.-.-.I 



APPLIED MECHANICS. 



[PREMU'.E INMi-ATORS. 



ipring-balance A (Fig. 161), graduated to the pressures 

 per square inch due to the spring. By turning a tiut B 



161. 



J 



n 



on the stem of the spring-balance, any required pressure 

 can be thrown upon the valve, which is kept down by 

 the spring acting on its lever. Should the pressure 

 within the boiler exceed that to which the balance is ad- 

 justed, the valve is opened, and a portion of the steam 

 escapes. The lock-up safety-valve consists of a valve 

 pressed down by a set of strong springs C, the whole en- 

 closed within a box under lock and key. While the 

 engine-driver has command over the spring-balance, 

 valve, so as to increase or dimmish the load at pleasure, 

 the lock-up valve is inaccessible to him, and opens when- 

 ever he has loaded the open valve beyond the pressure to 

 which the lock-up valve lias been adjusted ; thus serving 

 a* a check upon him in case of his working at a dangerous 

 pressure. 



In a boiler for an engine working at very low pres- 

 sures, there is frequently provided a vacuum valve, 

 which is a safety-valve opening inwards, and admitting 

 air into the boiler, in case the pressure within should fall 

 so far below that of the atmosphere without, that there 

 might be danger of collapse. 



n;l->-i 1:1; <;\i Q& The rtMio-gMge is aa tpp* 

 ratus generally fitted to boilers for indicating the pressure 

 of the steam. The safety-valve may be employed for 

 this purpose ; for if the weight be adjusted on the lever, 

 or the spring of the balance released, until the valve be- 

 gins to open and let steam escape, we know that the 



I'Mf. 101. 



weight or spring in that condition is a measure of the 

 proMuro. But u this mode of measuring the pressure 



require* personal attendance, it is letter to be provided 

 with some self-acting instrument which shall show at a 

 glance the condition of the .st.Min in respect to pressure. 

 r low-pressure boilers, tho mercurial steam-gauge is 

 generally employed (Fig. 1D2). It consists of an iron 

 pipe bent to a siphon form, connected with the boiler, 

 and containing mercury, on which floats a rod of wood 

 extending above tho mouth of the tube, and pointing to 

 divisions on a scale. As 2 cubic inches of mercury 

 weigh very nearly 1 Ib. , the rise of the wooden index 

 through 1 inch in height, indicates that the mercury in 

 1 limb of the siphon has risen 1 inch, ami fallen 1 inch 

 in the other, making a difference of level of 2 ii:. 

 equivalent to a pressure of lib. per square inch in the 

 boiler. Thus every inch on the scale corresponds to 1 

 Ib. pressure per square inch. 



For high-pressure boilers, the column of mercury 

 necessary would be inconveniently lii'_'h. and recourse is 

 therefore had to gauges of other kinds. Among the 

 most effective and ingenious of these may be mentioned 

 that of Bourdon. It consists of a flattened elastic tube 

 of copper or brass, bent into a spiral form. The pressure 

 within the tube tends to bulge it, and uncoil it a little 

 out of the spiral form ; and the slight movement thus 

 induced is communicated to an index, which points on a 

 dial-plate to the pressure marked thereon from the result 

 of experiments made for the purpose of determining the 

 proper graduation of the dial. 



CYLINDER. The steam generated in the boiler at 

 such pressure, and in such quantity as may be desired, 

 is conveyed by the steam-pipe to the i-ylinder, which is a 



vessel closed at both 

 ends, and fitted with 

 a piston E (Fig. 103), 

 capable of sliding 

 tightly from end to 

 end, and IIIIYM 

 rod F passing tightly 

 through one of tho 

 end covers, or the 

 cylinder Hi If we 

 suppose A and B two 

 pipes communicating 

 Mith the boiler, and 

 opening into the cylin- 

 der at opposite ends, 

 while two other pipes, 

 C and D, lead from 

 the ends of the cylin- 



fig. les. 



dor to the open air, or to any suitable pi ; iving 



these pipes to be provided with stop-cocks, wo can see 

 that by opening A and D, while B and C are closed, we 

 admit steam to press upon the upper surface of the pis- 

 ton 10, and force it to tho bottom of the cylinder, while 

 the contents of the part below the piston escape by D. 

 On the piston reaching the bottom, if we open IS and C, 

 while A and D are closed, the pressure acting on the 

 lower side of the piston forces it upwards, while the 

 steam above escapes. Thus, by alternately opening and 

 closing the four stop-cocks in proper order, an alternating 

 motion is given to the piston, and the force is communi- 

 cated by the rod to any suitable machinery. Tho amount 

 of force so communicated depends on the size of tho pis- 

 ton, or number of square inches in its surface on which 

 the steam -pressure acts, tho intensity of that pressure, 

 and tho velocity at which the piston is caused to travel. 

 If we suppose, for instance, that the d: i the 



cylinder is 1 foot, the circular area of which is 1 13 PC; 

 inches, that the pressure of tho steam is 2(1 1 !>.. per 

 square inch, and that the average speed of the piston is at 

 the rate of 200 feet per minute, the power communicated 

 through the rod is equivalent to 



Some of the first steam-engines hod cocks arranged as 



we h.v >l, which <li mai 'eiid- 



auco of a workman b < them at t 



times. But it suggested itself u,at apparatus conn 



