over the Disk of the Sun, &c. 227 
induce, as they often do, a considerable change in the tempera- 
ture of the different regions of air. See obs. 28. 
But, should they not be accompanied by such a change, there 
seems to be no reason why they should injure vision. See obs. 
29 and 30. 
The warm exhalations from the roof of a house in a cold night, 
must disturb the uniformity of the temperature of a small por- 
tion of air; so that stars which are over the house, and at no 
considerable distance, may be affected by it. See obs. 38 and 39. 
Sometimes the weather appears to be fine, and yet our tele- 
scopes will not act well. This may be owing to dryness occa- 
sioned by an easterly wind; or to a change of temperature, 
arising from an agitation of the upper regions of the atmosphere. 
See obs. 34 and 35. 
Or, possibly, to both these causes combined together. See 
obs. 36 and 37. 
If moisture in the atmosphere be necessary, dry air cannot 
be proper for vision. See obs. 2 6 and 27. 
And therefore, on the contrary, dampness, and haziness of 
the atmosphere, must be favourable to distinct vision. See obs. 
1, 2, 3, 4, 6 , and 8. 
Fogs also, which certainly denote abundance of moisture, 
must be very favourable to distinct vision. See obs. 9, 10, 1 i t 
12, 13, and 14. 
Nay, if the observatory should be surrounded by water, we 
need be under no apprehension on that account. Perhaps, were 
we to erect a building for astronomical purposes only, we ought 
not to object to grounds which are occasionally flooded ; the 
neighbourhood of a river, a lake, or other generally called damp 
situations. See obs. 5 and 7. 
Gg 2 
