84 Messrs. Paterson and Dudding on the 



Two facts should now be observed. Firstly, the tempera- 

 ture and humidity effects act against one another, and in 

 practice it is the difference between the two which is 

 operative. Secondly, as at Teddington an increase of one 

 unit of water-vapour is accompanied on the average by a 

 rise of 1°*6 2 C, the combined humidity-temperature coefficient 

 becomes 



0-187- (O0076 5 X 1-6,) =0*063, 



viz., the coefficient in equation (2). Now the coefficient 

 given by Rosa and Crittenden is 0'056 7 , and it is readily 

 seen that this would result from a prevailing climatic condi- 

 tion in which an increase of one litre of water-vapour per cub. 

 metre corresponds with an increase of 1°'7 C, instead of the 

 1 0, 6 2 C. observed at Teddington. It is obvious that too 

 great a significance must not be attached to the actual values 

 of the coefficients in equation (4), since they depend on the 

 assumption that a linear relation connects humidity and 

 temperature. 



The difference between the " humidity " coefficients deter- 

 mined (by neglecting the temperature coefficient) in England 

 and America being much larger than that which could be 

 attributed to the error of the experiments, tends to support 

 the suggestion of the authors that the pentane lamp has a 

 temperature coefficient, but that the usual method of making 

 the observations and deducing the results does not allow of its 

 determination. Thus, it would appear that the " humidity"' 

 coefficients determined for flame-standards are really com- 

 bined humidity-temperature coefficients. Whenever, there- 

 fore, a lamp is used under conditions of humidity and 

 temperature which approximate to those existing at the 

 locality where the original determination was made, the 

 constant so determined will apply rigidly. If, however, 

 the determination of the combined humidity-temperature 

 coefficient be made under different climatic conditions, a 

 slightly different constant may be expected. If, for instance, 

 the humidity at Washington tends on the whole to increase 

 at a different rate with temperature than it does at Tedding- 

 ton, a different factor for the combined effects would be 

 expected to result. The authors are not in a position to 

 know if this is actually the case, but it is not unreasonable to 

 suppose that differences of the order indicated might be 

 found to exist, and if this should be so it would afford 

 an explanation of the difference which has been found 

 between the " humidity " coefficients determined in the two 

 localities. 



