[ 2IO ] 



Take an Example each way: 



ift. 8<^ : 12H. M O T. : : 40' : I H. M. O T. : but i H. 

 M. G T.= iH. o'. 9", 83 fydereal time. 



2d. 12H. M. OT. = 12H. I'. 58', 283 fydereal time. 

 Therefore 8° : i 2H. r. 58", 283 : ; 40': iH. o' 9", 83. 



It is further to be obferved that though the two correaions I 

 have juft ftatcd are the principal ones that ufually occur in this 

 calculation, and indeed the only ones that for meridians of fhort 

 intervals afunder are fenlibly requifite, yet in order to difcufs 

 this queflion fully it may be neceflary to mention fome other 

 fources of irregularity, that at particular times and at places 

 confiderably diftant from each other in longitude, well deferve 

 the notice of the attentive and fcrupulous aftronomer. The ap- 

 parent dilk of the moon (as feen from the centre of the earth) 

 being a quantity perpetually varying in its dimeilfions, if its 

 change (liould be fenfible in the interval between the obfer- 

 vations at the iirft and fccond meridian, it is evident that in 

 proportion as the diameter of the moon is either contraded 

 or expanded during this interval, the enlightened limb which 

 iliould be a fixed boundary of menfuration for each place will 

 cither retire farther from or be protruded towards the ftar more 

 under the fecond meridian than it was under the former; and 

 thus, independent of all other caufes of error, vitiate the accu- 

 racy of the mcafurement. This alteration of the difk happens 

 from the moon's changing her abfolute diftance from the earth 



during 



