from the Indications of the Wet-bulb Hygrometer. 263 



resorted to with advantage in the manner already described 

 for ascertaining V by conjoint observations of hygrometer 

 and dew-point*. The principal recommendations to the me- 

 thod I have proposed are, 1st, that the assumed data (viz. the 

 law of dilatation of gases from heat, the latent heat of steam, 

 and the equality of me sum of the latent and thermometricheat 

 at different temperatures,) are all supported by concurrent ex- 

 perimental evidence with very trifling differences. 2nd, That 

 there are no calculations founded on disputed points (viz. the 

 capacity of air for heat, nor of vapour for heat, nor on the 

 changes of capacity of either air or vapour produced by 

 changes of temperature and pressure). 3rd, That the method 

 itself enables us to eliminate the effects arising from the prin- 

 cipal causes of uncertainty, and, in making the method more 

 accurate, furnishes valuable information on collateral subjects. 

 4?th, If at tolerably high temperatures (where V is large) we 

 should not be able to approximate (with certainty) within one 

 or two degrees of its real value, that the error that will ensue 

 in the determination of the dew-point at ordinary tempe- 

 ratures will be so small that it may be fairly disregarded. 

 I have given a table in which the 1st column represents the 

 temperatures of hygrometer, the 2nd column is Dalton's cor- 

 responding " tension of vapour," the 3rd column gives the 

 relative densities of the vapour of saturation of a given volume 

 at those temperatures as derived from formula (A.). The 

 4th column gives the relative quantities of heat requisite to 

 vapourize the moisture of saturation at those different tem- 

 peratures derived from formula (B.) ; and in the 5th column 

 I have attempted to give the values of (V) the fall of hygro- 

 meter in dry air. In forming this I have taken 51°'124 as 

 the value of V at 61, that being the mean value deduced from 

 the five sets of experiments on conjoint observation of dew- 

 point and wet-bulb (at 61°) in the table attached to Dr. Ap- 

 john's paper. Of course, I do not mean to press its accuracy; 

 but a comparison of the observed dew-points with those cal- 

 culated from it at the other temperatures of hygrometer would 

 lead me to think it not to be very far from correct. If the 

 plan now proposed were to be adopted, it would be desirable 

 to interpolate the values (for at least each quarter of a degree) 

 in the various columns of the table. 

 March 16, 1835. Henry Hudson. 



* We can thus ascertain by experiments, when temperature and pressure 

 are both alike, but the humidity of atmosphere different, what effect on 

 capacity of air, estimated by volume, may arise from this cause. 



