1865.] The Proposed Pendulum Operations fox India. 257 



time as the pendulum, so that their effect might be neglected in com- 

 puting the length of the simple pendulum. When in use, the knife 

 edge rested upon a steel plate. The number of vibrations per diem, 

 were ascertained by means of a clock, but Borda made a great improve- 

 ment on the old method of counting the coincidences. He fixed a 

 straight edge vertically, so as to coincide with the pendulum wire at 

 rest, when seen through a telescope placed opposite. A cross was 

 made on the bob of the clock pendulum, and the observation consisted 

 in noting the times when the wire and cross disappeared together 

 behind the edge. After a series of coincidences had been observed 

 the length of the pendulum was measured by means of a horizontal 

 steel plate, which was screwed up from below, so as just to touch the 

 ball : then the pendulum was removed, and a bar, whose length had 

 been carefully compared with a standard, inserted in its place. The 

 bar had a r head, of which the lower surface rested on the upper steel 

 plate, and a graduated rod, sliding on the bar, was adjusted to contact 

 with the lower plate. The diameter of the platinum ball was then 

 measured by means of the same slider, by placing it on the steel plate 

 for the purpose ; the brass cap and wire were then weighed. The 

 apparatus was enclosed in a glass case, and the temperature was care- 

 fully recorded. All necessary corrections were applied, excepting the 

 true one for buoyancy. The whole process, which required very great 

 delicacy, had to be repeated, and the length of the corresponding 

 simple pendulum computed after each series of observations. Borda's 

 pendulum was about 12 feet in length. 



His method was followed by M. M. Arago, Biot, and Chaix, at 

 Formentera, the southernmost station of the French arc, with this 

 exception that they used a pendulum of only 3 feet in length. These 

 observations were extended by Biot in 181 7 "to Leith, and Unst in the 

 Shetlands, and in conjunction with M. Mathien, he observed at 

 Dunkirk, Paris, Clermont, Bordeaux, and Figeac. From these opera- 

 tions, Biot deduced an ellipticity of g-^j. 



In about 1809, Captain "Warren made some observations at the 

 Madras observatory with a pendulum formed of a leaden ball suspended 

 by a fibre made from the plantain leaf. The vibrations were counted 

 and an assistant noted the times, from an astronomical clock. In order 

 to measure its length, he attached some glass plates to a Avail, and set 



