the Trigonometrical Operation . igy 
men laying hold of its radii, and the exterior being placed 
upon the interior axis, the cheeks at the top being at the fame 
time adjufted to their proper bearing, it turns round very 
fmoothly, and is perfectly, or at leaft as to fenfe, free from 
any central (hake. This mode of centering is one of the chief 
excellencies of the inftrument. From the ufe that has been 
made of it both years, it fee ms not to have fuffered in the 
leaft; and it is perhaps the only con Fraction that could have 
anfwered for a machine of fuch magnitude, undergoing fo 
many quick tranfitions from place to place, and fo often raifed 
to high fituations without any rifk of being thereby hurt. 
Art. III. Mahogany Planes under the injirument . 
By infpeftion of the plates, but more particularly the IIIcl, 
and the fe&ion towards the right hand in the Vth, it will be 
feen, that there are three planes of mahogany under the metal 
parts of the inftrument ; namely, that which forms the top of 
the ftand, which, although a fquare of about three feet four 
inches at bottom, becomes, by the feparation of the legs, an 
o&agon at top. In the center there is a circular opening of 
nine inches diameter, the ufe of which will appear hereafter. 
Over the top of the ftand lies another plane of mahogany, like- 
wife an oftagon, of fomewhat greater dimenfions than the 
former, with a circular curb running around it, about half an 
inch within the planes of its fides. This o£tagon hath in its 
center an open conical focket of brafs, three inches in diame- 
ter ; and on four of its oppofite fides there are fixed four ftrong 
brafs fcrews, one on each fide, which a&ing againft pieces of 
brafs inlaid into the oppofite fides of the top of the ftand, the 
o&agon plane, with every thing that refts on it, may thereby 
be moved in four oppofite dire£Uons, until the plummet fuf- 
Vol. LXXX. 1 T pended 
