new Syjlem of Wires in a f elf cope. 349 
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. And, 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 mull: 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 pofition 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 aftiftance to what was propofed the laft 
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 inftrument be found 
erroneous, the formula given by M. de la Lande in his 
Aftronomy will ferve to redtify the obfervation. Calling the 
larger interval between the pafifage of any oblique and the ho- 
2 , 2 
rary wire m, and the fmaller one n, will give the dif- 
ference of declination (in time, to be converted into degrees 
... v &. » 
and multiplied by the cofine of declination) from the angle 
where that oblique meets the horary ; and the dif- 
ference in right afcenfion from the fame angle. It mu ft fureiy 
be almoft needlefs to mention, that where the pofition is true, 
half the interval of time between a ftar’s paffing any two cor- 
responding obliques, converted into degrees, and multiplied by 
the cofine of decimation, will give the difference m declination 
of that ftar from the angle where thofe obliques meet, as the 
whole interval does in the rhombus. 
But 
