30$ 
On Signals made by Fire, 
to prevent its play. The head of the serew may extend 
come lines beyond the superficies of the plates, and in 
such a manner as that those tubes may turn, with their 
plates, about these screws, in order to direct them on the 
boards or screens p q , behind which the signals by fire 
are made, according to the different distances of the pla- 
ces where the signals shall be given. 
The tubes must be blacked within, in order that, when 
the eye is applied to one of their ends, it may not receive 
any reflected rays. There must also be placed about the 
end, on the side of the observer, a perforated ring, the 
aperture of which must be three or four lines ; and 
place at the other end two threads, the one vertical, and 
the other horizontal, crossing one another in the axis of 
the tube. 
In the middle of the beam a b must be made a round 
hole, two inches in diameter, in which must be fixed the 
foot l m no, which supports the whole machine, and 
round which it turns as on its axis. This machine may 
be called a rule and sights, though it differs from that 
which is applied to circumferenters, theodolites, and even 
geometrical squares, which are used to draw maps, take 
plans, and surveys, &c. but it has the same uses, which 
is to direct the sight. 
The person who makes the signal, and he who re- 
ceives it, must have the like instrument ; otherwise the 
man who receives the signal could not distinguish whe- 
ther the signals made are to the right or left of him who 
makes them, which is an essential circumstance, accor- 
ding to the method proposed by Polybius. 
The two boards or screens p q, which are to denote 
the right and left hand of the man who gives the signals, 
or to display or hide the fires, according to the circum- 
stance of the observation, ought to be greater or less, and 
nearer or further distant from one another, according ^s 
Yol. i. Qq 
