186 General J. T. Walker. On the Unit of Length [Feb. 13, 



platinum to the centre of the mass, I melted 900 parts of fine gold 

 with 100 parts of pure platinum, and, after repeated meltings, cast 

 this alloy into the same mould used for the experiments recorded 

 above. The result was, as in the previous cases, liquation of the 

 platinum towards the centre of the sphere, the gold and platinum in 

 1000 parts being as 900 to 098 on the exterior, against 845 and 146 

 at the centre of the mass (see diagram C). 



II. " On the Unit of Length of a Standard Scale by Sir George 

 Shuckburgh, appertaining to the Royal Society." By 

 General J. T. Walker, R.E., F.R.S. Received February 3, 

 1890. 



In the determinations of the length of the seconds pendulum, 

 which were made in London by Kater and at Greenwich by Sabine, 

 and are described in the ' Philosophical Transactions ' for 1818, 1829, 

 and 1831, the distance between the upper and lower edges of the 

 pendulum was measured off on a standard scale which had been con- 

 structed by Sir George Shuckburgh. The scale had not been com- 

 pared with any of the modern standard scales, but it had been 

 preserved with much care with the instruments appertaining to the 

 Royal Society. 



In the autumn of 1888, M. le Commandant Defforges, an officer of 

 the French Geodetic Survey, came to England to take a share in 

 operations for the determination of the difference in longitude 

 between Greenwich and Paris, and also to determine the length of a 

 French seconds pendulum at Greenwich. He kindly undertook to 

 comply with a suggestion which was made to him by me, to com- 

 pare the portion of Shuckburgh's scale which had been employed by 

 Kater and Sabine with one of the standard metre bars of the Inter- 

 national Bureau of Weights and Measures in Paris. The Council of 

 the Royal Society assented, and the scale was sent across to Paris 

 and brought back again by special messenger. 



The details and results of the comparison are given in the following 

 account by Commandant Defforges, from which it will be seen that 

 the scale was compared with the French metrical brass scale, N, at 

 the temperature of 48*7° F., at which the distance between Kater and 

 Sabine's divisions, and 39*4, of the Shuckburgh scale was found 

 equal to 1*0006245 metre. On reducing to the temperature of 62° F., 

 which was employed by Kater and Sabine, this distance becomes 

 1*0007619 metre, which is equivalent to 39*400428 inches if we adopt 

 the relation 1 metre = 39*370432 inches, which was determined by 

 Colonel Clarke, C.B., of the Ordnance Survey, and is given in his 

 valuable work on the Comparisons of Standards of Length. Thus 



