A REFRACTING TELESCOPE WITH A FLUID CONCAVE LENS. 43 



when thrown out beyond its perpendicular position^ as is necessary to bring the 

 instrument to small angles of elevation. 



This counterpoise is not, however, sufficient ; another therefore is introduced, 

 by suspending a chain from the swinging frame to the front stancheon ; to the 

 centre of which a 56-pound weight is suspended, passing over another pulley 

 on the brace into the same well-hole : by this contrivance the tension of the 

 chain increases as it approaches most to a straight line ; that is, when the 

 frame is thrown furthest out, and where its weight acts with greater force. 

 By a slight adjustment of the length of the chain and weight, we may thus 

 produce a perfect equilibrium in the whole machine, and the telescope is of 

 course obedient to the slightest power sufficient to overcome the friction. 



Things being thus equipoised, in order to render the motion as gentle as 

 possible, a wheel and pinion are attached to the barrel last mentioned, similar 

 to the one already described, but with four cross handles like those belonging 

 to the screw. The power thus gained is 2 to I by the pulleys, 4 to 1 by the 

 wheel and pinion, and about 9 to I by the handles, equivalent to 72 to I. The 

 slightest touch therefore of one of these handles will produce a change of ele- 

 vation of the telescope, either to increase or diminish it, accordingly as that 

 motion tends to pull in or out the swinging frame. 



The operation therefore of putting the instrument on a star is: first, to swing 

 round the whole stand towards the star, on the circular curb ; then to bring it 

 nearly to its proper altitude by the apparatus first described; then, being 

 seated at a proper height, the eye being applied to the finder, with the handle 

 belonging to the screw in one hand and that belonging to the altitude motion 

 in the other, the star is brought immediately to the centre of the field, and is 

 of course then in the large telescope. The observer is thus seated at perfect 

 ease and follows the star at pleasure, one of the four handles on each side 

 being always ready to receive a touch of the finger, which is sufficient for the 

 purpose. 



I have observed that this stand acts with considerable accuracy as an alti- 

 tude and azimuth instrument : it may therefore be proper to say a few words 

 on this subject. Such a purpose was not contemplated in its construction, and 

 therefore, notwithstanding the usual accuracy of millwright workmanship, it 

 could hardly have been expected to find the stand susceptible of such a degree 



.:J2imu'. g2 



