The proportions of steam.vessels were, originally, like those of sailing-vessels, short and full, their length being only about three or four times their breadth; while now the proportion of six breadths to the length is common in the best sea-going steam-vessels, and many of the fast river boats are still longer, ranging in length from seven to ten times their width. In America the proportion of length to breadth is sometimes made even greater than as ten to one ; and boats have been built, the length of which is twelve times their breadth. In larger steamers, the depth of the hull bears a greater ratio to the width than in smaller vessels. Different steam vessel builders adopt very different forms, ac cording as they wish to obtain a roomy, a sofe, or a swift boat; at the present day swiftness is regarded as almost an indispensable quality.
One of the important questions which has of late years attracted much attention in con nection with steam navigation, is that of the comparative advantages of iron and wood as materials for steam-vessels. Small vessels of iron have for many years past been used for river and canal navigation ; and recently steam vessels of considerable size have been built of the same material. Although different opi nions prevail as to the relative advantages of the two materials, it is certain that, owing to the snperior strength of iron, and its power of bearing strain in any direction, an iron vessel may be made much lighter than one of wood of equal strength ; the saving of weight being sometimes estimated at one half. Another great advantage of iron consists in the facility with which it may be formed into any shape. An iron hull is also superior to one of wood in its security from fire and its greater cleanli ness. Steam-vessels may be greatly strengthened by means of water tight bulk heads or trans verse partitions in the hull, the adoption of which is becoming very general in large steamers.
As an ample supply of cold water can al ways be commanded for the purpose of con densation, there is not much inducement for the use of high-pressure steam in marine en-' gines ; and this circumstance, with tho unfa vourable opinion generally entertained as to the safety of high-pressure boilers, has led to the almost universal adoption of low-pressure, or condensing engines, in European steam vessels ; although many of the steamers of North America are worked at a pressure of 140 lbs. to an inch, or even more. The kind of marine engine most commonly used in this country, is the sde-lever engine, where two distinct engines work separate cranks upon the same axle; but the steeple-engine, and other forms, are occasionally employed. The means adopted for condensing the steam is a matter of great importance in sea-going steamers. The most effectual method of con
densation is by the injection of cold water into the condenser, or by causing it to fall in a thin sheet, to which the steam is exposed. When sea water is used for this purpose, an incrus tation of salt forms inside the boiler, produc tive of much mischief. This evil can only be avoided by the use of fresh water in the boilers and condenser, and allowing the salt water to act only on the outside of the condenser. Hall's Condenser and Howard's Vapour Engine are two methods of attaining this ob ject; but neither of them has fullyrealised its intended purpose.
Notwithstanding the defects commonly im puted to it, and the great number of contri vances which have been devised for avoiding them, the common Paddle Wheel continues to be the chief means of propulsion used. It 1 consists of a number of flat boards, called teat-boards or paddles, bolted to the radii or arms of a light but strong iron wheel, which is fixed securely upon the crank axis of the engine ; and it should be so placed that the lowest float-board is entirely immersed in the water. Several improvements upon the com mon paddle-wheel have been introduced, which tend to diminish the defects attributed to it.
The substitution of the screw for the paddle wheel, is noticed under SCREW PROPELLER.
Sub-marine Steam Navigation has more than once attracted the attention of inventors. Dr. Payerne, whose contrivances in relation to sub-marine descent, are noticed under Drvism BELL, has lately (1851) formed a screw steamer which (according to the account given) will work at any depth down to 150 feet below the surface of the water ; while the contained air is rendered respirable by chemi cal means. The invention is said to be under examination by some of the silvans of France; if there is anything important or valuable in it, we shall soon hear more concerning it in England.
The steam ships of Great Britain now amount to a formidable number. Those which were registered at all the ports in the United Kingdom on Dec. 31, 1850, were Vessels. Tons.
Under 50 tons .... 520 .. 12,885 50 tons and above . 658 .. 154,327 The steam vessels that left and entered Brit ish ports, to and from foreign ports, in 1850, were as follow :— Inwards. Outwards.
England 10,782 .. 10,447 Scotland 4,467 .. 4,603 Ireland 4,340 .. 4,534 Isle of Man 225 .. 172 19,814 19,756 These numbers include the repeated voyages of each vessel. Besides the above, 399 steam ers arrived from, and 369 started for, the British colonies in the same year. Eighteen timber steamers, and fifty iron steamers, were built and registered at British ports during the same year.