VOL. XXXII.] PHILOSOPHICAL TRANSACTIONS. 64g 



and thickness. This head carries two cheeks, ll, about 13 or 14 inches in 

 height, their hinder edges, towards the lower end, extending 5 inches beyond 

 the axis of the pillar backward. Along the back of these cheeks, at equal 

 distances above each other, there are notches, tending obliquely downwards, 

 and answering each other in each cheek, to receive the pivots of a crooked 

 iron axis, c, fig. 13, on which the tube is placed. The notches are made at 

 different heights, to keep the eye-glass at a proper height for the eye, in dif- 

 ferent elevations of the object above the horizon. The figure of the axis an- 

 swers that of the three under sides of the tube. The axis of the tube lies 

 about 24- inches higher than the axis of the motion on these pivots, and the 

 centre of gravity, when the object metal is in, is about 3 inches backwarder. 

 To keep the tube from slipping back, when its fore end is raised, it has two 

 buttons fixed to it, which rest against the fore part of the axis. 



To keep the pillar from touching any of the sides of the round hole, in 

 which it turns, a cylindrical sector, containing about 65^^ or 70°, and about an 

 inch in height, is cut out on the back part of the pillar, near the upper end 

 D. In the square angle of this cavity is fixed a thin steel plate, bent across the 

 middle to the same angle. The internal angular edge, between the two parts 

 of this plate, lies in the axis of the pillar, and turns on the hardened edge of 

 a wedgelike iron, f, whose base, or board part, is fastened with two strong 

 screws on the top of the box, directly behind the round hole beforementioned. 



The upper parts of the cheeks are strengthened by two brackets, gg, leaving 

 room between them for the bottom of the tube to touch the upper edge of the 

 fore part of the head. The hinder part of the head is also hollowed, in the 

 manner represented in the third figure. 



The head on its fore part carries a flat arm, m, fig. 1 1, about 27 inches long, 

 a little taper towards the farther end, where it is 4 inches broad. This is 

 strengthened by a narrow slip, glued edgewise along the middle underneath, o, 

 and also by a brace or stay, n, reaching from the turning pillar to within g 

 inches of the end of the arm. The stay passes through a transverse opening 

 cut in the fore part of the box, p, which is long enough to allow room for a 

 sufficient motion of the pillar round its axis. 



On the other end of the bottom plank, transversely to its length, is erected 

 a board about 12 inches wide, and 26 or 27 high, q, the top of it reaching 

 within an inch and a half of the under side of the arm. This board is held 

 firm in its position by a spur, r, part of its upper end on the outside is pared 

 off toward the edges, to form it into the segment of a cylinder, whose axis 

 coincides with that of the pillar. Its use is to support a rest, ss, on which the 

 end of the fiat arm moves backward and forward. I'his rest being applied 



VOL. VI, 4 O 



