the top part cut all round, and inclined inwards so as to form an expanding belt, it presses upon the cone G, being supported at bottom by small springs or wedges, kk, which retain it in the required position, thus forming an adjust

179

ing gallery, whereby glasses of any ordinary length of shoulder (being otherwise properly formed) can be applied at once, and the air is forced to impinge upon the flame at the proper point. H is the glass chimney.

Sir,-A self-acting extinguisher is so generally useful, that I am rather surprised it is not an article of common sale. The little apparatus, of which the above sketches show one applied to a candle ready for action, and another after it has extinguished the light, I have had by me the last sixteen years. It acts by first retaining the extinguisher, supported by an almost upright stem, in its place by a thread passing from the extreme of the bent lever which carries the extinguisher, through a corresponding hole in the lower stem attached to the spring, or candle-clip, by which the whole is adjusted so as to extinguish the light at any fixed point. This thread then passes upwards through a notch cut in the side, or a hole drilled in the lower stem, or bracket, near the candle, and there terminates in an adjusting pin, having the head made sufficiently heavy to cause it to fall by its own gravity, the instant the tallow, in which it had previously been stuck, melts above it; in this state it may be seen in the figure representing the extinguished light.

No one should venture to read in a chamber, or leave lights behind them in bed rooms, without having such, or a similar contrivance, attached to the candle. If by chance a person falls asleep, the light is shortly after put out, and the

risk of fire very much diminished. Many persons, for the slight convenience of a light, leave a candle burning at home during their absence; in such, and many similar cases, it would be easy previously to attach to the candle a selfacting extinguisher. In hotels, clubhouses, boarding-houses, academies, and all establishments using a number of candles, much danger would be avoided by having recourse to this simple contrivance. The same, too, happens in the rooms of invalids, to whom it would often be desirable to have a light for a short part, but not the whole of an evening.

The apparatus recommended has the advantages of quick and sure action. A slow pressure is not so good; and therefore, the instant the pin is released by the softening of the tallow, its own weight causes it to drop, and the extinguisher falls smartly down on the candle.

This is a small matter in itself, but its value is perhaps greater than may appear on first reflection; indeed it almost requires to inspect the very ware-rooms of the Birmingham manufacturer to realize, even in imagination, the produce consequent on the introduction of a new style of article in anything of general utility, no matter how apparently insignificant. I am, Sir, &c. H. D.

Surrey, March, 1844.

N 2

It will be convenient to consider the work which steam is capable of performing under three aspects. 1st. the work performed whilst the steam is being formed; 2nd. the work performed by its expansion; and 3rd. the work by its condensation. The third source of power merges in the two former, upon considering that it is just equivalent to the atmospheric resistance. Conceive

the cubic foot of water to be placed in a cylinder whose piston contains 144,000 square inches; let heat be applied so as slowly to raise the piston with a pressure of P lbs., until the whole of the water is converted into steam, then the volume of this steam at this point will be expressed by the relation V = +m. AfP

n

ter this the steam will continue to perform work by its expansion, until its elasticity is equal to the pressure of the vapour above the piston plus the friction of the parts of the machine. The volume of the steam, at this limit, will therefore be found by the preceding formula, whence also the stroke of the piston will become known, and the units of work may then be calculated by 2. In calculating the

pressures at the different intervals of the stroke, it will be most convenient to use the relation P

=

24250

M

and the total units of work upon the pis64 079 × 144,000 9,227,000 nearly.

ton =

=

If a bushel of coal reduce 10 cubic feet of water to the state of steam, then, according to the foregoing calculation, the maximum duty of steam at 60 lbs. pressure will be upwards of 90,000,000; which is considerably more than any of the Cornish engines have yet attained. At the same time it is proper to observe, that a considerable portion of work is absorbed in these engines by the clearance. Any contrivance calculated to reduce the waste of steam thus occasioned would be well deserving public attention. By calculating, as in the preceding example, it will be found that the units of work in a cubic foot of water increase with the pressure at which the steam is generated.

12. To determine P so as to yield the greatest work from a given weight of steam,

We here propose to show, generally, that the units of work in a given weight of water increases with the increase of pressure at which the steam is generated.

Let P (see fig. 2) be the position of the piston when the steam begins to act expansively, and P, the position of the piston in another case, when the steam begins to act in the same manner. Then as the weight of the steam is the same in both cases, the pressures at and after P will be the same in both cases, therefore the units of work performed after that point will also be the same, but the work performed from O to P, is greater in the first case than it is in the second, because the pressures are greater in the former than they are in the latter case; hence, it follows that the work performed by a given weight of steam will increase with the pressure at which that steam is generated.

To appreciate the importance of this result it is necessary to observe, that since the sum of the latent and sensible heat of vapour is a constant quantity, it follows that there must always be the

==

64.079

same quantity of heat in the same weight of steam, whatever may be its pressure or temperature. Hence, therefore, the work performed by a given weight of fuel will be greatest when the pressure, and consequently the temperature, at which the steam is raised is greatest; hence the fallacy of the opinion entertained by some writers, that there is no advantage gained by using steam of high pressure.

It appears, from what has just been proved, that in order that an engine should perform the greatest work with a given weight of fuel, the following conditions must be observed. 1. The steam must be used of the greatest practicable pressure. 2. The piston must move, and the other parts of the engine must be constructed, so that the least possible change shall take place in the volume of the steam whilst passing from the boiler to the cylinder. 3. The steam must be cut off so that the whole of its work shall be taken up.

13.

To determine the point of the stroke at which the steam, working with a given pressure, must be cut off, so as to yield the greatest amount of work in a Doubleacting Engine.

The whole of the work will have been taken out of the steam when the sum of the resistances upon the piston in which there is no useful work, is equal to the moving pressure; that is, when the elasticity of the steam is equal to the elasticity of the vapour in the condenser, plus the friction produced by the motion of the engine. Let 7, therefore, be put for this resistance, and we shall have, by 10, 7 F + ƒL, + p, where p is put for the pressure of vapour in the condenser. As the relative dimensions of the cylinder cannot effect the work done, we have

h

vol. steam at P pressure H+ h vol. steam at pressure

But 8 (+) and S (+m)

m

are the volumes of the same weight of

$ (33 +m)..... (c) ;

183

N Kh

= S

144

n

whence P

Example 11. An engine makes 12 strokes per minute, the area of the piston=2000 inches, the steam is cut off at 3 feet of the stroke, and the water evaporated per minute cubic foot; required the pressure of the steam in the cylinder.

=

=

Here N=12, K=2000, h=3, and S= 24250 .. P = 25 lbs. 12 x 2000 × 3 × 144

65

15.-To find U in terms of S, in a Doubleacting Engine.

=

In order to effect this we have only to substitute the value of P, obtained in the preceding article, in equation 2. Then this value of U substituted in equation D, 10, will give the useful load. Example 12. If the length of the stroke 9 feet, the steam being cut off at 1 foot, the area of the piston=4000 inches, the number of strokes performed per minute 16, the clearance=6 inches, the pressure of the vapour in the condenser 3 lbs., and the water evaporated per minute=75 cubic feet; required the useful load, supposing the friction of the unloaded piston=8 lbs. per square inch, and the additional friction of the useful load.

3

H

=

by 14,

4