VOL. LXXVI.] PHILOSOPHICAL TfiANSACTIONS. 115 



axis, and confined the thermometer in its place, without the assistance of any 

 other apparatus. Through this stopple two other small holes were bored, and 

 lined with thin glass tubes, opening a passage into the cylinder, which holes were 

 occasionally stopped up with some stopples of cork ; but to prevent accidents, such 

 as I had before experienced from an explosion, great care was taken not to press 

 these stopples into their places with any considerable force, that they might the 

 more easily be blown out by any considerable effort of the confined air. 



Does humidity augment the conducting power of air ? 



To determine this question I made the following experiments, the weather 

 being clear and fine, the mercury in the barometer standing at 27 inches 8 lines, 

 the thermometer at 19°, and the hygrometer at 44°. 



(Exp. N° 17) 

 Thermometer N° 4. 

 Surrounded by air dry to the 44th degree 

 of the quill hygrometer of the Manheim 

 Academy. 



Taken out of freezing water, and plunged 

 into boiling water. 



Time elapsed. 



34* 



39 



44 



51 



6 



35 

 40 



not observed. 



Heat acquired. 

 80** 

 10 

 20 

 30 

 40 

 50 

 60 

 70 

 80 



0° to 70°. 



8 9 = total time of heating from 



(Exp N°18) 

 The same thermometer*(N° 4.) 

 Surrounded by air rendered as moist as' 

 possible by wetting the inside of the cylinder 

 and globe with water. 



Taken out of freezing water, and plunged 

 into boiling water. 



Time elapsed. 



6' 

 4 

 5 



9 

 18 

 26' 

 43 



45 



Heat acquired. 

 . 0° 

 10 

 20 

 30 

 40 

 50 

 60 

 70 

 80 



0° to 70°. 



1 51 = total time of heating from 



From these experiments it appears, that the conducting power of air is very 

 much increased by humidity. To see if the same result would obtain when the 

 experiment was reversed, I now took the thermometer with the moist air out of 

 the boiling water, and plunged it into freezing water ; and moving it about con- 

 tinually from place to place in the freezing water, I observed the times of cool- 

 ing, and compared the result of this experiment with those made with dry air. 



Though the difference of the whole times of cooling from 80° to 1 0° in these 

 two experiments appears to have been very small, yet the difference of the times 

 taken up by the first 20 or 30 degrees from the boiling point is very remarkable, 

 and shows with how much greater facility heat passes in moist air than in 

 dry air. 



Finding so great a difference in the conducting powers of common air and of 

 the Torricellian vacuum, I was led to examine the conducting powers of common 



a 2 



