Temperature. Thermometers 157 



altitude at which the temperature is 9 and the pressure 

 680 mm. ? 



Transforming the Centigrade temperatures into 

 Absolute temperatures, they become 291 and 282. 

 Since the volumes of gas are proportional to these 

 temperatures, the volume which 50,000 cu. ft. at 18 

 will have at 9 is 50,000 x ||f cu. ft. 



This would be the volume, assuming that no 

 pressure change were effected simultaneously with the 

 alteration in temperature, and the pressure change 

 must be allowed for by multiplying the volume, cor- 

 rected for temperature as above, by Jfg, as required 

 by Boyle's Law. Thus the volume which the coal-gas 

 would occupy at 9 and 680 mm. is 



50,000 x |ff x lif = 54,509 cu. ft. 



Assuming the capacity of the balloon to be un- 

 affected by its ascension, it is evident that 54,509 

 - 50,000 = 4509 cu. ft. of gas must have escaped from 

 it. 



The above method, in which the changes of volume 

 brought about by variations of temperature and 

 pressure are considered consecutively, and the law 

 governing each such change is directly invoked in 

 finding the necessary correction in each case, is recom- 

 mended in preference to methods involving the use 

 of memorised formulae, in which mere substitution of 

 the values of the quantities mentioned in the question 

 for the various symbols in the formulae takes the 

 place of reasoning based on knowledge of the properties 

 of gaseous bodies. 



