l62 



PHILOSOPHICAL TRANSACTIONS. 



[anno 177 If 



the windward of it, care having been also taken to keep it under a proper shade, 

 so long as the sun shone out. 



Register of the Thermometer, kept at the college of Glasgow, on Sunday, Jan. 3, 



and Monday Jan. 4, 1768. 



Sunday 

 morning 



10 o' clock + 5 deg. 



afternoon 



It was observable, that after 



The temperature of the snow ^^n setting, the atmosphere had 



on Sunday morning, at about a tendency sometimes to turn a 



IlA ten inches below the surface, i-^^i r ^ ■ . , , 



9h was near to 30 deg. ""le foggy, and agam quickly 



to clear up, balancing as it were 

 between these two different states. It is worthy of no- 

 tice, that the minute variations of the thermometer, as 

 set down in the above register, seemed to depend on 

 these different constitutions of the air ; the mercury al- 

 ways rising in the thermometer a small matter, when 

 the mistiness came on, and vice versa. 



In the intervals of observations, they made some other 

 experiments, which the present intensity of the frost 

 suggested ; particularly one relating to the evaporation 

 of ice, which was tried in the following manner. Mr. 

 W. took a square reflecting metal belonging to his 2- - 

 foot telescope, and exposed it on the ballustrade of the 

 observatory, till it had acquired the temperature of the 

 place, which was then at O deg. : after it was thus 

 cooled, he breathed on it repeatedly, till its polished 

 surface was covered over with an incrustation of ice or 

 frozen vapour, of a very palpable thickness. In this 

 condition the speculum was replaced in its former situa- 

 tion, having its incrusted surface exposed to the still open air , when, in a 

 little time, they found the frozen pellicle beginning to disappear at the outer 

 edge, all around, leaving the iDetal quite clear. Gradually more and more of 

 the speculum was bared in a regular progression, from the circumference towards 

 the centre; and at last, in about 50 minutes, the whole surface had parted with 

 its ice. This experiment was repeated when the speculum was defended from the 

 open air, by a large thin box, with a cloth over it. The event turned out the 

 same as before, only it required longer time. 



This progress of the evaporation, from the outer parts towards the centre of 

 the speculum, was probably owing to the original plate of ice being thickest to- 

 wards the centre, a circumstance which might arise from the manner of fixing 

 it at first breathing on it. Or perhaps it may be imputed to some more curious 



Monday 

 morning 



II 

 12 



I 



2 



3 



H 

 4 



*4 

 5 



I' 



7 



n 



8 



H 

 9 



9i 

 10 

 10} 



11 



iij 



12 



12J 

 1 

 2 



2i 



3 



3J 



4 



4j 



5 



7 



9 



10 



6J 



H 

 2 



n 



n 

 n 



— I 



-oj 



— I 



— 2 



— 1 



— 2 



— 2 



— 2 



— 



-oj 



— 1 



— 

 + 3 



6 



7 



9 



10 



12 



