223 Mr. Cartwright's new Piflon 
though not abfolutely neceffary, for the greater fecurity 
two other rings are fitted to the infide of thefe. On the 
upper rings is placed a plate of metal, alfo ground perfeétly 
flat, and of fuch a diameter as almoft to fit the cylinder: 
a fimilar flat plate is placed below the under ones; and the 
two plates with the rings between are attached to each 
other by meatis of the pifton rod that paffes through them. 
Ttis plain then, that fuippofing neither the outfide rings nor 
the cylinder to be able to wear one another, fuch a piftor 
would remain fteam-tight+ but as conftant fri€tion mutt in- 
evitably tend to widen the cylinder and diminith the dia- 
meter of the rings, the pifton after fome time would ceafé 
to fit, if a contrivance had not been fallen upon to remedy 
the evil. The rings are each of them cut into three pieces ; 
and, in cutting them, fuch a portion of the metal is taken 
away as to leave room to introduce, between two of the pieces, 
afpring in the form of the letter V, the open end of which is 
placed outwards, almoft clofe to the circumference ; by whiclt 
means the two pieces againft which the two fides of the {pring 
att, are prefiled, a the dire&tion of the circumference, againtt the 
ends of the third piece; fo that the three pieces are thus kept fo 
uniformly in contaé& with the cylinder, that the longer the ma 
chine is worked the better the rings mufi fit. ‘Yoprevent fteam 
from paffing through the cuts in the rings, the folid partsof the 
upper rings are made to fall upon ‘the divifions and fprings 
of the under ones. This is the method: contrived by Mr. 
Cartwright for making his pifton fit; which will be clearly 
underftood by looking-at Plate I. Vol. I. of this Magazine. 
df he had left it to <¢ a very moderate portion of mechanicak 
knowledge to fuggeft the manner in which the pieces might 
be made to recede outwards,, by means of fprings,’”” it might 
perhaps have been propofed to. make fuch {prings aét from 
the centreon the circumference: im that cafe, the preffure 
of the diferent parts of each ring could never have beer 
made)uniform, and the machine would foon: have workéd 
itfelf into inequalities, inftead of working itfelf more true. 
If 
