602 I'HILOSOPHICAL TRANSACTIONS. TaNNO 1795. 



rollers, 3 inches thick, and 4i in diameter; set in strong frames firmly united 

 to the sides of the tube, and resting on the flat face of the square axle which 

 carries the pivot in the centre. The middle of these rollers is applied about 2 

 feet 2 inches from the centre of the pivot; and being set so as to lose none of 

 the motion which they may have on the axle, we find that there is room for full 

 as much angular motion of the rollers on the axle, as there is for the tube be- 

 tween the sides of the ladders; and indeed more than can be wanted, as 10 mi- 

 nutes of time are general sufficient for viewing any object. 



The method of observing with this telescope is by what I have called the 

 front view; and the size of the instrument being such as would permit its 

 being loaded with a seat, there is a very convenient one fixed to the end of it. 

 The foot-board or fioor, is 3 feet broad, and 2 feet 2^ inches deep. The seat 

 is moveable from the height of l foot 7 inches to 2 feet 7 inches, not so much 

 for the accommodation of difl^erent observers, as for the alteration required at 

 different altitudes, and which amounts to nearly 12 inches. One half of the 

 seat fails down, to open an entrance at the back; and being inclosed at the front 

 and sides, a bar which shuts up the back after the observ&r is in his place, 

 secures" the whole in such a manner as to render it perfectly safe and convenient. 

 There are 2 strong iron quadrants with teeth, at the sides of the seat, in which 

 run 2 pinions fixed on a bar, with a ratchet and handle at the end of it. By 

 turning this handle, the seat is easily brought to an horizontal position, before 

 the observer enters it; or restored to it, when any considerable alteration in the 

 altitude of the telescope renders a change necessary. 



The focus of the object mirror, by its proper adjustment, is brought down to 

 about 4 iijches from the lower side of the mouth of the tube, and comes for- 

 ward into the air. By this arrangement, there is room given for that part of 

 the head, which is above the eye, not to interfere much with the rays that go 

 from the object to the mirror; the aperture of the speculum being 4 feet, while 

 the diameter of the tube is 4 feet 10 inches; especially as we suppose a night 

 observer will prefer some kind of warm cap to a hat, the rim of which might 

 obstruct a few of tKe entering rays. A long coarse screw-bar is confined in a 

 collet, which takes on and off, and may readily be put to the inclosing right side 

 of the seat, so as to present the observer with a short handle. The other end 

 of this bar passes into a nut, which, like the collet, moves on a double swivel, 

 so as to admit of every motion. The nut is planted on a machine which will 

 be described hereafter, and may be drawn up to any altitude, so as to bring the 

 nut on a level with the swivel of the handle. On turning the handle, the ob- 

 server will screw himself, the seat, and the telescope, from the ladder; and may 

 thus follow the object he wishes to pursue in its course, for as many minutes 

 as may be convenient. If indeed he is inclined to give up the meridian for some 



