upon the Collijion of Bodies. 349 
bearing part at 0 being femi-circular ; from which branching 
off on each fide the rod cc, pafifes to p, and fixes at each end 
into the wood at q. However, to clear the ratchet, which is 
necefiarily in the middle as well as the rod, the latter is per- 
forated ; and alio the block is cut away, fo far as to let the 
main lpring at e free of all obftacles that would prevent its 
play from the point B. The part jg is (hewn thicker than the red, 
by being covered with thin kid leather tight fowed on, to pre- 
vent a certain jarring that otherwife takes place on the meeting 
of the fprings in colliiion. 
Let us now return to fig. 1 . the marks upon the arch MN 
are put on as follows, op is an arch of a circle from the center 
/, and qr an arch of a circle from the center k interfedhng each 
other at s. Now the middle line of the marks /, v, are at the 
lame diffance from the middle line at s that the centers kl are ; 
lo that when each body hangs in its own free pofition, with* 
out bearing againft the other, the rod ej will cover the maik 
at /, and the rod gh will cover the mark at v. From the 
point S upon the arches S p and S q refpe&ively, let off points 
at an equal and competent diffance from S each way, which 
will give the middle of the mark w and x : and upon the 
arch Sp find a middle point between the mark v and w, which 
let be y ; and on the other tide, in like manner, upon theaich 
S q find a middle point for the mark z ; then fet off the diffance 
Sv or St from y each way, and from z each way; and from 
thefe points, drawing lines to the refpedtive centers / and 
they will give the place and pofition of the marks b, and 
c, d; and thus is the machine prepared for ufe. 
FOR TRIALS ON ELASTIC GOBI E S. 
For this ufe take out the pins and ratchets from each relpec- 
tivelv, and the fprings being- then at liberty, with a fhort bit 
of 
