Part 39

Footnote 66:

From this point of view, we see very clearly how imperfect is the steam-engine, even after all Watt’s improvements. For to “push the principle of expansion to the utmost,” we must allow the steam, before leaving the cylinder, to expand until its pressure is the same as that of the vapor in the condenser. According to “Watt’s law,” its temperature would then be the same as (actually a little above, as Regnault has shown) that of the condenser, and hence the steam-engine worked in this most advantageous way has in reality the very fault that Watt found in Newcomen’s engine. This defect is partially remedied by Hornblower’s system of using a separate expansion cylinder, an arrangement the advantages of which did not escape Carnot’s notice, although they have not been recognized extensively among practical engineers, until within the last few years.

Footnote 67:

I am indebted to the kindness of Professor Gordon of Glasgow for the information regarding the various cases given in the text.

Footnote 68:

In different Cornish engines, the pressure in the boiler is from 2½ to 5 atmospheres; and, therefore, as we find from Regnault’s table of the pressure of saturated steam, the temperature of the water in the boiler must, in all of them, lie between 128° and 152°. For the better class of engines, the average temperature of the water in the boiler may be estimated at 140°, the corresponding pressure of steam being 3½ atmospheres.

Footnote 69:

This number agrees very closely with the number corresponding to the fall from 100° to 0°, given in Table II. Hence, the fall from 140° to 30° of the scale of the air-thermometer is equivalent, with reference to motive power, to the fall from 100° to 0°.

Footnote 70:

It being assumed that the temperatures of the boiler and condenser are the same as those of the Cornish engines. If, however, the pressure be lower, two atmospheres, for instance, the numbers would stand thus: The temperature in the boiler would be only 121. Consequently, for each pound of steam evaporated, only 614 units of heat would be required; and therefore the work performed for each unit of heat transmitted would be 160.3 foot-pounds, which is more than according to the estimate in the text. On the other hand, the range of temperatures, or the fall utilized, is only from 131 to 30, instead of from 140 to 30°, and consequently (Table II.), the theoretical duty for each unit of heat is only 371 foot-pounds. Hence, if the engine, to work according to the specification, requires a pressure of only 15 lbs. on the square inch (i.e., a total steam-pressure of two atmospheres), its performance is (160.3)/(371) or 43.2 per cent of its theoretical duty.

Footnote 71:

If, in this case again, the pressure required in the boiler to make the engine work according to the contract were only 15 lbs. on the square inch, we should have a different estimate of the economy, for which see Table B, at the end of this paper.

Footnote 72:

These engines are provided with separate expansion cylinders, which have been recently added to them by Mr. M‘Naught of Glasgow.

Footnote 73:

377.1 = .2713 × 1390, 253 = .1820 × 1390 = (1)/(5.49) × 1390.]

Footnote 74:

Pressure 15 lbs. on the square inch.

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110 no appreciable change. (See no appreciable change. (See Annales de Ohimie et de Annales de Chimie et de

246 If, to abridge, we call N the If, to abridge, we call N the quantity (P)/(726), the quantity (P)/(267), the

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