off any dampness it may have contracted by slightly warming it, I then proceed, with a glass rod or a pellet of cotton, to coat its surface with ammonio-nitrate of silver as evenly as possible, and then dry it quickly, by holding it to the fire, or by pinning it up in a dry, darkish place. Dampness, either before or after the sheet is coated, is very apt to cause blotches, and hence it is advisable to use the sheet as soon as possible after it has been prepared.

The pressure frame I use is of the simplest construction. It consists merely of a cross-headed flat board, to which is attached by hinges a frame containing a square of plate glass; the pressure being given by a pinching screw. The ammonio-nitrate of silver is made as follows:

Nitrate of silver (crystallized), 110 grains.
Rain water (pure), 3 ounces.

Shake till all the crystals are dissolved, and then add liquor ammoniæ (fortissimus) in small quantities till the precipitate at first formed is almost entirely redissolved. Should too much ammonia be added, a few crystals of nitrate of silver will bring back the turbidity, in which condition I find it most suitable.

When the negative and sensitive sheet of paper underneath have been exposed to the action of the sun's rays long enough to make the copy a shade or two darker than it is intended to be when finished, the copy should be immersed as soon as possible into a bath of hyposulphite of soda to prevent the light from exerting any further influence upon it, or, as it is termed, to fix it. This bath is made thus :

Hyposulphite of soda, 2 ounces.
Water (common), 16 ounces.

To render this bath from the first capable of giving tints equal to an old bath, there should be added a dram or half a dram of chloride of silver, and 40 drops of chloride of gold solution, of the strength already mentioned. Those pictures, which were from the first rather faint, will be fixed after ten minutes' immersion; and darker ones may be allowed to remain as many hours, or until they assume the desired gradation of light and shadow. The pictures must then be

subjected to a thorough washing, so as to remove completely all traces of the hyposulphite of soda bath, which will otherwise be pernicious to the permanence of the colours of the photograph. The copies are then dried; and pressed, or polished on the back,

I have thus endeavoured shortly to describe the manner in which I practise the calotype process, and which I can confidently recommend for certainty and success. I have only given an account of one process, although several others might have been mentioned, being anxious not to confuse or render the description unnecessarily complicated.

The adjustment of the chemical materials to each other is of such importance, that the greatest accuracy is required in their preparation. All the manipulations of the process also require the greatest care.

In conclusion, I trust that my description is sufficiently clear to be understood, and that it may be of use in forwarding the progress of this art, and that it will be followed by accounts of the experience of others.

An Enquiry into the Principles which regulate the Action of Sails and Rudders, with some Practical Suggestions. By Rev. JAMES BRODIE, Monimail, Fife.*

While the practical skill of the British seaman secures for him the foremost rank in his hazardous profession, and while the superior attainments of many of our navigators entitle them to a distinguished place among the cultivators of science, there are not a few questions connected with sailing vessels that have as yet received no fully satisfactory solution. Among these there is none more important than that which forms the subject of our present inquiry; and the author of the following remarks indulges the hope that its importance will plead his excuse for bringing it before the notice of the Society :

* Read 9th January 1854; Silver Medal awarded, 1854. VOL. IV.

Y

Propelling and overturning Effects of the Wind.

The wind exerts in every case a double influence on sailing vessels; it impels them through the water, and it has a tendency, greater or less according to circumstances, to over

turn them.

When a vessel is running right before the wind, these two influences act in the same direction; the one urging it onward in its course, the other tending to depress its head in the water. The latter, however, is in this case so small in proportion to the various forces that tend to preserve the vessel's equilibrium, that no practical danger in ordinary circumstances can ensue from it.

When a vessel is sailing with the wind on the side, the action of the two forces becomes much more distinctly marked, and much more deserving of investigation. In consequence of the oblique position in which the sails are set, one portion of the pressure of the wind acts as an overturning force, the other becomes an impelling one. The buoyancy of the leeward side of the ship, and the weight of the windward side, are the forces that preserve its equilibrium, and, as these are comparatively small, the danger of overturning is much greater than in the former case.

One of the most important problems connected with our inquiry is that which determines the most advantageous angle of inclination for the sail. When the vessel is at rest, the position of the sail in which the impelling force of the wind is greatest is that which forms an angle of about 30° with the line of the keel. In this case, about one-third of the power of the wind becomes, theoretically speaking, a propelling force, the remainder tends to overturn the vessel. When the angle approaches more nearly to a right angle, a portion of the impetus of the wind is lost; when the angle is more acute, and the sail is made to lie more nearly in the line of the vessel's course, a still greater evil arises, for, while the impelling force is diminished, the overturning force is increased.

Another circumstance remains to be taken into account. While the angle of 30° is the most advantageous when the

vessel is at rest, it does not continue to be so when the vessel is in motion. In order to produce the greatest impelling power, the angle of the sail's inclination must vary according to the apparent angle which the wind makes with the vessel's direction, or, in other words, according to the comparative speed of the vessel's motion. If a ship, for example, be moving through the water as rapidly as the wind is blowing, the sail slips, as it were, from below the compressing column of air, so that one-half of the impelling impetus is lost. To determine accurately the degree of inclination that should in all cases be given is impossible, but the following rule accords with the results of experience, and is deserving of attention :-The more rapidly a vessel sails, in proportion to the velocity of the wind, in order to produce the greatest impelling force, the more acute should the angle be which the sail makes with the line of the vessel's motion, and vice versa.

This is the rule which experience has taught the mechanist to adopt in weathering, as it is called, the sails of windmills. Near the centre, where the motion of the arms is comparatively slow, the sail forms with the plane of its motion an angle of 20° or 30°; but the angle is gradually diminished towards the extremity, where the rapidity of the motion is great, and the angle is proportionally small, varying from 6° to 10°.

We must here recall to mind the remarks we made on the overturning and impulsive effects of a side wind. If the sails are set at too great an angle with the line of the vessel, there is simply a loss of impulsive power; if set at an angle too acute, there is at once a loss of impulse and an increase of the overturning force. Keeping this fact before us, and applying the rule which we have given, it appears evident, that if anything occurs to impede a vessel's progress, the sail should immediately be slackened, that is, placed more nearly at right angles to the line of the vessel's motion. A tightness of sail, which is at once safe and advantageous when the vessel is moving freely, may involve both danger and loss of speed when the rate of its motion is diminished. A melancholy illustration of this remark occurred some time ago in the Firth of Forth. Some fishermen, coming home

wards with their sails set, and the wind abeam, picked up a mast which had floated away from a wreek; they fastened it to the stern of their boat, and proceeded with their sails set as before; but the resistance caused by the mast they were towing behind them so much diminished the speed of their boat, and, consequently, increased so much the overturning power of the wind, that their bark was upset, and several of the crew were drowned.

(The remaining remarks on the Action of the Wind on Sails, with the diagrams illustrating them, are omitted, the author's inquiry into the subject being as yet incomplete.)

Principles on which the Action of Sails and Rudders depends in turning a Vessel round.

The power which the helm possesses is the effect of the water striking obliquely upon it as the ship moves along. This produces a pressure which, acting at right angles to the plane of the helm, pushes the stern to a side, and turns the vessel round. The action of the helm is entirely dependent on the momentum of the ship. When the ship is at rest, or merely moving with the current, the helm is useless. This fact is familiarly known, as is the effect produced on the helm by an increase of the vessel's velocity. The effect of the vessel's weight, including, of course, both ship and cargo, is not so generally adverted to. It is evident, however, that it forms a very important element in our calculations; for the greater the weight the greater will be the momentum; other circumstances being the same, the greater, consequently, will be the power of the helm, more especially in tacking, in which operation the impetus of the wind on the sails is very speedily lost. A ship of war, therefore, with guns and ammunition on board, or a trading vessel deeply laden, will answer the helm much better than a vessel in ballast, or one that is lightly loaded.

In considering the action of the rudder, the form of the hull must also be taken into account. The ease with which any floating body can be turned, depends principally on its shape. The longer and sharper a vessel is, the more difficult it is to turn. An old-fashioned ship, with round bows,