Jumna, showing the boilers.

The Jumna is of

4,175 tons burden, and is 366 feet long and 49 feet broad. Her engines, which are by Messrs. Maudslay, are of the return connecting rod type, with three cylinders, each 77 inches diameter, and 4 feet stroke. There are four boilers, two at each side of the stoke hole, and twenty furnaces each 7 feet 6 inches long, and 3 feet 4 inches wide. The boilers contain 2,780 brass tubes 2 inches outside diameter, and 6 feet 6 inches long.

PROPORTIONS OF BOILERS.

The proportions of boilers per nominal horse power are affected by two considerations-the first, what ratio it is intended shall subsist between the actual and nominal power; and the second, what amount of surface shall be allowed for the evaporation of a cubic foot of water in the hour. The greater the excess of the actual over the nominal power, and the less the expansion, the larger manifestly must be the surface per nominal horse power; and in proportioning boilers of every class the main thing to be had regard to is the number of cubic feet of water required to be evaporated in the hour. But in different classes of boilers very different quantities of surface are required to evaporate a cubic foot per hour. Thus in Smeaton's boilers a cubic foot was evaporated per hour with 5 square feet of heating surface, in Watt's land boilers with 9 or 10 square feet, in locomotive boilers a common proportion is 5

contrived that the volatile parts of the coal were distilled off as gas, and passed into the fire through hollow furnace bars perforated at the sides, to the end that the gases should be mixed with their proper proportion of atmospheric air before being mixed with the gaseous products of combustion, which are not only uninflammable themselves, but impart that property to combustible gases, if mixed with them in large proportion. The principle then adopted and promulgated by me was, that instead of endeavour. ing to burn smoke, we should rather endeavour to prevent it; and this doctrine, communicated by me to Mr. Charles Wye Williams, was subsequently preached by him with zealous iteration, and is now pretty generally accepted. In this boiler the draught was maintained by an exhausting fan-an expedient which did not originate with me, but had been previously introduced by Ericsson; and I subsequently employed such fans, as Ericsson also did in various steam-vessels. But on the whole, the conclusion I arrived at was, that a steam jet in the chimney is to be preferred. It has been conclusively shown by the experiments of Peclet and others, that an artificial draught is producible with a much smaller expenditure of fuel than a natural draught; and in steam vessels, where a great height of chimney is inadmissible, this is especially so, as the larger part of the exhausting column has to be discarded. The particular arrangement for separating the gases from the coke in the boiler referred to was defective in this

respect, that it entailed too much trouble upon the fireman; and mechanism should be substituted for hand firing in the case of all large boilers. But there is no doubt that the principle on which my boiler of 1838 was constructed was intrinsically correct; and I expect to see gas furnaces of analogous form come into use, the fuel-which may be petroleum-being introduced into a retort with water or steam. Gas tar was thus employed by me in 1834; and should liquid fuel take the place of coal to any extent as is probable-gas furnaces will become of easy application, and will bring many advantages in their train. An example of a common form of tubular boiler is given in fig. 9, which is a marine boiler with Beardmore's superheater introduced in the up-take. The flame and smoke, after passing over a brick bridge at the end of the furnace, return through the tubes to the front of the boiler; and the smoke then passes the chimney, but on its way thither encounters the horizontal tubes of the superheater. In this superheater there are two sets of tubes, separated by a diaphragm; and the steam passes back through one set of tubes, and forward through the other, so that it traverses a distance equal to twice the length of the superheater.

up

The best forms of tubular boiler do not differ materially from that constructed by me in 1838; and with the increasing pressures which are now used, it is inevitable that the rectangular shell at present employed should be discarded. The boilers intro

square feet of heating surface per nominal horse power. But these are exceptional cases: the usual proportion they give is 21 square feet per nominal horse power. Taking the evaporation as the measure of the power of the boiler, their present proportions for evaporating a cubic foot in the hour are as follows:

PROPORTIONS OF BOILER TO EVAPORATE A CUBIC FOOT OF WATER IN THE HOUR, 1868.

Total heating surface of tubes and plates. 10 square feet.

The quantity of water required to be evaporated to produce an actual horse power, depends of course upon the rate of expansion. Boulton and Watt usually put sufficient lap upon the valves to cut the steam off at half stroke, and then by the aid of the link, they are able to cut off at of the stroke if required.

The particulars of the proportions and performance of the boilers of a number of modern steam vessels, as also the proportion of a condensing surface per nominal horse power in the tubes of the condenser and other material facts, are recorded in the following table: