72 



Davies Gilbert, Esq., on the 



of the perpendicular, or nearly two degrees. With a mercurial 

 pendulum the arc of semivibration must be 1° 31' 44", or about 

 one degree and an half. 



And it is an extremely curious circumstance, that without any 

 reference to the attainment of this balance between opposite disturb- 

 ing causes, our best clocks should have been fortuitously made to 

 vibrate very nearly in the arc, which reduces them to an equality. 



Friction is supposed to be so very small in amount, and more- 

 over to possess such very trifling influence on these minute changes 

 as not to require attention. 



All thermometrical changes are corrected by the ordinary com- 

 pensation. Nor is it of any practical importance to estimate h t (the 

 height of the barometer from whence the variation t. is to com- 

 mence,) at any other standard than 30 inches, unless, indeed, the 

 barometer should be placed in some very elevated situation. — When 

 other tables should be constructed from the new values of t and s. 



The following table, reprinted from the former communication, 

 gives the effect of buoyancy on a brass pendulum. These numbers 

 multiplied by six- tenths, ( .6) become adapted to a mercurial pen- 

 dulum ; and the same numbers multiplied by the following factors, 

 with their signs changed, will give the effect arising from varia- 

 tions of the circular excess ; for degrees, and quarters of degrees 

 of semivibration. 



A Table, reprinted from the XVth vol. for correcting the time, as 

 shewn by a clock, having a brass weight, or ball to its pendulum, 

 for the variation of one inch in the height of the barometer. 

 Arguments. — The time elapsed since the last observation of the 



barometer. 



And the present observed height — 30 inches d~ J_ the variation 

 between the observations — 2 



Additive, if the sum is Plus. — Subtractive, if it is Minus. 



