APPENDIX TO THE PRECEDING.

During the progress or about the close of the above Report, I found that my friend and former pupil, Mr. A. Murray, had communicated a paper on a similar subject to the Institution of Civil Engineers, entitled The Construction and proper Proportion of Boilers for the Generation of Steam.'

Mr. Murray has had many opportunities of judging of the best forms and proportions of marine boilers, and, from the facilities afforded in his professional avocations at the Royal Dockyard, Woolwich, I am induced to

as to prevent the admission of too much cold air in that direction. This being accomplished, the fresh coal is then thrown on, commencing at the FIG. 21.

quote a few of his observations relative to the area of the flue, bridge, chimney, &c., which have in some degree been omitted in the preceding Report. In treating of the quantity of air entering into combination with the volatile products of pit coal, Mr. Murray states, that The quantity of air chemically required for the combustion of 1 lb. of coal has been shown to be 150.35 cubic feet, of which 44.64 enter into combination with the gases, and 105.71 with the solid portion of the coal. From the chemical changes which take place in the combination of the hydrogen with oxygen, the bulk of the products is found to be to the bulk of the atmospheric air required to furnish the oxygen as 11 is to 10. The amount is therefore 49.104. This is without taking into account the augmentation of the bulk due to the increase of the temperature. In the combination which takes place between the carbon and the oxygen, the resultant gases (carbonic acid gas and nitrogen gas) are of exactly the same bulk as the amount of air, that is, 105.71 cubic feet, exclusive, as before, of the augmentation of bulk from the increase of temperature. The total amount of the products of combustion in a cool state would therefore be 49∙104 +105·71=154·814 cubic feet.

'The general temperature of a furnace has not been very satisfactorily ascertained, but it may be stated at about 1000° Fahrenheit, and at this temperature the products of combustion would be increased, according to the laws of the expansion of aëriform bodies, to about three times their original bulk. The bulk, therefore, of the products of combustion which must pass off must be 154.814 × 3 = 464.442 cubic feet. At a velocity of 36 feet per second,* the area, to allow this quantity to pass

* See Dr. Ure's experiments, read before the Royal Society, June, 1836.

off in an hour, is 516 square inch. In a furnace in which 13 lbs. of coal are burnt upon a square foot of grate per hour, the area to every foot of grate would be ⚫516 × 13 =6.708 square inches; and the proportion to each foot of grate, if the rate of combustion be higher or lower than 13 lbs., may be found in the same way.

This area having been obtained, on the supposition that no more air is admitted than the quantity chemically required, and that the combustion is complete and perfect in the furnace, it is evident that this area must be much increased in practice where we know these conditions are not fulfilled, but that a large surplus quantity of air is always admitted. A limit is thus found for the area over the bridge or the area of the flue immediately behind the furnace, below which it must not be decreased, or the due quantity could not pass off, and consequently, the due quantity of air could not enter, and the combustion would be proportionally imperfect. It will be found advantageous in practice to make the area 2 square inches instead of 516 square inch. The imperfection of the combustion in any furnace, when it is less than 1.5 square inch, will be rendered very apparent by the quantity of carbon which will rise unconsumed along with the hydrogen gas, and show itself in a dense black smoke on issuing from the chimney. This would give 26 square inches of area over the bridge to every square foot of grate in which the rate of combustion is 13 lbs. of coal on each square foot per hour, and so in proportion at any rate. Taking this area as the proportion for the products of combustion immediately on their leaving the furnace, it may be gradually reduced as it approaches the chimney, on account of the reduction in the temperature, and, consequently, in the bulk of the gases. Care must, however, be taken that the flues are nowhere so contracted, nor so constructed, as to cause, by awkward bends or in any other

way, any obstruction to the draught; otherwise similar bad consequences will ensue.'

From this statement it would appear that 26 square inches of area over the bridge is about the correct proportion for the combustion of 13 lbs. of coal per hour on each square foot of grate-bar. Now these proportions are rather more than are given in stationary boilers, as the mean of a number of experiments, taken where the combustion was most perfect, gave about 18 square inches over the bridge, and about 28 square inches as the area of the flues to every square foot of grate-bar.

These data may not at first sight appear important; they are, however, of great value in practice, as the economy of the fuel and the efficiency of the furnace in a great measure depend upon the height of the bridge behind, which operates as a retarder of the currents, in the same way as the damper is used for checking the draught of the chimney in the flues.

Mr. Murray further treats of the temperature of the furnace, flues, &c., but these points having already been experimented upon and fully discussed in the Report, it will not be necessary to notice them in this place.

223

LECTURE X.

METALLIC CONSTRUCTIONS*- -ON IRON SHIP

BUILDING.

To the student in architecture, engineering, and building, there is scarcely any acquirement more essential to professional success than a knowledge of the properties of the materials used in construction. It is equally important in the art of design as it is in correctness of proportion: whether the structure be a house, a ship, or a bridge, we must, before entering upon its construction, and before we can attain a due and correct idea of proportion, as a preliminary inquiry, make ourselves acquainted with the material of which it is composed. We must also make ourselves acquainted with its powers of resistance to the varied strains of tension, torsion, and compression; and further, we should know something of its elasticity, and its powers of restoration under the varied tests and changes to which it may be subjected.

* At the commencement of these Lectures it was intended to have given a series of communications on construction, where iron, as a material, has been chiefly employed. The subject was, however, found to embrace such a large field of inquiry, and my time was so closely engaged in other professional duties, that I was forced to abandon the idea, and content myself with the inquiry into the strength and other proportions of the iron ship. Whether another opportunity will present itself for a more extended investigation, I am unable to say; I will, however, bear it in mind, and I trust the time may shortly arrive when my engagements may admit of sufficient leisure for effecting that object.