708 PHILOSOPHICAL TRANSACTIONS. [aNNO 1/80. 



there was always a remarkable fall of the mercury, not less than 6°. In shifting 



the instrument from the pole to the ballustrade, it was commonly laid on some 



hoar-frost f of an inch deep, which had settled on a piece of thin board which 



had been for years exposed to the weather. Some fragments of the hoar-frost 



were also made to touch the upper part of the ball; which was done by i)ushing 



them on with a long frozen straw. 



Exper. 8. When the thermometer, taken At Nidu Therm, on Therm in 

 ^ . i-ii. •.. 1-] tlie snow. the air, 



from the pole as m last experiment, was laid on ^^g>, + 5 + 0" 



pieces of stone, from which the hoar-frost had yj + 5 + 8 



been brushed away for some time before, the jq^' ' _' [ ' ' ' ' ^ f^' '..'...X 10 



mercury sunk but very little by such a change 11 + 6" + 9 



of situation. Next night, beingthat of Sun- \2^'.\\'.'.\\'.X 5.'.'.'.'.'."+ 8 



day Jan. 13, the thermometers were placed in 12^ + 4 -1- 7 



their former station below, when they pointed 1 j"^"'"' + 1 t \ 



as annexed. 



From these observations it appears, that the cold now was very moderate when 

 compared to that of the 14th, and somewhat more moderate than that of the 

 preceding night. Experiment 7th was again repeated with a similar result, 

 though the difference of temperature was not now so great. This night Mr. W. 

 made another experiment with a view to the evaporation, not so liable to objec- 

 tions as those of the bellows and the fan, as follows. 



Exper. g. When the thermometer in air at the lower station had contracted a 

 considerable film of frozen matter all over the ball, it was swung round at the 

 end of a pack-thread, about a yard and a half long. On stopping the motion 

 at the expiration of 1 minutes, and making the servant who waited approach 

 quickly with a lighted candle, he found the mercury had got up 2°. In this 

 experiment, which was repeated 4 times with the same result, care was always 

 taken to keep the instrument to windward of their bodies, and of the lighted 

 candle. 



The 2 following experiments afford some grounds for believing that no kind 

 of evaporation was going on at the time the remarkable excess of cold in the 

 snow and hard frost was observed. 



Exper. 10. On Sunday morning, Jan. 23, before 1 o'clock, Mr. W. repeated 

 the experiment with the metal speculum which was tried herein 1 768. A large 

 spare metal of a 2-foot telescope was laid out to cool, after which a film of ice 

 was imparted to its polished surface by breathing on it 4 or 5 times. It was then 

 exposed as before, and in half an hour the whole film disappeared in the way of 

 evaporation. But when the experiment was again repeated, antl a thicker film 

 imparted, some of this, towards the middle of the speculum, remained fixed, 

 and would not go off after long exposure. The speculum was next warmed, 



