TRANSIT OF MERCURr. J'f 



But, when a froft, though very fevere, becomes Tetlled, the 

 mirror will foon accommodate itfeif to the temperature ; and 

 we fhall find our telefcopes to ad well. See obfervation 16, 

 20, 21, 22, 21-, and 25. 



This explains, with equal facility, why no telefcope j u ft or of expofure j 

 brought out of a warm room can a6t properly. See obferva- 

 tion 40 and 41. 



Nor can we ever expccJt to make a deh'cate obfervation, obfervatjons 

 with high magnifying powers, when looking through a door, ^''°"' ^ ""^'"^ 

 window, or flit in the roof of an obfervatory ; even a confined 

 place, though in the open air, will be detrimental. See obfer- 

 vation 42 and 43. 



It equally (liows, that windy weather in general, which muft windy weatherj 

 occafion a mixture of airs of different temperatures, cannot be 

 favourable to diftind vifion. See obfervation 31, 32, and 33. 



The fame remark will apply to Aurora Boreales, when they aurora boreaIe«5 

 induce, as they often do, a confiderable change in the tempera- 

 ture of the different regions of air. See obfervation 28. 



But, Qiould tliey not be accompanied by fuch a change, there 

 feems to be no rcafon why they fiiould injure vifion. See ob- 

 fervation 29 and 30. 



The warm exhalations from the roof of a houfe in a cold the roof of an 

 night, mufi difturb the uniformity of the temperature of a fmall ^°"^^' ^ 

 portion of air ; fo that ftars which are over the houfe, and 

 at no confiderable difiance, may be affeded by it. See obfer- 

 vation 38 and 39. 



Sometimes the weather appears to be fine, and yet our tele- weather appa- 

 fcopes will not a<5t well. This may be owing to drynefs occa-''^""J' ^"^* 

 fioned by an eafterly wind ; or to a change of temperature, 

 arifingfrom an agitation of the upper regions of the atmofphere. 

 See obfervation 34 and 35. 



Or, poffibly, to both thefe caufes combined together. Se« 

 obfervation 36 and 37. 



If moifture in the atmofphere be neceflary, dry air cannot 

 be proper for vifion. See obfervation 26 and 27. 



And therefore, on the contrary, dampnefs, and hazinefs of damps, haze j 

 the atmofphere, mufl be favourable to diftind vifion. See 

 obfervation 1 , 2, 3, 4, 6, and 8. 



Fogs alfo, which certainly denote abundance of moifture, fogs, &c. 

 muft be very favourable to diftind vifion. See obfervation 9, 

 10, 11, 12, 13, and 14. 

 Vol, VI.— -StiPTEMBER, C Na/, 



