95 



Excluding all except the results of the comparisons of U with 

 the two platinum troy pounds, U = Sp-|- 0*0083 grain. 



The temperatures were determined by means of three thermo- 

 meters by Bunten, having centesimal scales etched upon the tube, 

 and two thermometers having arbitrary scales traced upon the tubes 

 with a diamond point. The zero-points of these were determined 

 at distant intervals. They were often compare^ with each other, 

 and, lastly, with an excellent standard thermometer constructed at 

 Kew under the directions of Mr. Welsh, in order to form tables of 

 the errors at any point of their scales, and to determine the position 

 of their zeros at any given time. The barometer employed was a 

 portable cistern barometer by Ernst of Paris, the scale of which was 

 divided into millimetres. It was compared first with the standard 

 barometer of the Paris Observatory, and afterwards with a standard 

 barometer, having a tube of very large bore, belonging to the Taylor 

 Library of Sidney Sussex College, Cambridge. 



According to Eitter (Memoires de la Societe de Physique de Ge- 

 neve, t. iii. p. 361), the observations of Regnault show that in Paris, 

 lat. 48 50' 14", 60 metres above the mean level of the sea, a litre of 

 dry atmospheric air, containing the average amount, 0*0004 of its 

 volume, of carbonic acid, the density of which is 1-529 of that of 

 atmospheric air at Cent., under the pressure of 760 mm. of mer- 

 cury at Cent., weighs 1-2934963 gramme. If G be taken to 

 denote the force of gravity at the mean level of the sea in lat. 

 45, the force of gravity in lat. \, at the mean level of the sea, 

 = 6(10-0025659 cos 2X) (Baily, Mem. Ast. Soc. vol. vii. p. 94). 

 The force of gravity in a given latitude at a place on the surface 

 of the earth at the height z above the mean level of the sea 



= { 1 (2 2 -)"} x force of gravity at the level of the sea in the 



same latitude, where r is the radius of the earth, p its mean density, 

 and p' the density of that part of the earth which is above the mean 

 level of the sea (Poisson, Traite de Me'canique, t. ii. p. 629). 



According to Regnault, the expansion of air under constant pressure 

 from to 100 Cent., is 0-36/06 of its volume at Cent.; also at 

 50 Cent., the mercurial thermometer is about 0-2 in advance of 

 the air thermometer (Memoires de PInstitut, t. xxi. p. 91. p. 238, 

 Annales de Chimie, 3 serie, t. v. p. 99). Hence, density air at 



