. 
Practical astronomy was on the Continent far be- 
hind its state in England at the period of which we 
speak. The various national observatories contri- 
buted comparatively little to the progress of science ; 
but there were of course exceptions, a few of which 
we will here briefly notice. 
The discovery of the four small planets? Ceres, 
Pallas, Juno, and Vesta, the first by Piazzi, the second 
and fourth by Olbers, the third by Harding, gave cele- 
brity to those astronomers, of whom Piazzi and Olbers 
were farther distinguished bymany important labours. 
To the latter we owe the discovery of several comets, 
and one of the best methods of calculating their or- 
bits from observation. THe was a person of much 
simplicity of manner, made his observations with the 
most unpretending means in the attic of his house, 
and died at a great age generally respected. His 
firmest friend was Baron von Zach, who had no slight 
share in the discovery of the new planets, owing to 
his having instigated the association of twenty-four 
astronomers, chiefly in Germany, for the express pur- 
“3 
1 The experiments of Cavendish are related in the “ Phil 
phical Tra 
8” for 1798 ; those of Reich in a separate small 
work, entitled “ Versuche iiber die mittlere Dichtigkeit der Erde,” Freiberg, 1838; those of Baily in the “ Memoirs of the 
Astronomical Society,” vol. xiv. 
A still more recent series of experiments by Reich gives 5°58, a close approximation to Baily’s 
result.—(Phil. Mag. March 1853.) From observations with a pendulum in Harton coal-pit in 1854, Mr Airy has obtained a 
density higher than any of the above. 
as 
2 In the Fifth Dissertation, page 789, Harding instead of Olbers is named as the discoverer of Vesta. 
(159.) 
Maske- 
lyne’s im- 
provements 
at Green- 
wich, 
(160.) 
Its superi- 
ority to 
Continent- 
al observa- 
tories at 
that time. 
(161) 
Astronomy 
on the Con- 
tinent. 
Olbers— 
Piazzi. 
