STEAM ENGINE. 



such a cylinder must be one cubic foot, or 1728 

 cubic inches. Now if heat be applied to the bottom 

 of this cylinder, and the water made to boil, the 

 steam from that water will fill the whole of the in- 

 terior of the cylinder. Suppose that a piston were 

 fitted into the cylinder so that the piston must 

 rise before the steam can fill the cylinder, and that 

 there is no weight pressing down the piston, it is 

 phiin that the piston must be pressed up to the top 

 of the cylinder with an energy or force equal to 

 the force of the steam upon its under surface, which 

 at this temperature of 212 is equal to the pressure 

 of the atmosphere, or 15 Ibs. upon the square inch. 

 To determine therefore the effect of one cubic inch 

 of water, converted into steam, we have only to 

 multiply 144 square inches, or one square foot, the 

 surface of the piston acted on by the pressure, which 

 is 15 Ibs. on each square inch, and the product 144 

 X 15 = 2160 Ibs. through the height of one foot 

 by the steam formed from one cubic inch of water. 

 But this number, 2160, comes so near to the number 

 of pounds in a ton, i. e. 2240 Ibs. that, for the 

 sake of round numbers, we may consider that one 

 cubic inch of water, converted into steam at a tem- 

 perature of 212 will raise one ton weight one foot 

 high. 



This is no doubt low pressure steam ; and the 

 question naturally arises, can we obtain any more 

 force from high pressure steam, raised from the 

 same quantity of water ? No, we cannot. For let 

 us take the hollow cylinder spoken of in the last 

 paragraph, with its piston, and containing its cubic 

 inch of water to be converted into steam ; let the 

 upper side of the piston be so loaded, that it cannot 

 be forced up by the pressure of the steam below 

 until it acts with a force of 30 Ibs. upon the square 

 inch, then when the whole of the water has been 

 formed into steam, it will have raised the piston up 

 with a force of two tons, but only to the height of 

 six inches ; for the steam having double the elastic 

 force of the former, occupies only half the bulk. 

 The effects from the high and low pressure steam, 

 when formed from the same quantity of water, are 

 therefore equal; for it is quite the same thing, so 

 far as mechanical effect is concerned, to raise one 

 ton one foot, or two tons through half that height. 

 The power of an engine will therefore depend upon 

 the quantity of water converted into steam to 

 supply it for a given time. It is also worthy of 

 remark, that it will require the same quantity 

 of fuel to convert a given quantity of water 

 into steam, whether it be of the low or high 

 pressure kind ; so that the power of an engine 

 will depend on the quantity of fuel consumed in its 

 working. 



In the common low-pressure steam engine of 

 Watt, steam is admitted into the cylinder whose 

 elastic force is somewhere about that of the atmos- 

 phere, which we have all along supposed to be 15 

 Ibs. to the square inch ; but imperfect vacuums, 

 even in the best engines, diminish this pressure to 

 about 12 Ibs. on the square inch, or 9-4 on the cir- 

 cular inch. But from the power required to over- 

 come the friction of the several parts, and to work 

 the air and other pumps, there is only about 6 Ibs. 

 of effective pressure remaining for each circular inch 

 of the piston. 



Mr Tredgold gives the following tables, as an 

 estimate of the loss of power that takes place in 

 high pressure and condensing engines. In an en- 

 gine which has no condenser : 



The pressure on the boiler being . . lO'OOO 



1. The force necessary for producing motion of 



the -i. Mm in the cylinder . . . -069 



2. By cooling' in the cylinder and pipes -180 



3. Friction of piston and waste . . 2-000 



4. The force required to expel the steam into 



the ittmosphere . . . -Q69 



5. The force expended in opening the valves, 



and friction of the parts of an engine . -622 



6. By the steam being cut off before the eud of 



the stroke ..... 1-000 



Amount of deductions - 3-S20 



Kffective pressure . 6-080 



In one which has a condenser : 



The pressure on the boiler being . . 1 -000 



1. By the force required to produce motion of the 



007 



OK; 



125 



IW7 



steam into the cylinder 



2. By the cooling in the cylinder and pipes 



3. By the friction of the piston and loss 



4. By the force required to expel the steam 

 through the passages 



5. By the force required to open and close the 

 valves, raise the injection water, and overcome 



the friction of the axes . . . -063 



6. By the steam being cut off before the end of 



the stroke ..... -100 



7. By the power required to work the air-pump -050 



Amount of deductions -308 



Effective pressure . -632 



The diameter of a given cylinder being 24 inches, 

 its area, and consequently the area of the piston in 

 circular inches, will be, 24 8 = 576. 



We have seen that steam just sufficient to balance 

 the atmosphere yields a power equal to 6 Ibs. on 

 the circular inch ; hence the area of the piston 576 

 X 6 = 3456 the resulting force of the steam on the 

 whole surface of the piston. And say that the length 

 of the stroke is five feet, and the engine makes 44 

 single or 22 double strokes in a minute, then the 

 piston will move through a space of 22 X 5 X - 

 = 220 feet in a minute ; and from what has been 

 said before, it will not be difficult to see, that the 

 power of the engine will be equivalent to a weight 

 of 3456 Ibs. raised through 220 feet in a minute. 



But in place of steam equal to the pressure of 

 the atmosphere, it is usual to employ steam 2, 3 or 

 4 Ibs. above the pressure of the atmosphere. So 

 that the piston will have nearly that much more 

 pressure on each circular inch of the piston. 



This is the most certain measure of the power of 

 a steam engine. It is usual, however, to estimate 

 the effect as equivalent to the power of so many 

 horses. This method, however simple and natural 

 it may appear, is yet, from differences of "opinion as 

 to the power of a horse, not very accurate ; and its 

 employment in calculation can only be accounted 

 for on the ground, that when steam engines were 

 first employed to drive machinery, they were sub- 

 stituted instead of horses ; and it became thus ne- 

 cessary to estimate what size of a steam engine 

 would give a power equal to so many horses. 



There are various opinions as to the power of a 

 horse. According to Smeaton, a horse will raise 

 22,916 Ibs. one foot high in a minute. Desaguliers 

 has 27,500 ; and Watt makes it 33,000. There is 

 reason to believe that even this number is too small, 

 and that we may add at least 11,000 to it, which 

 gives 44,000 Ibs. raised one foot high per minute. 



Now, in the case above, we found that the engine 

 of 24 inch cylinder, would raise 3456 Ibs. through 

 the space of 220 feet in one minute ; and it is easily 

 seen that it could raise 220 X 3456 Ibs. through 

 one foot in the same time ; therefore 220 x 3456 

 760320 Ibs. raised through one foot in one minute, 

 is the effective power of the engine ; and from 

 these considerations it will be easy to find the 

 power according to the different estimates of a 

 horse's power. For, 



