The popularity of the Scotch boiler is due mainly to its great reliability. In capable hands, its life is from 15 to 20 years. The watertube boiler is not a serious competitor in the mercantile marine,

and whilst it has the advantage of being lighter, and the ability to raise steam in 2 to 4 hours, compared to the 20 to 24 hours required with the Scotch boiler, it has, for other practical reasons, been adopted in merchant ships to only a limited degree.

The internal combustion oil engine has been fitted in only a few

coasters so far, and the experience gained has not been sufficient yet to say whether it is likely to be more freely adopted in the future or not. The usual speed of coasters is about 8 to 10 knots.

Coast Liners.

These embrace all the cross-channel or coasting steamers running on regular routes; for instance, between the main seaports of Great Britain, or to ports in Ireland and Isle of Man. A typical vessel of this class is shown in Fig. 3. These ships are provided for the carriage of general merchandise, cattle, passengers, and mail. We have, therefore, now shown a different type of ship, having her machinery and boilers amidships, with holds forward and aft, and with 'tween decks fitted to give more deck space for the use of cattle and goods of every description. This class might be further subdivided into

1. The ships which sail regularly right round the coasts, calling at all the principal seaports. These handle general cargo mainly. The passengers carried are mostly travelling for pleasure, and therefore the number of cabins in these ships is moderate. Vessels engaged thus do not as a rule have a greater speed than 12 to 14 knots, and are fitted with steam reciprocating engines and coalfired boilers. It may be noted that coal is the most economical and readily obtained fuel for ships plying entirely between Northern European ports.

2. The ships, for instance, which maintain a regular service between British and Irish ports. Whilst the passage generally would not exceed 12 hours, the voyage is usually made through the night for the sake of economy of time, and accommodation is provided in a very ample manner for all passengers travelling. Some years ago, all such accommodation was usually placed right aft, under the poop, whereas in the modern ship it is arranged on the upper and bridge decks amidships, where much finer rooms can be arranged, with greater facilities for heating and ventilating, etc. As the mails are also carried, such vessels are usually very much faster, having speeds as high as 18 to 20 knots. Many of them are now fitted with steam turbine machinery, having single reduction gear.

3. The vessels which make daylight trips only, linking up island

or other towns which have no railways or other connection with the mainland. Such have no sleeping accommodation, and are fitted only with day cabins, dining saloons, and open deck spaces. Originally, this work was almost entirely carried on by paddle steamers, but now many are fitted with high-speed steam turbines. The speed of ship aimed at is usually from 16 to 20 knots.

Tramps.

We now come to the class of vessel which comprises the greatest proportion of the world's tonnage. As the name implies, these vessels are built generally to carry any kind of cargo between any two ports on the ocean seaboard. These have increased in size very much in modern times. Not so very many years ago a vessel which could carry 4,000 to 5,000 tons deadweight was considered very large, whilst nowadays such a ship would be looked on as very moderate, and the standard of "big" ships has quite doubled. There are many different designs, ranging from the single decker, with large accessible holds for the carriage of bulk cargoes such as coal, ore, grain, phosphates, etc., to the two and three decker, with suitable space for general merchandise in bales or crates. Attention might also be drawn to the fact that in very large tramps the number of decks comes into the strength calculation of the design, and influences also the freeboard of the ship, so that a single decker would naturally be limited in size.

The smaller tramp used in general trade between, for example, United Kingdom and Mediterranean ports, is usually built with a single deck, clear holds, and self-trimming hatches for carrying coal outwards from Britain and grain, iron ore, or other cargo homewards. Bulky cargoes like cork, etc., require large capacities, and the ships employed are usually fairly broad, and are correspondingly slow. The larger tramp of modern type is illustrated in Fig. 4. The example shown is a vessel capable of carrying about 8,000 tons, and has two decks with large hatches. The ship has also a long bridge deck amidships in way of the machinery spaces. This erection, as it is called, adds considerably to the strength of the ship as a whole, besides covering the light and air hatches to the engine and boiler rooms, and contains all the living quarters for the officers. This vessel might also be called a well-decker. It will be

noticed that the three erections-forecastle, bridge, and poophave between them wells in way of the cargo hatches. If this deck were continuous, with portable covers over the wells, it would be a shelter-decker, and if it were completely watertight, with hatches, it would be a spar-decker. All these erections, or extra decks, with their relative proportions to the structure as a whole, are taken into consideration in determining the freeboard of the ship. Besides these general cargo carrying tramps, we have ships which sail on more established routes, such as India, China, Australia, etc., and each has special features suitable for the trade. Two of these are worthy of notice. Vessels which are engaged regularly in the dead-meat trade have all the holds insulated and refrigerated by special machinery, so that the carcases are kept frozen during all the voyage. Vessels carrying fruit, on the other hand, as well as being refrigerated, are very specially ventilated and have holds all well protected with wood to avoid damage to the cargoes, with additional shelving and divisions, etc., arranged as found most suitable.

Tramps, as a rule, are not built for any exceptional speed; from 8 to 10 knots is the average. It might be remarked that speed can only be obtained in a ship by fitting her with high-powered engines (with, therefore, a large consumption of coal) and by making her fine in the water lines. Neither of these attributes, the first entailing much expenditure in fuel, and the second involving a reduction in deadweight carrying, would contribute to the economical working of a tramp steamer, where, after all, speed is not necessarily a very great consideration in such work.

Tramp Liners.

Where certain routes have become well established, some tramps now cater for a moderate number of passengers. They are able to carry passengers more moderately in cost, and able oftentimes to take them direct to more out-of-the-way places,' where regular passenger liners do not call. Such ships might be called tramp liners, and would be designed to have a greater sea speed, say up to 14 knots.

The great majority of the tramp class of ship is propelled by steam multiple expansion engines, of the reciprocating type, with