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burners, fancifully termed a bat's-wing, to the gasometer of oil gas, with 7-8ths of an inch pressure. There was a very intense light, accompanied by a great deal of smoke, and the quantity consumed, which was accurately measured, scarcely exceeds two feet per hour; while in coal gas it burns at least five feet. The way in which the difference between oil and coal gas is estimated is, that in lamps giving equal light, the one will consume one and one-third foot per hour; while the other will burn five feet in the same time.

It is impossible from the statement given in the Report, to draw any correct comparison between the advantages of oil and coal gas; yet if we take the most favourable part of each Company, and compare it with oil gas, we shall find the superiority of the latter to be quite as great, if not greater, than I have before mentioned. The average quantity of gas consumed by each burner of the Chartered Company, allowing for waste, cannot be less than 5 feet per hour. By an accurate account kept of the quantity of gas consumed each hour during the night for several nights at the Oldford Works, and taking the hours between two and four, the time when only the public lamps were alight, and the number could be correctly ascertained, the average quantity of gas for each lamp was from 14 to 14 foot per hour, making it rather less than 1 to 4. This quantity is a pretty near average for the private lights. Comparing this with the other two Companies, it would be nearly as 1 to 51: my experiments were always as 1 to 4; but I calculated only as 1 to 3; in both I have been within the mark.

In the amount of capital I will draw a comparison with the City of London Company. This is very little more than half the Chartered, supposing their statement to be quite correct--that they are quite clear of debts-and that their whole expenditure has not exceeded the sum stated in the Report. To produce the same number of lights with oil gas, a sum of 15,000l. for all necessary erections, apparatus, &c. law charges, and other contingencies, exclusive of mains, would be ample for every expence. The cost of 50 miles of main would not exceed 35,000l. so that a capital of 50,000l. would be sufficient. This is somewhat more than one-third compared with the above-mentioned Company, but with the Chartered, and their statement carries the greatest appearance of correctness, the difference would be considerably more. From the comparative small capital which is required for an oil gas establishment, it is clear if the same profit be derived from every light, the amount of dividend upon the money advanced must be, taking the average, three or four

times as much in the one instance as in the other.

There is also another great advantage attendant on oil gas establishments which I have not sufficiently dwelt upon, and that relates to the current expences. In the Chartered Company, the

cost for producing 12,000 feet of gas, supposing that quantity to be procured from a chaldron of coals, is 37. 17s. 5d. of which the cost of materials is 17. 10s.; and the remaining 27. 7s. 5d. is for labour, wear and tear, management, &c. To produce 3000 feet of oil gas, the cost of materials will be, oil being 251. per ton (the Oldford Company have never yet paid more than 227. including casks); 3l. for oil, and 51. for coals; while the cost for labour, wear and tear, &c. would not exceed, taking the average of full and slack work, 4s. per 1000, or 12s. for the 3000 feet. The profit upon that quantity by the tables in the Report will be 21. 3s. 11d.; and in the Oil Gas Company, allowing one-third of the quantity consumed to be for public lights, which pays about one-half, would produce the sum of 21. 3s. Now whether there be a large or small demand for gas, the current expences do not vary much. The establishment must be kept up, and where there are many occasional lights, they must be always in readiness to supply them, if they should be wanted. The interest on the capital too always remains the same. In the Oil Gas Establishment, both the one and the other are comparatively very small, and the greater expence, the cost of materials, ceases when no gas is required; while in the other, the smaller expenditure, namely, the cost of materials, ceases; but the greater ones, the current expences, and the interest on capital, continue, whether a small or large quantity of gas is required. Thus in every way the advantages of oil gas are most clearly manifested.

I am aware that the foregoing observations will not afford any more satisfaction to the advocates for coal gas than my former statements have done. Whether that dissatisfaction has been expressed in any of the monthly journals, I have but few opportunities of knowing, as I seldom see them, though I understand it has been inserted in some of the provincial newspapers, accompanied by insinuations which it is not worth my while to notice. I state what appears to me to be facts. If I am incorrect, let me be proved so by direct argument, and the public, or that part of it who are interested in the subject, will judge between us. Yours truly,




Essays on the Construction of Sea Harbours.
By Mr. J. B. Longmire.

(Continued from vol. v. p. 182.)

(To the Editor of the Annals of Philosophy.)

Troutbeck, May 20, 1823. On the following phenomena of fluids, in direct and reflected motions, depend the disposition of the piers; which is perfect, when the water in the harbour is still, or nearly still; the surf, as little increased as possible by the piers; and when they interfere not with the lines of approach.

a. Straight waves, driven by the wind, directly through an opening into the still water of an harbour, assume curved figures; which, as they advance, become nearly semicircular, increase in length, and decrease in depth, till they are quiescent. Waves obliquely driven through a given opening do not agitate the interior water so much as direct waves; for the length of the waves that can pass through decreases as the angles of obliquity increases.

The waves, in moving over the surface, give the water under them impulses that create an agitation, extending much beyond them. This, which is here designated lateral agitation, to distinguish it from the surf, acts in and near harbours through the whole depth of the water, and appears on the surface in flat and slow undulations.

If a part of the harbour be separated by an inner pier, which only leaves a small entrance, not far from the principal one; then small parts of the waves pass into this bason; but being previously much decreased in height and velocity, and the lateral agitation considerably weakened, they very little disturb the water in it.

The surf, driven by a strong gale, through an entrance facing the sea, and sixty yards wide, provided the pier heads be opposite and parallel, requires a space equal to 1,200 square yards, to be quieted so much, that the rest of the bason have not undulations larger than one foot high.

When a gale commences, the waves but slightly disturb the interior water; yet by reiteration, the agitation greatly increases. Now the art of stilling the water in a harbour that admits the surf is to allow space for it to dissipate, and so to proportion an entrance to the area of an inner bason, that when the agitation is at its utmost, the undulations at the surface, where vessels lie, shall never exceed 15 inches, nor be repeated oftener than five

times in a minute. Any increase to this rate would injure vessels in the ebb tide, when they strike the ground.

Fig. 3.

b. The surf is raised, and driven forward by the wind, and of course takes its direction. If a part of the surf be stopped by the outside of the pier, the adjoining part continues to move forward, leaving the water on the innerside undisturbed, except by lateral agitation, for a given distance forward. Let fig. 3, represent this position. 1, 2, are parts of the piers of a harbour, rejecting the surf. Let b, 1, be the direction of the wind, and a b d a line of waves moving in this direction. The part a b, is stopped by the pier; but the part b d continues to move forward, say to h; beyond this point, the waves lengthen out towards the pier 2; and are limited on this side by the curved



line hl. Thus while the exterior sea 3 is covered with a high surf, the part 4 has only the lateral agitation; and the harbour 5, being acted on by the same force only through the entrance h 1, is nearly quiescent.

The passing surf affects the harbour 5 least in the direction ad; as the sea within this line, in the parts 3 and 4, is comparatively smooth and most when coming in the line 0,2; as then the part 4 exterior to the line 2,0 is equally covered with the part 3, by the surf; and of course a stronger lateral agitation is forced through the entrance. The surf indeed almost enters the harbour when driven in the line c h, and would disquiet it more than in either of the former courses; but that a gale in this line, making only a small angle with the lee shore, never raises a high surf at the harbour, and the surf is least in a gale directly into the entrance hi; for as it faces the calmest quarter, or, in other words, as the lee shore, at no great distance, projects into the sea, and covers the entrance, this gale comes over too small a range of sea, to force an injurious surf into the harbour. This description developes the mode of forming a harbour that rejects the surf.

c. When a wave strikes a pier at right angles, it rebounds directly back; but if it strike obliquely, the angle of rebound is equal to that of percussion. In oblique percussion, the reflected surf is greater at the leeward than at the windward end of the pier, by the amount of such surf collected through the whole, or a part, of the length, according to the strength or frequency of the impinging waves. The surf that strikes the pier at an angle of about 25°, sends the greatest quantity of reverberated water to the leeward end; and that impinging under an angle of 45°, disturbs the sea in front, to the greatest distance forwards.

The terms leeward and windward ends are used relative to the

direction of the wind, and change at the same end as the wind passes from one into the other of the two quarters in front of the pier. This alternation in the direction of the surf makes it difficult to adjust the direction of the enclosing piers so as to prevent the strong surf in any gale from passing along such piers to, and accumulating at, the entrances.

I am, Sir, your very humble servant,

(To be continued.)


On the Temperature of Mines. By M. P. Moyle, Esq.

(To the Editor of the Annals of Philosophy.)


Helston, May 11, 1823. NEARLY twelve months have now elapsed since the temperature of many parts of Huel Abraham, Crenver, and Oatfield Copper Mines, in this county, were taken, an account of which you did me the favour to publish in the Annals for January last. Many of the experiments were a few days since repeated in precisely the same spot, and under similar circumstances, as before, and nearly with the same results, excepting the temperature of the water accumulated at the bottom of Oatfield engine shaft below the depth of 182 fathoms from the surface, in consequence of the pumps being drawn up from below that level.

The coldest part of this water, ten months ago, at the depth of 1,164 feet from the surface, and in 12 fathoms of water, was 66°. Last week, at precisely the same depth, it was only 59°; while the water at the surface of this shaft was 77°. This increase of temperature at the surface is to be attributed to the immense quantity of warm water sent from distant parts of the other mines to this shaft to be drawn out; and although it falls several feet into this shaft, which keeps the water in a constant and great agitation, yet it does not effect the temperature at the above-mentioned depth so much as might be expected.

I regret much that the registering thermometer could not be sunk to a much greater depth, and quite out of the influence of the falling waters, as I am inclined to think that it must ere this have arrived, or nearly so, to the mean annual temperature, or 53°.

I have before shown that by admitting the gradual increase of temperature (according to our descent) after a certain ratio, the temperature of this depth ought to be, at the lowest calculation, 70°. How comes it then to be less by 11° and 18° minus since this place was in the full course of working?

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