from the Indications of the Wet-bulb Hygrometer. 187 



temperature, the quantities of caloric which it loses and gains in 

 a given time are perfectly equal. This requires no demonstra- 

 tion. The caloric lost also is entirely employed in converting 

 the water into vapour; but the whole of the acquired caloric is 

 not necessarily derived, though this is assumed to be the case, 

 from the air cooled by contact with the bulb of the instru- 

 ment. In fact, the hygrometer is in the predicament of a 

 cool body placed in a warm medium, and it must consequent- 

 ly receive from surrounding bodies by radiation a greater 

 amount of caloric than it imparts to them in virtue of the 

 same process. To the d grains, therefore, of moisture con- 

 verted into vapour by the heat given out by 4195 grains of 

 air in cooling through d degrees, we should add the addi- 

 tional quantity vaporized by the heat which the bulb has in 

 the same time received by radiation. Where t—t^ is small, 

 this quantity may probably be safely neglected, but it will 

 sometimes, I make no doubt, be of sufficient magnitude to 

 exert an appreciable influence. I regret my inability to 

 assign any means of determining its amount, and shall merely 

 add that the neglect of this correction will always tend to 

 make the calculated dew-point somewhat higher than the 

 true. 



Having disposed thus rapidly of the theory of my method, 

 I shall conclude by subjecting the results which it affords to 

 the test of experiment: I shall not at present refer to my own 

 observations, though I have amassed a considerable number 

 on the hygrometer and dew-point. As a more unimpeach- 

 able criterion I shall compare my formula with the observa- 

 tions of others, and shall select for this purpose, it being the 

 nearest to hand, a table published in the last number [Oct. 

 ]834] of Prof. Jameson's Edinburgh New Philosophical 

 Journal. The differences, it will be seen, between the cor- 

 responding numbers of the fourth and fifth* columns, are so 

 small that we may consider them as almost entirely due to 

 errors of observation. I may add, that as in the original table 

 there is no notice taken of the barometer, the formula in its 

 most complete form could not be applied, so that a perfect 

 coincidence between calculation and observation was not in 

 this instance to be expected. 



January 3, 1835. James Apjohn. 



* The numbers in the fourth column are the observed dew-points, and 

 those in the fifth the dew-points deduced by ray formula. h 



2B2 



