70 LECTURES TO SCIENCE TEACHERS. 



33. But the weight of a given volume of air is necessarily 

 greater or less according to the pressure of the atmosphere, 

 the temperature of the air and other conditions affecting it. 



In comparing two standard weights, the weight of air 

 displaced by each is computed from the following data : 



As regards the weight of air, 



(1.) The observed mean barometric pressure during the 

 comparison, reduced to 32 F. and corrected by deducting 

 the pressure of vapour and of carbonic acid gas in the air. 

 (2.) The mean temperature of the air. 



As regards the volume of the standard weight displacing 



the air, 



(3.) The density of each weight. 



(4.) Their mean temperature and coefficient of cubic expan- 

 sion. 



(5.) The actual weight of each standard. 



34. From data 1 and 2 the ratio of the density of the 

 air to the maximum density of water must be ascertained. 

 This ratio is also affected by the latitude of the place where 

 the comparisons are made, and its height in relation to the 

 mean level of the sea, as the force of gravity differs accord- 

 ingly, as I have before explained to you. For determining 

 this ratio, tables have been prepared by Professor Miller, and 

 are used at the Standards Office. They are printed in 

 Appendix V. to the Fifth Eeport of the Standards Commis- 

 sion. These tables are also available at other places by 

 making a slight correction for difference of latitude and 

 height in relation to sea level. They are based upon the 

 weight of a litre of dry atmospheric air as deduced from 

 M. Regnault's observations at Paris, viz. : 1'2932227 gramme 

 at C., in latitude 48 50' 14", 60 metres above sea-level, 

 with a barometric pressure of 760 millimetres of mercury ; 

 and assuming that atmospheric air contains on an average 

 0-0004 of its volume of carbonic acid, and that the pres- 

 sure of vapour in comparing rooms is on an average two- 

 thirds of the maximum pressure due to the temperature, the 

 density of the vapour of water being 0*622 of the density 

 of air. 



35. As regards the other data, the first to be considered is 

 the density of each weight. 



The density of a body is defined as the mass contained in 

 its volume when referred to a uniform standard. Its density 



