CHAP. III., 1.] 



ASTRONOMY. MASKELYNE DELAMBEE. 



37 



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 ? 



Michell imaginedfor this purpose the balance oftor- 

 on sion, which was re-invented by Coulomb (who probably 

 first executed it) forthe purpose of measuring electrical 

 forces. Mich ell'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 arc 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 ( J^ of a grain, their 

 centres being 8-85 inches apart, and he thence com- 



iesult. puted the density of the Earth to be 6-48 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 



Daily's re- O f physics a more beautiful and more important ex- 

 periment. It has been repeated since by Reich of 

 Freiberg and Baily of London. The former obtained 

 5-44, the latter 5-66 for the Earth's specific gravity; 



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. 1 



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- 

 nac. 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 Rbmer), 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 2 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 by many 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. He 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- 



(159.) 

 Maske- 

 lyne's ira- 

 provementi 

 at Green- 

 wich. 



(160.) 

 Its superi- 

 ority to 

 Continent- 

 al observa 

 tories at 

 that time. 



061.) 

 Astronom] 

 on the Con 

 tinent. 

 Olbers 

 Piazzi. 



1 The experiments of Cavendish are related in the " Philosophical Transactions" 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. 



2 In the Fifth Dissertation, page 789, Harding instead of Olbers is named as the discoverer of Vesta. 



