L 
of applying the Screw . 6 5 
object and eye-glafs, the ferews may be contained within the 
fulcrum by which the micro fcope is fupported. 
The machine (fig. 1.) may be applied as a jack to raife great 
weights a little way from the ground, by fubftituting two crofs 
hand-fpikes for the handle Ckl ; or a vertical ! handle may bd 
employed in the Following manner. Let a (fig. 5.) be a pinion 
turned by the handle Ab, which We fuppofe a foot in length. 
Let the pinion a have' 4 teeth, and move the wheel cd of 16 
teeth. The ferew ef of 4 threads in an inch is fixed in this 
wheel, and turns round along with it. Within it plays the 
ferew fg of 5 threads in an inch, and which we fuppofe pre- 
vented from following the motion of EF : it terminates in fuch 
a fhoulder as that reprefented at g, and being continued to H 
ends in a foot as in the figure. The whole is inclofed in a ftrong 
frame. The pinion a muft be connected in fuch a manner with 
the wheel cd as to rife within the frame along with it, which 
may eafily be done by making its axis play in a piece of wood or 
metal, which is connedted by the end to the ferew ef. Or, if 
this fliould be deemed inconvenient, as the rifing of the pinion 
mull; raife the handle ab, the wheel cd may be hindered from 
rifing, and at the fame time turn the ferew ef, by a contrivance 
fimilarto that ufed with the index c (fig. 3.) in the micrometer. 
In either cafe, the axis of the pinion fliould be continued through 
the oppofite fide of the frame, and armed with a heavy fly to 
regulate the motion. When the machine is to be applied to 
ufe, the bottom of the frame refting on the ground, if the body 
to be lifted is already as high as the top g, that top is applied 
below it ; but if it is clofe to the ground, we put below it the 
foot h ; then, if the handle ab be turned once round, the 
wheel cd and ferew ef will turn \ part round, and the point f 
will rife (|x|z) -J_th of an inch. The point g or h will 
Vol. LXXL K therefore 
