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for Papiu's Digester. 163 



tode of it, but allowing the vapour to pass. The upper 

 part (a) is closed by a small cover screwed ovi to it, and 

 perforated with a hole through which the piston-rod passe? 

 easily. This cover serves the double purpose of guiding 

 the rod, and preventing it from being blown oul. The 

 piston-rod is furnished with a shoulder, g,g, which serves 

 to support different weights, vvv, which can be changed at 

 pleasure. In the side of the cylinder there are holes, e, e, 

 as small as possible, placed above each other at the distance 

 ©f about a line; but this distance, as well as the number of 

 them, is a matter of indifference. 



To give an idea of the effect of this small apparatus, let 

 us suppose the piston lowered, and loaded witl\ any weight, 

 and that a fire is kindled under the boiler. When the va- 

 pour has acquired sufficient elasticity to raise the weights, 

 the piston will ascend, and, having passed the first hole, 

 some vapour will escape. If this aperture be of sufficient 

 size for the passage of the quantity of vapour continually 

 produced, the piston will remain there stationary, and in a 

 state of oscillation ; if not, it will ascend above the second, 

 third. Sec. hole, and, if the intensity of the fire is sufficiently 

 strong, above the last, which must be made larger, that, by 

 giving the proper means of escape to the vapour, all acci- 

 dents may be prevented. It is here evident, that though the 

 greater or less elevation of the piston, as well as the num- 

 ber of the holes open, depends on tlie variations and dif- 

 ferent intensities of the fire ; these variations, however, 

 have no influence on the interior heat, and the elasticity of 

 the vapour contained in the digester, since their force is al- 

 ways proportioned to the weight with which the piston is 

 loaded, and which is constant. This safety-piston seems 

 likely to afford, for delicate experiments, greater exactness 

 than the usual safety-valves, with or without levers charged 

 with weights, hitherto employed. For, in the whole course 

 of the space which the cylindric piston passes over in as- 

 cending, the state of the elasticity of the vapour is th^ 

 same ; whereas, when the conical valve in common use is 

 once raised up, nothing indicates v, hcther, or how much, 

 the present state of the vapour surpasses the first effort it 

 'made to open a passage, i'esides, the diameter of the piston 

 being once known, the force of the vapour requisite for 

 each experiment can be easily regulated and determined. 

 .'If we suppose, for example, that the lower surface of the 

 piston is -jJjth of a square inch, each ounce of we.dit on the 

 shoulder,^, v/ill be equivalent to the pressure ot a pound 

 on each stjuare inch of surface, and so on in proportion. 

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