J04 ftESCR'lP-Hd* ibF A tACHdSfETE*. 



riiry at the axis wlien the cup is revolving with the {greatest 

 velocity, the upper part M M X Y of the cup should be of 

 such a form, as to have the gides cdvered only with a thin 

 film of the fluid ; and secondly, that for thg purpose of 

 "raising the siniill cfuantity of rtiercury to the level L L, 

 which may support a proper height of spirits when the cup 

 is at fest; the cavity of the cup should be in a great mea- 

 sure occupied by the block K K, having a cylindrical per- 

 foration in the middle of it for the immersion 5f the tube, 

 and leaving sufficient room within and around it for the 

 mercury to move freely both along the sides of the tube and 

 of the vessel. 



Tlie block tt K*is preserved in its pro{^er position in the 

 cup or vessel X Y Z, by means of t!',iee narrow projecting 

 slips or ribs placed at equal distances round it, and is kept 

 frori) vising or floating upon the mercury by two or three 

 small iron or steel pins inserted into the underside of the 

 cover, near the aperture thi-ough which the tube passes. 

 Form of the It would be extremely diflficuU, however, nor i"s it by any 

 *"P* • means important, to give to the cup the exact form, which 



would reduce the quantity of mercury to its minirtium ; but 

 we shall have a sufficient approximation, which may be 

 executed with great precision, if the part of the cup above 

 M M is made a parabolic conoid, the vertex of the gene- 

 rating parabola being at that point of the axis to which the 

 mercury sinks at its lowest depression, and the dimensions 

 of the parabola being determined in the following manner : 

 Let V" G, fig. 3, represent the axis of the cup, and V the 

 ' point, to which the mercury sinks at its lowest depression; 

 at aniy point G above V, draw G H perpendicular to V G; 

 let n be the number of revolutions, which the cup is to per- 

 form in 1" at its quickest motion; let v be the number of 

 inches, which a body would describe uniformly in l", with 

 the velocity acquired in falling from rest, through a height 



V 



zz toG V; and makeG ^^t;:— r-- Then, the parabola. 



3 14 n 



to be determined is tlVat which has v for its vertex, V G 



for its axis, and G Fl for its ordinate at G. The cup has a 



Vk\ to prevent the mercury from being thrown oiit of it, an 



evept whicfi would take place with a very moderate velocit';^ 



'^•'" ^of 



