292 



vapours are perfectly gaseous. The following is a summary of the 

 results : — ■ 



Fluids jEther. Bi-sulp. of Carbon. Alcoh. 1. Water. 



Boiling points 36° cent. 46° 78° 100" 



Volume of one lb. of 

 vapour as compu- 

 ted— cubic feet. cubic feet. cubic feet. cubic feet. 



From latent heat 5-3968 5*4689 9-366 26 36 



From composition..,.. 5-3874 5-4643 9900 2718 



In article 74, the close coincidence of the results of the above 

 computations for aether and bisulphuret of carbon is stated to be a 

 confirmation of the principles deduced from the mechanical theory 

 of heat, and also a proof that the vapours of aether and bi-sulphuret of 

 carbon may be treated in practical calculations, without sensible 

 error, as perfectly gaseous, when at pressures not greatly exceeding 

 one atmosphere. The following are the values of some of the con- 

 stants for these fluids : — 



^ther. Bi-sulp. of carbon. 

 P^Vo 10,110 ft. lb. 9902 ft. lb. 



Specific beat of liquid for centigrade I 718-4 443-3 



scale, I " " 



Specific heat of vapour at constant l 668-4 218-9 



pressure for centigrade scale, J " " 



In article 75, the differences between the results of the two 

 methods of computation for alcohol and water are considered as the 

 effects of deviations of the vapours of these fluids from the perfectly 

 gaseous condition, — deviations which in the case of steam have long 

 been anticipated. 



On an Inaccuracy (having its greatest value about 1") in the 

 usual method of computing the Moon's Parallax. By 

 Edward Sang. 



When, as in the usual operation, the moon's observed zenith dis- 

 tance is corrected for the effects of atmospheric refraction, the zenith 

 distance so obtained is that of the rectilineal part of the ray of light 

 between the planet and the upper surface of the air ; and on apply- 

 ing that correction, as at the Observatory, we do not obtain the direc- 

 tion of the moon as it would have been seen if there had been no 

 atmosphere, but that of a line drawn parallel to the first part of the 

 ray, and therefore passing below the moon. The true direction of 

 a straight line drawn from the observer to the planet, must differ 



