Mr, N Pigott’s Obfervation of the Tranft of Mercury . 385 
obfervation of Mercury. The rate of the clock, and the appa* 
rent times thence deduced, were obtained by equal altitudes of 
the fun, taken with the quadrant. Thefe were the only in- 
ftruments I had, and therefore fuch obfervation s as are not 
dependent on the mzafure of time , are to be confidered as made 
by eftimation ; however, the mo ft important, the internal and 
external contacts of Mercury, and hence the egrefs of his 
center and the interval of time between the two contacts, were 
made in a very fatisfadtory manner. About fix o’clock, when I 
attended for the obfervation, there being a great number of 
folar fpots. Mercury might eafily have been miftaken for one ; 
but his motion foon removed every doubt in that refpedt. Flying 
clouds obfcured the fun at intervals ; but during the laft half 
hour, the weather was fine, the Iky clear, the limb of the 
fun well defined ; Mercury round and very black. There 
feems to have been fome miftake, in refpeft of this phamome- 
non, either in the calculation or the printing of the Con~ 
noijfance des "Temps of this year: the emerfion of the center of 
Mercury is there fet down at 19 h. 45' apparent time at Paris; 
whereas, by my obfervation, the egrefs of the center at Lou- 
vain was at 20 h. \ f 2 8" or 29'' apparent time. Taking here 
no other equation into conlideration, except the difference of 
meridians between Paris and Louvain, which, by a great 
number of obfervations, I determined in 1775 to be f 37^ in 
time the emerfion of the center at Paris muff have been at 
20 h. 3/ / 51" or 52", which differs nearly 53' from the com- 
puted time. By the fame reafoning, 1 fihould fuppofe, that 
the emerfion of the center of Mercury at Greenwich was ol> 
ferved at 20 h. 28' if' or 36"* Mercury being fo very near 
* See Philofophical Tranfa&ions, vol. LXVIII. p. 654. 
the 
