and short in proportion to its breadth, requires much less force to pull it round than a sharp-built clipper. Some may object to this statement, and tell us that vessels of the modern shape are as easily steered as those of a more antiquated form. We must keep in mind, however, that vessels of the newer build sail much faster than the others, and that their superior velocity gives greater power to the helm; but, whenever this speed is checked, the difficulty of turning attendant on their length and sharpness of form becomes a matter of very serious importance, as has been shown in not a few of those melancholy shipwrecks that have lately occurred.

It is also proper to remark, that the longer a vessel is in proportion to its draught of water, the more uncertain is the action of the helm in consequence of its liability to be thrown almost, if not entirely, out of the water, when the stern is in the trough of the wave.

The action of the sails in turning a vessel round, in some respects, is similar to that of the rudder, in others, it is very different. The power of the rudder depends on the motion. of the vessel through the water, that of the sails depends on the motion of the wind past the vessel. The pressure of a side wind on the foresails has a powerful effect in turning the vessel's head to leeward, while the action of the sails at the stern is precisely the reverse. When the ship is to be turned completely round to another tack, as in wearing and tacking, the sails are uniformly employed as assistants to the helm.

It is necessary to keep in mind the distinction that is to be drawn between the action of the rudder and that of the sail, and more especially to advert to the fact, that though the vessel's head-way be lost, and the rudder, consequently, have lost its power, the action of the sails continues unimpaired, and may be sometimes employed as a means of supplying its place.

We find inadvertencies in regard to this subject even in the treatises of Dr Robison, one of our most distinguished authors on nautical subjects. In the article on Seamanship in the Encyclopædia Britannica, after describing the opera

tion of tacking, he says,-" It frequently happens that in this conversion of her course the ship loses her whole progressive motion. It is evident that in this case there is little hope of success, for the ship now lies like a log, and neither sails nor rudder have any action. This is called missing stays." He then describes the evolutions required either to bring the vessel back to her former course, in order to "try again," or to wear her round, that is, " turning her head away from the wind, going a little way before the wind, and then hauling to the wind on the other tack." It is needless to say that he falls into a mistake when he says that "the sails" have lost their action.

This inadvertence of the doctor we should not have noticed if it had not gone to confirm the idea that when the vessel loses way there is nothing to be done but either to "try again" or "wear her round "-operations which require both time and space, and which circumstances often render impracticable. We feel persuaded that, if it could be efficiently applied, the action of the wind upon the hinder sails would be sufficient of itself to accomplish the object in view.

Looking to the subject theoretically, it is evident that if the sail next the stern, whether we call it a mizen or a spanker, were gradually brought more and more to windward as the vessel's head came round, its effect in tacking would be precisely the same as that of the rudder.

In practice we find that it is sometimes thus employed in boats which are provided with such a sail, and some instances have been mentioned of its being thus employed on board a revenue cutter; but in general the force that is required to bring it round, and the number of the crew that must at the same time be attending to other operations, renders it impracticable.

To obviate this difficulty, the following arrangement is suggested:

Make the boom of the mizen-sail, a b, and the gaff, cd, as long as convenience will admit; if made to project beyond the stern and sides of the vessel, the greater will be its power. At the ends a and c fasten swivels; to these swivels fasten yards, eƒ and g h, and to these yards fasten the sail.

The swivels are not placed exactly in the middle of the yards, but a little nearer to g and e than to h and f, so that

the line ca divides the sail into two parts, nearly, but not altogether, equal. The yards are fastened to the boom and gaff by means of ropes at ƒ and h.

When it is desired to employ the sail as a means of bringing round the vessel in tacking, the ropes at ƒ and h are slackened, the sail wheels round and presents its edge instead of its side to the wind, and the boom is hauled to windward with little difficulty. When brought as far round as may be desired (as in fig. 2), it is fastened, and the sail is then brought round again to offer its side to the wind. This operation requires comparatively little force, as the action of the wind on the one part of the sail is nearly balanced by its effect on the other. It should, however, be done as rapidly as possible, as the sail in the act of turning retards the vessel's motion. When thus brought round it is first brought into a position parallel to the line of the keel, and gradually turned to windward as the vessel's head comes to the wind.

N.B.-The author has not yet been able satisfactorily to test his invention. The result of some experiments on a small scale may be given, in the words of the Committee appointed to examine his paper :

"We have also witnessed experiments tried with his lifeboat, in which the principles it was his object to establish were practically applied; and while in such a boat they wrought at a great disadvantage as compared with one of the ordinary construction, the result was perfectly satisfactory, and established the practicability of his suggestion, at least when applied on a small scale; and altogether we think the principle well worthy of being tested on a larger one, and believe that the owners of such vessels as clippers (which are often difficult in stays) would find it to their advantage to do so."

Description of a Semi-Revolving Light. By J. T. THOMSON, F.R.G.S., F.R.S.S.A., Hon. Mem. Nat. Hist. Soc. Newcastle-upon-Tyne.

Revolving lights, when placed on small islands, or rocks in open seas, obviously perform to the greatest advantage, for in such situations their light requires to be seen from every direction; but when placed on the coasts of large islands or continents, it is necessary to the requirements of navigation that they should be seen only from seaward. In the latter case, which is a very general one, the revolving light, while performing its circuit, makes half of its power only available, and the other half is dissipated uselessly to landward; thus a constant double expenditure of oil and stores takes place, and the original cost of the apparatus is also greatly enhanced.

This defect of the revolving light has drawn considerable attention, and several plans have been proposed with the view of saving the useless expenditure. So before laying the description of my apparatus before the Society, I will shortly mention what had been done previously.

The first semi-horizon-eclipsing light brought to the notice of the public was the invention of Captain Smith of the Madras Engineers, and has been denominated by him a reciprocating light. The account of this light was published in the Transactions of the Institution of Civil Engineers, vol. ii., page 196, where it will, on reference, be seen the inventor states that the apparatus fulfils the condition of illu

minating one-half the horizon at five-eighths of the expense of revolving lights. The invention was brought to practical use in or about the year 1843 by being placed on the Madras Lighthouse. I understand that an apparatus worked on the same principle has since been placed on the Saugor Lighthouse, lately erected at the mouth of the River Hooghly, in Bengal. The reciprocating light, like the revolving one, shows a brilliant eclipsing light; but here the similarity ends, and the most essential distinctive property of the latter, viz. the equality of periods, is wanting. The reciprocating light, owing to the principle on which its machinery is constructed, varies its duration of obscurity in different azimuths, so that it cannot be distinguished by its periods in the manner of revolving lights; this defect cannot be considered of much consequence at the remote places where it has hitherto been applied, but it is sufficient to debar its introduction on the coasts of most parts of Europe or America where lighthouses are necessarily numerous, and must have plainly distinctive properties.

The next invention for an eclipsing semi-horizon light is that of Mr Thomas Stevenson, and as a description of it appears in an appendix to his account of the holophotal system, which is printed in this Society's Transactions, I will require to do nothing more than mention it. The apparatus is most ingeniously simple, and therefore most valuable; it is calculated to perform rigorously the required conditions of illuminating 180 degrees of the horizon with the same appearances, periods, and distinctions (whether coloured or otherwise), as can be obtained by the common revolving apparatus, and it dissipates none of its power on the other half of the horizon. Mr Stevenson has named the invention a reversing light; and possessing, as it does, the features and capabilities above mentioned, it is a great improvement on Captain Smith's reciprocating apparatus.

At the time Mr Stevenson published the account of his reversing light he was so kind as mention that he had already seen plans of my form of apparatus. My attention was first directed to the subject of semi-horizon-eclipsing lights by reading Captain Smith's account of his. This was in the year 1844, at which time I was engaged in designing a light