VOL. XXXII.] PHILOSOPHICAL TRANSACTIONS. 651 



to its due strength. The spring on the left hand, n, draws the left side of the 

 pillar backwards in the same manner. These pins are placed on the pillar a 

 little higher than the tops of the springs, that being drawn a little downwards, 

 as well as turned round its axis, the pivot in its bottom may not be raised out 

 of the hole in the brass plate, when the rest bears hard against the rollers at 

 the end of the arm. Each of these springs draws with a force equal to about 

 18 or 20 lb. weight, when the end of the arm is carried close to the small head, 

 k, fig. 11, and consequently, the semidiameter of the pillar being an inch and 

 half, and the distance of that head from the axis about 28 or 2Q inches, the 

 end of the arm will be carried by the united forces of both the springs, towards 

 the other end of the rest, with a force equivalent to the weight of about 2 lb. 

 Each of the pegs, y and z, turns in a hole made in a piece of wood, 1, fastened 

 to the under side of the arm ; and the pieces being slit with a saw from one end 

 through the hole, and about half an inch beyond it, the separated parts are 

 drawn together by a screw, m, till the end of the peg is griped between them 

 with a due degree of force. By these pegs, with the help of the telescope h, 

 the tube is easily directed to any object, and made to accompany a celestial one 

 in its diurnal motion, while the eiid of the arm moves the whole length of 

 the rest. 



If it be desired, that when the object is found, the turning of one peg 

 should carry the tube along with the motion of the heavens, so as to keep the 

 object always in sight; this may easily be effected in various manners. 



The open air has commonly an undulating motion in its parts, especially in 

 the day time, which occasions the rays of light to deflect a little from the 

 straight lines, in which they ought to move, in order to render the species 

 perfectly distinct. The effect of this, though insensible to the naked eye, or 

 even through a small telescope, becomes considerable, when the object is very 

 much magnified. The instrument, when tried at an object inclosed, so as to 

 secure it from this inconvenience, seems to bear an aperture of 54- inches, 

 with the deepest of the forementioned eye-glasses, as well as the common te- 

 lescopes do the usual charge and aperture given to them, except that in these 

 the objects appear a little brighter. 



Fig. 11, represents the instrument placed on the machine, to be applied to 

 use. Fig. 12, represents the inside of the slider, with the rest of the apparatus 

 belonging to the oval plane and eye-glass. Fig. 13, represents the hinder part 

 of the machine, the back, and one side of the box, being taken away, to show 

 the turning pillar and springs on the inside. Fig. 14, represents Saturn, as it 

 appeared in June, 1720, by this telescope. 



4 o 2 



