that it is the obtaining the service of the heated products by an adequate run of flue, with sufficient time and surface, that characterises the Cornish boilers. In these, the main feature consists in generating, by slow combustion, no more heat than can be taken up, and transmitted to the water. In this respect, then, it is the direct reverse of the tubular system. In the former, there is slow combustion,-a continuous small development of combustible gas,-a long run,abundant absorbing surface,-a moderate rate of current,— free access of the water to the flues,-and sufficient time to enable the surface plate to do its duty;-added to the adoption of every possible means of preventing the loss of heat, externally, by clothing the outside of the boiler. In the marine tubular boiler, on the other hand, everything is the reverse. There is the most rapid combustion,-the largest and most irregular development of gas,-a rapid current, a short run,-a restricted and imperfect circulation of the water, and a total inadequacy of time for the transmitting and absorbing processes, with a great waste of heat by radiation from the boiler. Another serious evil of this tubular system, and its short run, which carries the heat away so rapidly, is, the over-heated state of the funnel and steam-chest; and the consequent danger to the part of the vessel in their immediate contiguity. The cause of such heat in a situation where it can be of no avail for the purposes of evaporation, has not been sufficiently inquired into. To this circumstance, without doubt, was attributable the destruction by fire of the Amazon steam-ship. The excessive heat of the lower part of the funnel, the take-up and steam-chest,-both of which were in that vessel under deck,-created a source of danger which does not exist in vessels where both are above the main deck. With reference to the availability of the tubular surface, even the horizontal position of the tubes, and their being ranged in tiers above each other, is peculiarly unfavorable. The lower tiers presenting the nearest opening for the escape of the heated gaseous products, are first occupied, and at an accelerated rate of progress. Mr. Atherton has given the most convincing proof of the waste and danger of the tubular system. Speaking of the combustible gases evolved from coal, he observes that "after having passed through the tubes, the proceeds from all the different furnaces become collected in the up-take and funnel; and being there combined and mixed together, they burst into useless combustion, frequently making the funnel red hot." This is unquestionably true, but it only shews that the combustible gases must have passed through the tubes unconsumed; and having, in the smoke box, encountered the air (which should have been supplied earlier), "then burst into useless combustion." But there is a more important reason for the large sectional area" in the flues which has not been referred to by writers on the subject, viz., that it is absolutely essential, chemically, to the completion of the process of combustion and the disposing of the products, of which water is so large a one, as will be shewn hereafter. In calculating the effective surface, then, it should be taken with reference to the length of the road, so to speak, along which the heated products have to travel in their hurried course, rather than to the breadth, or enlarged areas, which may be laterally or accidentally provided, but which are practically not used or available. the gas is reduced in temperature by cooling agencies, as will hereafter be shown. Luminosity is not an element in the generation of flame. It is merely the result of the presence of some other and solid matter-the degree of luminosity being in proportion to the quantity and temperature of such solid matter. In the combustion of coal gas, flame is caused by the union of the hydrogen with atmospheric oxygen, the heat produced being intense, raising the carbon, if present, to the state of incandescence, and producing the effect of luminosity. Here we may admire the wonderful adaptation of nature to human wants. Without the hydrogen there would be no heat, and without the carbon there would be no light. The luminosity of the incandescent carbon, then, is the mere result of that high temperature which is essential to its own subsequent combustion, or chemical union, with oxygen. Let this fact, then, be borne in mind, as it indicates the cardinal point of the whole process in our furnaces, namely, that it is not the gas, but the mixture, the compound of gas and air, that is ignited, and which produces the flame, with its heat and luminosity. This necessary condition of mixture clearly exposes the error of supposing that the gas may be ignited or consumed by being made to pass over, or in connection with the red hot fuel. Sir H. Davy has show that no degree of heat will consume gas-combustion being, not the heat in the gas, but the chemical union of its constituents with the oxygen-mixing being but the preliminary operation of bringing those constituents and the supporter of combustion into atomic contact, or within the sphere of chemical or electric action. * The two essentials of combustion being laid down by Sir H. Davy, viz.-temperature and contact, he then considers the management or treatment of the flame, and the means by which it may be effected or extinguished. He states, that on mixing one part of carbonic acid with seven parts of the mixture of gas and air; or one part of nitrogen with six |
