ANALYSIS OF QUANTITATIVE PHENOMENA. 405 



in which it was graduated c . There is no end to the 

 number of minute corrections which may ultimately be 

 required. A very large number of experiments on gases, 

 standard weights and measures, &c. depend upon the 

 height of the barometer ; but when experiments in dif- 

 ferent parts of the world are compared together we ought 

 to take into account the varying force of gravity, which 

 even between London and Paris makes a difference of 

 *oo8 inch of mercury. 



The measurement of quantities of heat is a matter of 

 great difficulty, because there is no known substance 

 impervious to heat, and the problem is therefore as 

 difficult as to measure liquids in porous vessels. To 

 determine the latent heat of steam we must condense a 

 certain amount of the steam in a known weight of water, 

 and then observe the rise of temperature of the water. 

 But while we are carrying out the experiment, part of the 

 heat will have escaped by radiation or conduction from 

 the condensing vessel or calorimeter. We may indeed 

 reduce the loss of heat by using vessels with double sides 

 and bright surfaces, surrounded with swan's-down wool or 

 other non-conducting materials ; and we may also avoid 

 raising the temperature of the water much above that of 

 the surrounding air. Yet we cannot by any such means 

 render the loss of heat inconsiderable. Rumford ingeni- 

 ously proposed to reduce the loss to zero by commencing 

 the experiment when the temperature of the calorimeter 

 is as much below that of the air as it is at the end of the 

 experiment above it. Thus the vessel will first gain and 

 then lose by radiation and conduction, and these opposite 

 errors will approximately balance each other. But Reg- 

 nault has shown that the loss and gain do nojb proceed by 

 exactly the same laws, so that in very accurate inves- 

 tigations Rumford's method is not sufficient. There 



c Balfour Stewart, ' Elemeutary Treatise on Heat/ p. 16. 



