extraordinary Degree of Cold at Glafgow. 467 
over with the fame fort of hoar-froft which was attached 
to other bodies which had been long expofed. 
The two following experiments afford fome grounds 
for believing that no kind of evaporation was going on 
at the time the remarkable excefs of cold in the fnow 
and hoar-froft was obferved. 
EXPERIMENT X. 
On Sunday morning, Jan. 23. before one o’clock, Df- 
crawford and I repeated the experiment with the me- 
tal fpeculum which was tried here in 1768. A large 
fpare metal of a two-foot telefcope was laid out to cool, 
after which a film of ice was imparted to its polifhed 
furface by breathing on it four or five times. It was 
then expofed as before, and in half an hour the whole 
film difappeared in the way of evaporation. But when, 
the experiment was again repeated, and a thicker film 
imparted, fome of this, towards the middle of the fpecu- 
lum, remained fixed, and would not go off after long ex- 
pofure. The fpeculum was next warmed, and its po- 
lifhed furface made quite clean, and then laid out for two 
hours and a half. Before the expiration of this time it 
began to draw frozen matter from the air, which fettled 
all over the polifhed furface in long parallel lines which 
Vol, LXX. Q q q gradually 
