347 
Steam JEngihe. 
ii^g down the packing any time* The parts are '^o 
clearly expressed in the plates that no further 'description 
is necessary to make any person comprehend it. 
The other method is similar in principle, but a little 
different in construction. Instead of having several 
screws all worked down by one motion, there is in this 
but one screw, and that one is a part of the piston-rod s 
on this is placed a wheel of a convenient diameter, the 
centre of which is furnished with a female screw. This 
wheel is turned round, L e, screwed down by means of 
the pinion, which is furnished with a square projecting 
head rising into a recess of the kind already described. 
The ring is prevented from turning with the wheel by 
means of tXvo steady pins* ^ 
Description of a Safety Valve ^ containing a Vacmum Valve 
in the same Hole of the Boiler, By Sir A. N. Edel- 
CRANTZ.f 
IN large boilers or coppers, where l^oiling fluids are 
enclosed, a safety valve is generally used to prevent their 
bursting, from an unexpected excessive force of the elas- 
tic steam, and, besides, a vacuum valve ^ to prevent their 
being compressed or crushed by the weight of external air, 
in the case of a sudden condensation of the vapoury. 
These two valves are commonly fitted ill two different 
holes in the boiler ; but as a more simple, and conse- 
quently more eligible, method seems to be that of join- 
ing them together, I lake the liberty to submit to the Soci- 
ety for the Encouragement of i\rts, &c. the following 
Contrivance for that purpose 
a by Plate IIL Fig. 2. is a common conical safety vab e, 
* There is a plate describing this in 26 Phil. Mag-. SI /, whicn 
i have not thought it necessary to copy. T. C. 
t From the Memoirs of thp Society of .Arts fo^* I 80 t Tlic 
jd.}v€r medal was awarded forthis inventioir. 
