Mecijurement of a Bafe on Hon nflow- Heath. 44^ 
end is attached to a bell -metal button. The outward fur face of 
this moveable button is fpherical, defcribed on a radius of 
about two inches ; while the inward furface, like that at the 
fixed end, would apply clofely to the ground end of the glafs 
tube, but fhould not be pufhed fo 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 
infide of the button by fmall lerews, 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 Ane interfec- 
tion cut upon it near the inward end, made black to render it 
more conspicuous. Thus, two rods being brought into contact, 
and the Axed button of one being prefled againft the moveable 
button of the other, the interfeftion is thereby puAied for- 
wards until it coincides with the diamond line on the interior 
furface of the tube ; whofe length is fo adj lifted, as that, when 
the coincidence is perfect, the diAance between the plane fur- 
face of one button, and the fpherical furface of the other, is 
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 Ade Aiews the relative filia- 
tions of the extremities of the fecond and third rods, before 
the ivory is brought to coincidence with the diamond line, the 
Aide being then puAied out by the adlion of the fpiral fpring 
within the cork. 
Every rod has four wheels, two at each end. They are two 
inches in diameter, and connected by a common fleel axis, 
which rifes and falls in a vacuity prepared for its admiflion in 
the mahogany end*pieGes ? the under part of which vacuity is 
afterwards filled up. 
A brafs 
1 
