Meafurcmeni of a Bcif en Hon nilow- Heath. 44 j 



end 15 Jittached to a bell -metal bntton. The outward furface ot 

 this moveable button Is fpherical, defcribed 011 a radius oF 

 about two inches ; while the inward furface^^like that at the 

 fixed end, would apply clolely to the ground end of the glais 

 tube, but Ihould not be pulhed io far forward as to touch it. 

 A circle and narrow Aide, cut from, a folid cylinder of ivory, 

 fitted originally to enter eafily the glafs tube, is attached to the 

 in fide of the button by fmall fcrews, and permits the neck to 

 pafs through a hole made on purpofe in the circle. The Aide 

 is about eight -tenths of an inch long, and has a fine interfec- 

 tion cut upon it near the inward end, made black to render it 

 inore confpicuous. Thus, two rods being brought into conta6:, 

 and the fixed button of one being prefled again ff the moveable 

 button of the other, the interiedrion is thereby pufhed for- 

 wards until it coincides with the diamond line on the interior 

 furface of the tube ; whofe length is fo adjufted, as that, when 

 the coincidence is perfect, the diftance betw^een the plane fur- 

 face of one button, and the fpherical furface of the other, i» 

 exactly twenty feet. The left-hand fide of the plate repre- 

 fents the relative pofitions of the extremities of the firfl: and 

 fecond rods, when the ivory is in coincidence with the dia- 

 mond line. And the right-hand fide fhews the relative fitua- 

 tions of the extremities of the fecond and third rods, before 

 the ivory is brought to coincidence with the diamond line, the 

 flide being then pufhed out by the action of the fpiral fpring 

 within the cork. 



Every rod has four wheels, two at each end. They are two 

 inches in diameter, and conneded by a common fteel axis, 

 which rifes and falls in a vacuity prepared for its admiflion in 

 the mahogany end-pieces, the under part of which vacuity is 

 afterwards filled up. 



A brafs 



