INDICATION OF PRESSURE .] 



APPLIED MECHANICS. 



847 



sure of 2 atmospheres, or 30 Ibs. per square inch, is ad- 

 mitted so as to force the piston through 2 inches of its 

 stroke ; we have 2 cubic inches of steam at 2 atmospheres, 

 which are equivalent to 1 cubic inch at 4 atmospheres, and 

 would be generated from the same quantity of water, 

 jjjyth cubic inch, at nearly the same expenditure of heat- 

 ing power. We should in this case find the average pres- 

 sure on the piston, throughout the stroke of 4 inches, to 

 be about 24 Ibs. , or equivalent to a weight of 96 Ibs. moved 

 through 1 inch. At first sight it appears startling, that by 

 an expenditure of no more heating power in the first case, 

 we should have obtained 45 per cent, more power than 

 in the second ; but such is the fact, nevertheless : and 

 that it is so may be clearly seen by the diagram (Fig. 

 148). Let A B represent the stroke 4 inches long, A C 



Fig. 148. 



the pressure, 4 atmospheres (represented by 4 inches in 

 height), continued through 1 inch of the stroke, as shown 

 by the line C D ; then from D let the pressure-curve be 

 drawn till it terminates at G, B G being 1 inch high, 

 representing the terminal pressure 1 atmosphere. Then 

 the area of the whole compound figure A C D F G B 

 represents the total power developed in the first case by 

 the expenditure of a quantity of steam represented by 

 the area of A H D C. Again, let the pressure in the 

 second case of 2 atmospheres, represented by 

 A E, 2 inches high, be continued through 2 

 inches of the stroke to F ; a portion of the 

 same pressure. curve F G will enclose the 

 pressures during the rest of the stroke. In 

 this case the power is represented by the area 

 of the shaded figure A B G F E, the quantity 

 of steam expended being as the area of A K F E. 

 Now the area of A K F E, or 2 x 2 = 4, is 

 precisely the same as that of A H D C, or 

 1X4 = 4; while the area of ABGDCexceeda 

 that of the shaded figure by the portion E F D C 

 so much clear gain of power by the higher 

 initial pressure, and greater range of subse- 

 quent expansion. We have here supposed 

 that no resistance is offered to the movement 

 of the piston. If it be conceived to move in 

 opposition to the pressure of the atmosphere, 

 the gain by expansion of the steam is even more marked. 

 A line G L represents the constant atmospheric re- 

 sistance, and cuts off from both figures an area A B G L, 

 which leaves the effective overplus of force impressed 

 on the piston so as to move machinery, represented in 

 the one case by the area of L G F E, and in the other 

 by L G D C. Numerically, since we found the total 

 force in the two cases to be equivalent to 90 Ibs. and 

 140 Ibs. respectively moved through 1 inch, and as the 

 atmospheric resistance is 15 Ibs. through 4 inches, or 

 60 Ibs. through 1 inch in both cases, the effective forces 

 are as 14000, or 80, to 96 00, or 36 ; that is to say, 

 the one is more than double the other. 



This mode of graphically representing forces is 

 not only of an interesting, it is also of a most useful 

 character. Watt invented an instrument which, being 

 applied to a steam-engine, could draw upon a card 

 a figure such as we have employed, representing 

 the power developed by the action of the steam 

 in the engine. He, however, seemed to consider it 



only as an interesting toy ; but of late years this 

 instrument has been found to be of the greatest 

 utility. It is called the indicator, and is capable 

 of not only representing with great accuracy the 

 power developed in the engine to which it is applied, 

 but' also of exhibiting defects in design and construction, 

 and of suggesting improvements. We shall have occa- 

 sion hereafter to enter more particularly into the details 

 of its construction and application. 



THE BOILER. In all steam-engines, the boiler or 

 apparatus for generating the steam is a part of prime 

 importance. In devising a good boiler, the problem is 

 to obtain the greatest quantity of steam, or to boil off 

 the greatest weight ot water with the least weight of 

 fuel consistently with due simplicity, durability, strength, 

 and economy of material and labour in its construction. 

 It must be a vessel capable of containing water, and 

 affording space for steam generated from it : every part 

 of it being exposed to the pressure of the steam within, 

 it must be capable of resisting this bursting force ; and in 

 its construction, precautions must be taken for safety in 

 case of the pressure tending to exceed the strength 

 provided to resist it. A certain portion of its surface 

 must be exposed to the action of the fire ; and as the 

 materials which we have to use in its construction, suffer 

 when exposed to excessive heat, and as we cannot, con- 

 sistently with economy, afford to apply any portion of 

 our fuel ineffectually, we must make provision for 

 having the interior of the fire-surface covered with water 

 to receive the heat communicated through it. 



The most simple kind of boiler is one of cylindrical form, 

 with hemispherical ends (commonly called the egg-ended 

 boiler (Fig. 149), placed horizontally, with a tire ar- 

 ranged under it, so that the direct heat of the fire, and 

 of the heated products of combustion in their passage to 

 the chimney, act through the metallic casing on the 

 water within. The water only partially fills the boiler, 

 leaving a space above it for steam, which is conducted to 

 the engine by a pipe leading from the top of the boiler. 



Fig. 119. 



It is found by experiment, and indeed it seems to be 

 a reasonable conclusion, that, within proper limits, 

 having a certain quantity of fuel to dispose of, the larger 

 the surface of water over which the heat developed from 

 combustion is spread, the greater will be the heating 

 effect produced, or the larger will be the quantity of 

 water turned into steam. In fact, it becomes the object 

 of the engineer to allow as little as possible of the heat 

 to escape oy the chimney, and consequently to arrange 

 his boiler in such a manner as to make the water absorb 

 the greatest possible quantity of heat, by exposing the 

 largest possible surface to the combustion, and disposing 

 that surface in the best manner. 



The strongest form in which a vessel can be made, 

 when it is intended that it shall resist internal pressure, 

 is that of a spherical shell. If, then, in the construction 

 of steam-boilers strength alone were studied, the spherical 

 form would be generally adopted. But of all forms of 

 vessels, the spherical is that which has the smallest sur- 

 face in proportion to its capacity, and it is consequently 



