ASTRONOMICAL PAPERS. 



i6i 



From the above meafures, and the appulfes of the Sun 

 and planet to the wires of the telefcope, a projedion of the 

 tranfit might be made, were it neceffary. But the chief 

 advantages to be derived from obfervations of a tranfit of 

 Mercury, are the pcrfcdling the theory of his motions, and 

 fixing the longitude of places on the earth. For the firft, 

 the lead diftance of the centers, and the diameters of q and 

 ^ , which may be got from the foregoing obfervations, are 

 the moft material elements; and for the fecond, the con- 

 tads and their exad times are fufiicient, 



Witli refped to the theory of mercury's motion, the late 

 Dr. Halkf hath left but little to be fettled. He obferves 

 (Philofophical Tranfadions, vol. VI. No. 39.) a remark- 

 able period in this planet's motion, wherein he makes 191 

 revolutions about the Sun, and correfponding tranfits over 

 his diJL Thus, if at the afcending node^ the planet hath 

 paffed over the Sun, it will, in 46 Julian years, 4 hours, 

 51 minutes, (if there have been 12 intercalations) pafs 

 over the Sun again, only i'. 22^^ more northerly; or in 

 one day more, if there have been but 1 1 intercalations. 

 At the defccnding node^ the period is 46 years, 7 hours, 14 

 minutes, or one day more, according as the intercalation 

 requires. Thus, If one tranfit of any fcries or clafs hath 

 been obferved, the times of the following correfpondent 

 ones are obtained by addition only; and all we have to do, 

 is examine the theory by the obfervations, to fee if it needs 



corredion. 



The firft time that ever vierciiry was. obferved on the 

 Snn^s diflc, was by Gafiendus, at Paris, Odober 28th, 

 1631, O. S; and the late tranfit of November 9th, was 

 the fourth in that clafs the two intermediate ones, each at 

 46 years diftance, being obferved by Dr. Halky^ in 1677 

 and 1723. This clafs, therefore, will aft'ord as good a 

 comparifon as any. 



Thus, at Paris, 28th Odober, 1631, at 10^^-28' viam^ 

 Gafltmdus obferved the laft external contad. Whence the 

 middle reduced to the meridian of Greenwich was, in the 



aftronomical 



