1836.] of the Wet-bulb Hygrometer. 421 



pressure will, by the experiment, be equal to d -f .27 [<* — — d I 

 instead of d + { rf_-rf j (or simply d~p~ )• I will not seek to en- 

 quire the cause of this deviation from theory ; or whether it be pecu- 

 liar to the form of apparatus I employed ; or whether the effect will 

 be constant under all circumstances : — I will merely suggest that the 

 supply of heat from extraneous sources — the brass tube (only half inch 

 diam.) radiation, &c. could not fail to reduce the cooling effect of the 

 mere current of air ; and here we have the measure of their united 

 disturbing power, which it is satisfactory to find constant throughout. 

 Let us now see whether the same constancy can be traced in the more 

 elaborate experiment with common air (10-11.) The first thing neces- 

 sary is to calculate the percentage of moisture for each step. Now, as 

 under 30 inches the centesimal tension was found to be '58 by the 

 dew-point, and as no source of fresh supply was at hand, the tension 

 at any other pressure should be directly as the pressure, or inversely as 

 the volume ; since it is evident that a double space, for instance, will 

 require twice as much aqueous vapour to bring it to a given state of 

 humidity ; the aqueous tension, therefore,will be '58 X ~ for this 

 series of experiments. Again, from our table of depressions, (from the 

 diagram or from the formula) can be obtained, with the reading at 

 these variable states of humidity, the depression either in dry air or 

 in air of the initial tension '58. I have, in fact, given both in the fol- 

 lowing table, and have set in the three last columns the calculated de- 

 pressions by the expression just found of d + .27 ( d30 _ d \ 



Tab. VIII. — Experiment 10-11, reduced to a constant hygrometric state. 

 Barometer. 



inches. 



30 

 28 

 26 

 24 

 22 

 20 

 18 

 16 

 14 

 12 

 10 

 8 

 6 

 5 

 4.4 



Centes. 



Tabular 



Observ- 



Deduced 



Deduced 









aqueous 



centesi- 



ed de- 



depres- 



depression 

 for dry 



Calculated 



Calculated 



Calculated 



tension 



mal de- 



pression 



sion for 



depression 



depression 



depression 



calc. 



pression 



variable 



aq. ten. 



air, D = 



for varia- 



for aq. tens. 



for dry air. 



H X 2- 

 30 



corres- 

 ponding 

 T 



aq. tens. 

 d 



.58 



d X ;jr 



dX—f 



ble aq. tens, 

 of second 

 column. 



.58 













o 







o 



o 



o 



.58 



32 



12.0 



12.0 



37.5 



12. 



12.0 



37.5 



.54 



34 



13.1 



12.2 



38.2 



13.1 



12.3 



38.2 



.50 



37 



14.6 



12.5 



39.0 



14.5 



12.6 



39.1 



.46 



42 



17.0 



12.8 



40.1 



17.0 



12.8 



40.1 



.42 



44 



19.1 



13.8 



43.3 



18.2 



13.3 



41.4 



.39 



47 



21.5 



14.7 



46.0 



20.1 



13.7 



42.8 



.35 



50 



24.5 



15.7 



49.0 



22.3 



14.3 



44.6 



.31 



54 



26.8 



15.9 



49.8 



25.6 



15.0 



47.4 



.27 



58 



30.6 



16.9 



52.7 



28.8 



15.9 



49-7 



.23 



62 



34.5 



17.8 



55.5 



33.2 



17.1 



53.5 



.19 



66 



38.9 



18.6 



58.2 



39.2 



18.7 



58.9 



.15 



72 



45.3 



20.2 



63.3 



49.4 



19.4 



69.6 



.11 



76 



53.1 



22.3 



69.8 



63.0 



25.7 



80.4 



.096 



78 



61.4 



25.1 



78.5 



71.0 



29.1 



91.0 



.085 



79 



66.1 



26.7 



83.5 



78.0 



34.8 



99.6 



