[ I04 ] 



motion to it, except a pendulous vibration affecting the whole 

 mafs. The Reverend Mr. Auftin, for whofe judgment in thefe 

 'things I had due refped, advifed me to contrive the machine on 

 this principle ; and accordingly I made it in the following fafliion, 

 which, all circumflances confidered, I found to afford the moft 

 commodious general ufe of it, and to render it fufficiently por- 

 table, as the whole may be contained in a round brafs cafe nine 

 inches in length, and lefs than three in diameter, fuppofing that 

 of the mirror to be two inches and a half, nor need it be much 

 heavier than the glafs planes commonly ufed, together with the 

 quickfilver, &c. required with them, even when mnde of a fize 

 as lars;e as here propofed. It is reprefented fig. 8. in a vertical 

 feQion through the middle and one third of the intended fize; 

 though I think, if it were made of only twice the dimenfions in 

 the figure it might be fufficient. 



MN is a cylindrical veffel or cafe, made of brafs or wood, 

 which contains the level : in the middle of the bottom of it, 

 (which fhould not be liable to bend) is firmly fcrewed a ftrong 

 brafs rod E E, tapering upward ; and in the upper end of this 

 is inferted the needle L, on whofe point the mirror rells : the 

 plate Ah the mirror, B the brafs plate cemented to it, in which 

 is fixed the conical cap that receives the needle : this plate B is 

 immoveably fcrewed and foldered to the end of the pipe CC, 

 which pipe is feven inches and a half long, and widens downward, 

 having a hole through its fide juft under the projeding part at^, 

 to prevent the condenfation of air within it, when it is immerfed 



in 



