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duced by Messrs. Penn in the Hydra are represented in figs. 10 and 11; and these boilers have the advantage of a cylindrical shell, and may be worked with safety to a pressure of 40 lbs. on the square inch. But the furnaces are too small; and cylindrical furnaces are very inconvenient for enabling a proper slope to be given to the fire-bars, as the width of the

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BOILERS OF H.M.S. HYDRA, BY MESSRS. JOHN PENN AND SON. Transverse Section, showing Tubes. Transverse Section through Furnaces.

bars at the back end is necessarily contracted; and the bars must either be made taper, or taper pieces must be cast to fill the vacuities at the front. is also too little room for the ashes.

There

The species of water-space boiler known as Lamb and Summers' boiler has now been widely introduced, and has this special feature of advantage, that it enables a rapid circulation of the water to take

place. This boiler is shown in fig. 12, which is a transverse section of the boilers of the steamer Ripon.

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BOILERS OF STEAMER RIPON ON LAMB AND SUMMERS' PLAN.
Transverse Section.

Fig. 13.

In this boiler the smoke, instead of being returned through small cylindrical tubes, is returned through a row of very narrow flat-sided flues; and in order to prevent these flat surfaces from being forced to

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BOII ERS OF COLLINS' LINE OF ATLANTIC STEAMERS, WITH DOUBLE-FURNACES AND UPRIGHT TUBES. Longitudinal Section.

gether by the pressure of the steam, they are strutted asunder by short struts, so that in the interior of the boiler, the whole heating surface of these flues is clear of stays or other obstructions.

The same benefit of an effectual circulation that is obtainable in Lamb and Summers' boiler is also obtained in the boiler with upright tubes invented by that remarkable genius the late Earl of Dundonald, and of which an example is given in fig. 13, which is a longitudinal section of one of the boilers of the steamer 'Atlantic.' Boilers on this principle are not much used in this country, except in the case of the hay-stack boilers invented by Mr. David Napier, and which are very generally employed in the river steamers plying on the Clyde. In these boilers upright tubes, with the water within them, are also employed.

The importance of maintaining a rapid circulation in the water of the boiler is not yet sufficiently recognised. Not only will a rapid circulation add to the durability of the boiler by preventing the plates from being overheated, but it will materially increase the efficiency of the heating surface; and in the boilers of the 'Atlantic,' it was found that by inserting a short piece of tube in the mouth of each upright pipe, whereby the length and consequently the velocity of the ascending column was increased, a sensible advantage was gained in the performance of the boiler. In many tubular boilers the tubes are set so closely together, that the circulation of the

water amongst them is greatly impeded; and it has in consequence been found, that the evaporative power of such boilers has been increased by removing some of the tubes altogether the loss of a part of

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BOILERS OF THE TROOP SHIP JUMNA, 1867.

the heating surface being more than compensated by the increased efficacy of the rest.

An example of the form of tubular boiler commonly used in first-class steamers in 1867 is given in fig. 14, which is a cross section of the troop-ship

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