10 TRANSIT OF MERCURV. 



OBSERVATIONS AND EXPERIMENTS RELATING TO THE 

 CAUSES WHICH OFTEN AFFECT MIRRORS, SO AS TO 

 PREVENT TKEIR SHOWING OBJECTS DISTINCTLY. 



The aftion of It is well known to aftronomers, that telefcopes will aft very 

 refleaing tele- difjgrently at different times. The caufe of the many difap- 



fcopes is very . ^ , ■ , ■ , • , r . . 



different at dlf- pointments they may have met with in their oblervations, is 



fcrent times. however not fo well underftood. 



Sometimes we have feen the failure afcribed to certain tre- 

 mors, as belonging to fpecula ; and remedies have been pointed 

 out for preventing them. Not unfrequently again, the tele- 

 fcope itfelf has been condemned ; or, if its goodnefs could not 

 admit of a doubt, the weather in general has been declared 

 bad, though poffibly it might be as proper for diftindl vifion as 

 any we can exped in this changeable climate. 



The experience acquired by many years of obfervation, will 

 however, I believe, enable me now to affign the principal 

 caufe of the difappointments to which we are fo often expofed. 

 Unwilling to hazard any opinion that is not properly fupporled 

 by fads, I (hall have recourfe to a coUedion of occafional ob- 

 fervations. They have been made with fpecula of undoubted 

 goodnefs, lb that every caufe which impeded their proper ac- 

 tion muft be looked upon as extrinlic. I (ball arrange thefe 

 obfervations under different heads, that, when they have been 

 related, there may remain no difficulty to draw a few general 

 conclufions from them, which will be found to throw a con- 

 liderable light upon our fubjedl. 



Moifture in the Air. 



Whether moif- (1 .) Oftober 5, 1781. I fee double ftars, with 460, com- 



-"'^ dVthV" P^^tely well. The air is very damp. 



adtion of tele- (2.) Nov. 23, 1781. li** 30'. The morning is uncom- 



fcopes. monly favourable, and I fee the treble flar ^ Cancri, with 460, 



in high perfedion. The air is very moift, and intermixed with 

 paffing clouds. 



(3.) Sept. 7, 1782, I viewed the double ftar preceding 

 12 Camelopardali?,* with 932. In this, and feveral other 

 fine nights which I have lately had, the condenfing moifture 

 on the tube of my telefcope has been running down in ftreams; 

 which proves that damp air is no enemy to good vifion, 



* See Phil. Tranf. Vol. LXXV. Part I. page 68 ; II. 53. 



(4.) Dec, 



