of applying the Screw . 6 $ 
©bje<f and eye-glafs, the fcrews may be contained within the 
fulcrum by which the micro fcope is fupported. 
The machine (fig. r.) may be applied as a jack to raife great 
weights a little way from the ground, by fubf ituting two crofs 
hand-fpikes for the handle ckl ; or a vertical handle may be 
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 1 6 
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 ri 
ends in a foot as in the figure. The whole is inclofed in a ftrong 
frame. The pinion a rnuf 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 connected by the end to the ferew ef. Or, if 
this fhould be deemed inconvenient, as the rifing of the pinion 
muft raife the handle ab, the wheel cd may be hindered from 
rifing, and at the fame time turn the ferew ef, by a contrivance 
fimilar to that ufed with the index c (fg. 3*) in the micrometer. 
In either cafe, the axis of the pinion fhould 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 
life, the bottom of the frame refill g 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 
wall rife (| x | =: ) -UAh of an inch. The point g or h will 
Vol. LXXI. K therefore 
