1836.] 



of the Wet-bulb Hygrometer. 



429 



a higher temperature, equal to what would be the boiling point of a 

 similar solution. Here the saline solution at 2 1 2° cannot support 

 a tension of/' (=30 in.) ; deposition therefore takes place with conse- 

 quent disengagement of latent heat, until the tension of the salt at 

 t + x finds itself in equilibrio, or = 30 inches*. 



6. Perkins has observed, that when water is thrown upon a heated 

 metal not visibly red, it flashes into steam suddenly : but when placed 

 upon iron, silver or gold at a much higher heat, it takes a considerable 

 time to evaporate. Here would seem to be an indication that at or 

 about 1200 Farh. the evaporation point gradually rises to exactly 

 212°, and that beyond this it becomes negative, or, the depression 

 becomes so great that it falls below the boiling pointf. 



This is surely a more rational explanation than Perkins's, who 

 supposed the liquid to be prevented from evaporating from the enor- 

 mous pressure on its surface : — how could such a false equilibrium hold 

 with free space around for the vapour to expand into ? 



Many other illustrations might be brought forward, but I forbear 

 from exhausting the patience of my readers, and will here conclude 

 with the tables for the depression of the wet-bulb at temperatures from 

 30° to 180o under the constant pressure of 30 inches. For other states 

 of the barometer the small table below will be found sufficient, until 

 my friends in Nepal, Dehra Dun, or the Nilgiris may furnish better 

 data for its correction. 



Table of Multipliers, to convert the following Tabular Depressions at 30 

 inches (T000) into the depressions at any other pressure of the atmosphere. 



Barome- 



Density of 



Multipli- 



Barome- 



Density of Multipli- 



Barome- 



Density of Multipli- 



ter 



the air. 



er 



ter 



the air. 



er 



ter 



the air. 



er 



inches. 



e. 



l+.27"e^l 



inches. 



e. 



l+.27e^T 



inches. 



e. 



l+.27e^l 



29-5 



l'Olfi 



1-004 



24-5 



1-224 



1-060 



19-5 



1-538 



1145 



29-0 



1-034 



1-009 



24-0 



1-250 



1-067 



190 



1-579 



1-156 



28*5 



1-053 



1-014 



23-5 



1-277 



1-075 



18-5 



1-621 



1-168 



28-0 



1-071 



1-019 



23-0 



1-304 



1-082 



18-0 



1-666 



1-180 



27-5 



1-091 



1-025 



22-5 



1-333 



1-090 



17-5 



1-720 



1-194 



27-0 



1-111 



1-030 



22-0 



1-364 



1-0.98 



17-0 



1-765 



1-206 



26-5 



1-132 



1-036 



21-5 



1-395 



1-107 



16-5 



1-818 



1-221 



26-0 



1-154 



1042 



21-0 



1-428 



1-115 



16-0 



1-875 



1-236 



25-5 



1-176 



1-048 



20-5 



1-463 



1-125 



15-5 



1-935 



1-252 



25-0 



1-200 



1-054 



20-0 



1-500 



1-135 



15-0 



2-000 



1-270 



Notk. — When the depression in attenuated air has been observed, divide it by 

 the multipliers here given, before entering the table following to find the aqueous 

 tension. 



* A new source of error in the wet-bulb is hence suggested, in the substance 

 with which the bulb is coated : — flannel, linen, and cotton may have different 

 hygrometric affections. This is a fit subject for inquiry. 



f The very slight modification required in the theoretical curve of depressions, 

 to produce the effect alluded to in the text, is shewn by a dotted line in Fig. 5 of 

 PI. XXI. Apjohn's temperature of evaporation only reaches 212° at 2800°. 

 Leslie's at 2600° ; beyond which it would continue to rise. 



