Mr. HtrNTER on a new Method of applying the Screw * 59 
3. That the machine be as Ample as is confident with other 
conditions. 
4. That it he as portable and as little trotxblefome in the ap- 
plication as poffible. 
5. That the contrivance be fuch that the moving power 
may be applied in luch a way as to a£l to the greateft 
advantage ; and that the motion ultimately produced may have 
that dire< 9 :ion and velocity which is moft adapted to the execu* 
tion of the defign propofed by the whole. 
6. Of two machines, equal in other refpe&s, that deferves 
the preference in which the friftion lead: diminifhes the effect 
propofed by the whole. 
It will eafily appear, that fome of thefe conditions, if car- 
ried to an extreme, will be inconfiftent with fome of the others* 
Here the proper medium confifts in adapting them to each other 
in fuch a manner, as that the refult of the advantages of both 
may be the greateft, and that of the defers the leaft, that is 
poffible. 
The following method of applying the fcrew, I think, may 
in certain cafes, be attended with fome of thefe advantages to a 
greater degree than by tliofe commonly practifed. 
Let ab (fig. 1.) be a plate of metal in which the fcrew 
€D plays, having a number of threads in an inch equal to a . 
Within the fcrew cd there is a female fcrew, by which is re* 
ceived the fmaller fcrew de of a -f 1 threads in an inch. This 
fcrew is retained from moving round along with the fcrew CD 
by means of the apparatus at Afgb. 
Now, if the handle ckl be turned a times round the fcrew, 
cd will advance upwards an inch, and if we.fuppofe the fcrew 
de to move round along with cd, the point e will alfo advance 
an inch. If we now turn the fcrew de a times backwards, the 
I z point 
