the only cure would be another stopper upon the same principle as the fore one, viz., in a plane with the vessel's bottom, and flush with it; but I am afraid that considerable difficulties would present themselves in shipping and unshipping these stoppers, so far under water as well as under the vessel.

Since my last letter upon this subject, I have seen a letter in your magazine, stating that after various experiments, it was found that a screw worked within a tube was the most effective arrangement; and so far this bears out Mr. Gachet's theory, but no mention was made of the length of the tube, and this appears to me to be the cream of the

advantages of protecting the screw from all external dangers, and enabling it to be made of any length, without inconvenience, and thereby obtain a firm hold upon the water, the same as the driving-wheels of a locomotive engine upon the rails." I do not see the analogy, but supposing it to exist, he must calculate upon his old enemy-friction, which would make sad havoc when exerted upon a screw (in addition to a tube) of any length, and I am afraid he would not tain a very great speed, and rival every screw of the present construction;" but hoping that I may be mistaken, and that your correspondent's bird may turn out a VERITABLE SWAN, I am, Sir,

July 13, 1846.

66

at

Your old subscriber, T. W.

IMPROVEMENT IN THE COMPOSITION FOR

PRINTERS' ROLLERS.

Sir,-Most pressmen will have observed that the rollers by age become unpleasantly hard, and, if left unused for some time, likewise turn mouldy. Both these bad qualities are obviated by adding a very little of the cheap chemical salt called chloride of calcium (muriate of lime) to the mixture, while warm, and stirring it well about.

The deliquescent property of chloride of calcium causes a regular even moisture throughout, and, being of a preservative nature, (like nearly all other neutral salts,) it prevents putrefaction taking place. So that the old stuff will be found equal to the new, on filtering it through open muslin while hot, to remove the indurated ink and dirt. Of course, too much chloride would render the roller softer than would be desired ;-a trial or two will soon put any one into this.

There is another purpose the same salt can be applied to in the art of letter-press printing, viz., to preserve the parchmentof the tympan, and keep it perpetually in the desired state of moisture. I remain yours, &c.,

B. CRESSWELL. Mainpoint, Edinburgh, June, 1846.

AMERICAN OPINIONS ON MARINE STEAM

ENGINES.

(Continued from page 93.) From Report of Mr. Charles W. Copeland, Steam Engineer, U. S. N.

Horse Power.-It is a practice almost universal in Europe, to make the power of the engines some given proportion to the tonnage of the vessel, without regard to the relative dimensions or model of the vessel, or the depth of its immersion. The proportions in most general use are the following:

For coasting steamers running about 250 to 400 miles-one horse power to two tons.

For coasting steamers running about 600 to 1000 miles-one horse power to three

as might have been anticipated, as it is well known that when the distance which a steamer is to run, without a new supply of fuel is increased, the size of the vessel must also be increased, in order to render it capable of accomplishing the distance, which, taken in connection with the fact that the resistance of a vessel to the propelling power, other things being equal, will be increased only as the square of the linear dimensions, while the tonnage increases as the cube of these dimensions, give sufficient reasons for these proportions having resulted from long experience.

That this rule of apportioning the power to the tonnage is very erroneous and without foundation, I will attempt to show. To do this it will be necessary first to consider by what the resistance to the passage of the vessel through the water, which the propelling power has to overcome, is produced. This resistance is determined by the following conditions, supposing the general power of the midship section in all cases the same, viz. :-The area of the immersed section. The angle of entrance, and the fineness of the run. The length of the vessel.

The error of the former method will be best shown by supposing a case. According to the rule given, a vessel of 600 tons would have an engine of 200 horses power. Suppose the dimensions of this vessel in one case to be

Length of keel for tonnage 127 feet,
Breadth of beam

30,,

And allowing the draught of water in both cases to be the same,-say load draught 9 ft.: it is evident, assuming the angle of entrance, run, and general model of the vessel the same in both cases, that the resistance, and consequently the propelling power to produce the same speed, will be nearly in proportion to the immersed section, or (30 × 9270 and 26 × 9=234) as 270 to 234, (calling the immersed section a parallelogram; therefore if the same power is used in both vessels, that of 26 feet beam will have the greater speed, and this speed would be increased if the angle of entrance and run of the vessel of 26 feet beam was improved, as it might be from its increased length.

From the foregoing reasoning we arrive at this conclusion, viz. :-that in order to make use of the minimum amount of power, consequently attain the greatest economy of fuel, and also to obtain the greatest amount of room for stowage of fuel, stores, &c.,

Two engines 40 inch. cylinder, 8 feet stroke, 105 horses power each; or one engine 54 inch. cylinder, 9 feet stroke,=210 horses power.

These three vessels are supposed to have an easy angle of entrance and a fine run; the proportion of power to tonnage is one horse power to 3.3 tons; but if the vessels were to be built with a full bow and stern, or the length was made less in proportion to the beam, it would become necessary to increase the power of the engine in proportion to the tonnage of the vessel; but how much this increase would be, could not be determined until the dimensions and general model of the vessel were fixed.

I have been induced to enter into the discussion of this question more fully than I should otherwise have done, as the efficiency of a steamer must, in a great measure, depend upon the power being properly proportioned to the vessel; for the reason, that if the power be too small the efficiency of the vessel is in a great measure destroyed in a head wind, or running head to the sea; and, on the contrary, if too great, the expense of maintaining and running the vessel is much increased, without any adequate benefit being received.

2nd, "The kind of engine." The form of engine in general use for the British sea steamers, is that known as the English marine engine, with side levers. An engine with a long stroke is generally considered by American engineers to have advantages over, and to be superior to, one of a short stroke,

for several reasons; and it is one objection to the kind of engine mentioned above, that the length of stroke is determined by the depth of the vessel. The cast-iron framing in use for this form of engine is also considered objectionable, as from the nature of the materials, it possessing no elasticity, they are very liable to accident or derangement from the working of the vessel in a sea; and to provide as much as possible against these accidents, they are necessarily made very heavy.

The other objections are, the expense of building and keeping in repairs great weight, and straining the frame of the vessel.

A form of engine used in the English naval steamers, Gorgon, Cyclops, &c., and known as the Gorgon engines, are liable to some of the objections to the English marine engine; viz., the length of stroke being very short, those of Gorgon and Cyclops being only 5 feet; the depth of hold of the vessels being 23 feet; and the action of the engine tends to work the frame of the vessel. The connections also are very short, and work at great angles..

The engines of a kind called vibrating or oscillating engines, of small dimensions, are much in use on the river Thames, and in some instances have been used at sea, but their use has generally been confined to vessels of less than 400 tons, and with engines of less than 60 horses power each. On account of some practical difficulties, it is doubtful whether they would be suitable for vessels of a larger class.

There is a form of engine which has recently been brought out in this city, that I consider preferable to any of those already mentioned. It is called the right-angled engine; the two cylinders being placed each forward or aft of the other, and upon an angle of 45° with the keel; so that when the connecting-rods are attached to the same crank-pin, the engines will be working at right angles to each other. The boilers are intended to be placed on each side of the engines.

This plan of engine is more suitable for the merchant than for the naval service, and is only applicable to vessels of the largest class. The objections to it for the naval service are, that the boilers being placed alongside and outside of the engines, there is nothing to protect them from shot but the side of the vessel itself. The engineers, when working the engines, will be at a distance apart, of from fifty to seventy feet; a difficulty in arranging the flues of the boilers, so that they may be brought into one chimney; also a difficulty in producing the requisite ventilation, and the length of stroke dependent upon the depth of the vessel.

I am of the opinion that the inclined engines