]g TRANSIT OF MERCURY. 



focal Length. 



ObfervaMons in (i-V.) Nov. 14, 1801. The focal length of my 10-feet 

 which the focal ^jfj-o^ increafes by the heat of the fun. I have often observed 



]ength and figure •' 



of the fpeculum tliis before; the dilTerence, by feveral trials, amounts to 8 

 was altered by hundredlhs of an inch. 



(48.) Dec. 13, 1801. The focal length of my 10-feet 

 mirfor, while I was looking at tlie fun, became fliorter, con- 

 trary to what it ufed to do ; but, there being a ftrong froft, I 

 guefs that the objedt metal grows colder, notwithftanding its 

 expofure to the fun's rays. 



(49.) Nov. 9, 1802. 10" 50'. The focus of my 7 -feet glafs 

 mirror became 18 hundredths of an inch (liorter, on being ex- 

 pofed for about a minute tu the fun. The figure of the fpe- 

 culum was alfo difiorted ; the foci of the infide and outfide 

 rays differing confiderably, though its curvature, by obferva- 

 tions on the ftars, lias been afcertained to be flrictly parabo- 

 lical. 



12''0'. The fame mirror, expofed one minute to the action 

 of the fun, became 21 hundredths (liorter in focal length. 



The focus of a 10-leet metalline mirror, when expofed one 

 minute to the fun's rays, became 13 hundredths of an inch 

 longer than it was before. 



(50.) January 9, 1803. When I looked with the glafs 7- 

 feet mirror, feveral times, a minute or two at the fun, it (hort- 

 ened geiierally ,24, ,26, and ,30 of an inch, in focal length. 



The obfervations which are now before us, appear to be 

 fuliicient to eflabliili the following principle; namely. 

 General prin- " That in order to fee well witli telefcopes, it is required 



ciple. Uniform jj^^t the temperature of the atmofphere and mirror thouid be 

 rnoifturem the uniform, and the air fraught with moifture." 

 air are requlftte TWi^ being admitted, we (liall find no difficulty in account- 



ij w •vijioft. .^_^ ^.^^ every one of the foregoing obfervations. 

 Thlsdoarine If an uniform temperature be necelfary, a froft after mild 



applied to, weather, or a thaw alter froft, will derange the performance of 



our mirrors, till either the froft or the mild weather are fuffi- 

 cientiy fettled, that the temperature of the mirror may accom- 

 f'Mcn changes fn<^'^ate itfelf to that of the air. For, till fuch an uniformity 

 cf tctrpsrature ; with the open air, in the temperature of the mirror, the tube, 

 the eye-glaifes, and I would almoft add the obferver, be ob- 

 tained, we cannot expcdl to fee well. See obfervation 15, 

 17, 18, 19, and 23. 



But 



