propofed trigonometrical Operation. 2 1*5 
lights are alternately exhibited, and again covered by an extin- 
guifher prepared for the purpofe, which operation may be 
repeated feveral times the fame evening;* it is certain, that a 
juft mean being taken between the inftants fo marked by the 
refpettive clocks, well regulated before-hand, the difference of 
time between the two extreme ftations will thereby be ob- 
tained to a very confiderable degree of accuracy, and probably 
more to be relied upon than that refulting from the comparifon 
of the obfervations of the heavenly bodies. 
Rockets have been propofed by fome to be applied in this 
way ? but even the largeft fort, carrying up too finall a body 
of white light, could, I think, only be depended upon for 
fhort diftances ; and fince the a&ual error of obfervation would 
be the fame on a fhort as on a long diftance, remote ftations 
only fhould be chofen for conclulive experiments- of this na- 
ture. It is imagined, that by means of a balloon, a fet of 
light-balls might be fent up of fuch a volume as to be feen, 
from their great elevation in the air, at ftations very remote 
from the place of explofion. The balloon would afeend with 
a burning fuze attached to the firft ball, and having attained 
the wifhed-for height, it might be made faft below, by 
means of a cord fixed to it for the purpofe *. The firft ball, on 
its explofion, would communicate fire to the fecond, and fo in 
fucceffion, at intervals of time proportionable to the lengths of 
* It is to be underftood r that the practicability of fattening the balloon 
to the earth by means of a rope mutt be previoufly afeertained : for if that fhould 
be found impoffible, without the wind forcing it down again to the ground, then it 
mutt be fent up detached with burning fuzes fixed to it, of ftill greater dimenfions. 
By means of thefe it is imagined, that the track of the balloon in the air might be 
pointed out to the remote obfervers, who might perhaps be better er^bled 
thereby to watch for. and dittinguifli the fuccefiive inttants of the burtting of the 
white lights. 
the 
