2i 6 Gen. Roy’s Account of a 
the fazes. Thus 8 or io inftants might be marked from the 
experiment of a fingle balloon, which might then be hauled 
down to be reloaded for a repetition. But whatever might be the 
mode adopted as the bed: for conducing experiments of this 
nature, the obfervers mud: not only be very attentive and dili- 
gent, but alfo quick-lighted, have their clocks nicely regulated 
indeed, and the trials mud: be many times repeated before the 
uncertainty, even in this way, which feems to be the bed: 
mode, could be reduced to lefs than -^th part of a fecond of 
time, to which it may infallibly be brought by trigonometry. 
Having in this manner fliewn what probable degree of exa£f> 
nefs may be expected in the various, but ufual, ways of afcer- 
taining the difference of longitude between the Obfervalories 
of Greenwich and Paris, and compared the refults with the 
uncertainty that feems yet to exilf in this matter from the date 
of adronomical obfervations ; let us next fee how Mr. Rams- 
den’s indrument is likely to perform, when actually applied 
to the determination in quedion, by the obferved angle be- 
tween the pole dar in its eadern or wedern azimuth, and a 
very remote dation, whofe didance from the indrument is 
known by the feries of triangles, and didinguifhable by the 
Indian lights at night, for the purpofe of this particular 
obfervation. 
With an indrument, carrying telefcopes fo good that the 
pole dar may be feen in daylight, it is obvious, that the bi- 
fefted angle between the dar in its eadern and wedern azi- 
muths will give at once the polar didance of the dar, and 
the true meridian of the place, as referred to any known Ra- 
tions vidble at the time of obfervation. But as cloudy wea- 
ther may often prevent a complete obfervation of this fort 
from being obtained, and fince much time might be lod in 
3 attempting 
