new Syjlem of Wires in a T^elefcope, 34c> 



caufe, if any part of it be thought doubtful, its tallying or not 

 tallying with the known extent of the field will fhew whe- 

 ther there be any error, or where it lies. i\nd, in each of 

 them, the parallel wires will tell you whether the placing of 

 your inftrument be true or faulty ; becaufe, if truly made and 

 truly fet, the fame ftar mufl: take the fame time in paffing from 

 one wire to its correfponding parallel ; which will differ confi- 

 derably, and in every ftar the fame way, if the poiition be 

 faulty. 



Some of thefe latter remarks might have been fpared, but 

 that they may ferve as hints to fuch gentlemen as may be in- 

 clined to lend their affiftance to what was propofed the lafl 

 year, and who may not have confidered the many helps to be 

 derived from a crofs examination of theobfervations they make. 

 For their ufe alfo it may be proper to add, what indeed is no- 

 thing new, that if the pofition of the infh'ument be found 

 erroneous, the formula given by M. de la Lande in his 

 Aflronomy will ferve to redify the obfervation. Calling the 

 larger interval between the paflage of any oblique and the ho- 

 rary wire w, and the fmalleronew, — , ^ will give the dif- 

 ference of declination (in time, to be converted into degrees, 

 and multiplied by the cofine of declination) from the angle 



where that oblique meets the horary; and — i — r- the dif- 



ference in right afcenfion from the fame angle. It mufl: furely 

 be almoft needlefs to mention, that where the pofition is true, 

 half the interval of time betwesn a ftar's paffing any two cor- 

 refponding obliques, converted into degrees, and multiplied by 

 the cofine of declination, will give the difference in declination 

 of that ftar from the angle where thofe obliques meet, as the 



'whole interval does in the rhombus. 



But 



