[ 17 ] 



height, then the fhort arm prefles againft the bottom of the 

 azimuth plate, and any additional force now applied overcomes 

 the fridion at the centre ; fhould the motion of the arm be 

 required in the oppofite diredlion, this is effeded by making the 

 fhort arm prefs againft a pin placed below it, with a fimilar 

 effed to the former, the force applied overcoming the central 

 fridion as above ; and thus the long arm may be brought parallel 

 to the telefcope in whatever pofition it is fet for obfervation. 



Fig. V. reprefents the lever and glafs-frame ; the azimuth 

 plate is reprefented by dotted lines. If it be required to move 

 the long arm from the pofition there reprefented to a vertical 

 fituation for an obfervation in the zenith ; then by depreffing 

 the handle x, the fhort arm a, a is raifed and puthes up 

 the glafs-frame, until a, a prefTes againft the bottom of the azi- 

 muth plate, where, as it can go no higher, any additional force 

 in that diredion overcomes the fridion at the central pin (d) and 

 the long arm is readily brought into a vertical pofition, the 

 fhort one remaining at the azimuth plate j and in this new pofi- 

 tion, the lever has the fame power of regulating the defcent of 

 the frame as before: if it be required to put the arm into an 

 horizontal pofition, the prefTure of the fliort arm againft the 

 pin (p) inferted in the pillar below it, overcomes the central fric- 

 tion in this diredion ; h reprefents the hole in the pillar through 

 which the light comes ; n, n the brafs fram? ; b, b the green 

 glafs. 



In juftice to Mr. Ramfden I muft obferve, that it was not 

 without fome difficulty that I prevailed upon him to introduce 



D this 



