304 Of Telegraphic Communication . 
a b is a beam about four or five feet long; five or six 
inches broad, and two or three inches thick. At the ex- 
tremities of it are, well dove-tailed and fixed exactly per- 
pendicular in the middle, two cross pieces of wood, 
c d, e f of equal breadth and thickness with the 
beam, and three or four feet long. The sides of these 
cross pieces of timber must be exactly parallel, and their 
upper superficies very smooth. In the middle of the sur- 
face of each of these pieces, a right line must be drawn 
parallel to their sides ; and consequently these lines will 
be parallel to one another. At an inch and a half, or two 
inches, distance from these lines, and exactly in the mid- 
dle of the length of each cross piece, there must be driven 
in very strongly, and exactly perpendicular, an iron or 
brass screw, whose upper part, which must be cylindri- 
cal, and five or six lines in diameter,* shall project se- 
ven or eight lines above the superficies of these cross 
pieces. 
On these pieces must be placed two hollow* tubes or 
cylinders, g /i, i k, through which the observations are 
made. These tubes must be exactly cylindrical, and for- 
med of some hard, solid metal, in order that they may 
not shrink or warp. They must be a foot longer than the 
cross pieces on which they are fixed, and thereby will 
extend six inches beyond it at each end. These two tubes 
must be fixed on two plates of the same metal, in the 
middle of whose length shall be a small convexity, of 
about an inch round. In the middle of this part must be 
a hole exactly round, about half an inch in diameter ; so 
that applying the plates on which these tubes are fixed 
upon the cross pieces of wood c d, e f this hole must 
be exactly filled by the projecting and cylindrical part of 
the screw which was fixed in it, and in such a manner as 
* Twelfth part of an inch. 
