Transit of Mercury over the Sun, 291 



the true inlernal contact at the exit of Mercury. Mr. 

 Richard Best*, who was observing near nie with a five-feet 

 achromatic telescope, made the same observation exactly 

 at the same time. The regular circumferences of Mercury 

 and the Sun appeared in contact about two seconds later, 

 the small ligament joining the limbs of Mercury and the 

 Sun being for that space of lime plainly visible. 



* ' The external cbntact of Mercury and the Sun wds otDserved 

 at 14^ 53"* \6^'S or 23'-^ 59"" P*l apparent time by myself, 

 14 33 13-8 23 58 58'1 by Mr. Best. 



T. FlRMINGER." 



Method employed in reducing the Measures of the Distances 

 taken lij the Micrometer^ hetween the nearest Limbs of 

 the Sun and Mercury. (See Plate VIII.) 



LetO (Fig. 3.) be the centre of the Sun, AC the apparent 

 path of Mercury, A O, BO, C O, any three observed distancesj 

 C D a perpendicular to AC, B F a perpendicular to A O, 

 and B E a right line parallel to C O meeting A in E. Then, 

 because the differences of times between the observations are 

 given, and the planet's relative motion is nearly uniform, the 

 proportion of A C to A B, or of C O to B E, or A O to A E, is 

 given^ being as the time between the first and third obser- 



* To this gentleman, who is extremely fond of astronomy, I was greatly 

 indebted for the assistance he afforded me during the whole time of the transit : 

 his readiness in reading off the scale of the micrometer enabled me to pro- 

 cure at least double the number of measures I could otherwise have done; 

 Throughout the whole of the measures, every possible care was taken in 

 making the exterior circular limb of Mercury to touch the nearest interior 

 limb of the Sun ; but in the middle of the observation, and at a time when 

 these distances were most wanted, I was prevented from, proceeding with so 

 much accuracy, or acquiring so great a number as I could have wished, in 

 consequence of a part of the micrometer coming up against one of the sup- 

 porting pieces of the object end of the telescope -, a Circumstance I was un- 

 prepared to meet, having never used the micrometer before, or understood 

 that such a defect existed. I mentioned this to Dr. Maskelyne when he re- 

 turned, (he being in Wiltshire at the time of the observation,) who said he 

 had often met with the same circumstance when observing the lucid parts 

 of the Sim in a solar ecUpse. This defect was soon afttf.- remedied by Mr. 

 George Doliond, by a very simple plan I suggested to him of making the sup- 

 port of the object end of the telescope to shift, by conneciing it to a move- 

 able collar; 



T 8 vaiions 



