have seen its idea reduced to fact in one of our own naval departments. Practical knowledge and seaman-like experience preside over one of our most important Boards; and the modern ships which grace our navy are the best illustrations of the soundness of the remark of the distinguished architect whose views I have cited.

CHAP. III.

WINDS.

THE Composition and extent of atmospheric air, its nature as far as regards subjection to the law of gravity, the absence of cohesive attraction among its particles, and their equality of pressure at every point, have already been noticed. We have now to consider its weight, compressibility, elasticity, mobility, and the different. purposes to which these properties are applied in connection with naval architecture. Air is of different degrees of density throughout; a cubic foot weighing on the average 1.2 oz. The particles of the lower strata, yielding to the pressure of those above, become so much reduced in bulk that there is usually as much air within 3 miles of the earth as there is altogether beyond that altitude (Fig. 5.); and the whole body exerts a pressure of 15 pounds on every square inch of the globe's surface, equal to 15 tons on the human frame. Air expands in proportion to the diminution of pressure, and when entirely extracted from a close vessel, a vacuum is formed. To compress air into twice its density requires a force of 15 lbs. 3 oz., into four times its density, 45 lbs. 9 oz., and so for other diminutions less one. In a thoroughly exhausted receiver animals soon expire, vegetation and combustion cease, gunpowder will not explode, smoke descends, bodies of different densities fall with equal rapidity, water and other fluids turn to vapeurs, sound is indistinctly heard, and heat imperfectly transmitted. The nature of a vacuum, the elasticity of air, and the force of atmospheric pressure will readily be understood by alternately closing and opening the vent during the common operation of washing out a gun barrel.

Air conveys, but is a bad conductor of, heat. The solar rays pass through it, and their heat is radiated from those bodies (as earth and sea) on which they fall. Hence, the lower strata are first heated, and, being thus rarefied, necessarily ascend, their place being supplied by colder ones. Some quarters of the globe as the equatorial regions - receive more heat than others. Land derives more heat from the sun during the day than sea, and cools more rapidly during the night. Currents of air are thus created, and this is the chief cause of such periodic winds as trade winds, monsoons, land and sea breezes. Sir Humphry Davy says, "the immediate cause of the phenomena of heat is motion, and its laws are precisely the same as those of the communication of motion:" and another writer remarks that "every body moved in a right line and continually reacted upon, is necessarily turned into a circle; every atom projected with velocity into a medium of atoms, is necessarily turned into an orbit, greater or less as the force of the projection, or the intensity of the atomic motion called heat. When reduced in bulk, the orbits are reduced, and their motion or heat imparted to other bodies around."

This rotatory motion, gathering force from opposition, from collision with currents of different densities and gravities, or from the results of rarefaction, is presumed to be the leading cause of whirlwinds and hurricanes. In application of the subject of hurricanes to the management of vessels, Colonel Reid recommends that a ship caught on the right side of a hurricane be put on the starboard tack, and when caught on the left side, on the port tack, as the wind will gradually draw aft, and the danger of the ship being taken aback by its heading will be thus avoided. Admiral Fitzroy also supplies a concise rule for avoiding the centre or strongest part of a hurricane, cyclone, typhoon, tornado, or circling storm. Thus:

"With your face toward the wind, in north latitude, the centre of the circling storm will be square to your right. In south latitude, square to your left.

"The apparent veering of the wind, and the approach or retreat of the dangerous centre, depend on your position in the circular whirl or sweep.

"Draw a circle; mark the direction of the rotation by an arrow with the head towards the left hand (against the movements of

a watch's hands) in north latitude; but towards the right (or with the hands of a watch) if in south latitude. The direction of the wind, and the bearing of the centre, show your position in the meteor, for such it is, though perhaps hundreds of miles in diameter; and the veering of the wind, or the contrary, and its change in strength, will show how the meteor is moving bodily -over a region of the world, revolving horizontally-or inclined at a certain angle with the horizontal plane.

"If the observer be stationary, in north latitude, and the centre pass on his polar side, he will experience a change of wind from southward by west towards north; but if it pass between him and the equator, the change will be from southward by east towards north, but otherwise in south latitude, as his place in circles sketched will show more clearly than words." It would carry us beyond the limits of this work to enter at greater length on the consideration of the important subject of the nature and laws of these revolving storms. Nor is it necessary to do so, as that subject will be found lucidly treated of in a short pamphlet published by order of the Lords Commissioners of the Admiralty*, and also in a work written by a distinguished American officer.†

The subject of the velocity of wind as demonstrated by its perpendicular pressure on the square foot, has come under the experiments of Mr. Smeaton. To him we are indebted for the following table, intended to show these relative proportions, and so to give an adequate conception of the force of air in motion. The first column shows its velocity, and the second its power of pressure.

The "Physical Geography of the Sea," by M. F. Maury, LL.D., Lieut. U. S. Navy. In this admirable book, the author has included a philosophical account not only of the winds, but also of the currents of the sea, its temperature and depths, the wonders that are hidden there, and the phenomena that display themselves at its surface; and this in a manner which must not

The principal use to the sailor of that well-known instrument the Barometer is to announce atmospheric changes, and so prognosticate storms. Many instances are on record of seamen having been, by its motions, warned of the approach of danger when there was little or nothing in the appearance of the sea or sky to excite alarm. It may therefore be instructive to devote a few pages to the explanation of its principles and construction; as well as quote some practical observations and rules respecting its use as a weather glass; premising that in, or rather at the approach of foul weather, the atmosphere is lighter, and in fair weather heavier.

The weight of the atmosphere may be ascertained by placing an exhausted tube in a vessel filled with fluid. The fluid being forced by the atmospheric pressure on its surface, will mount into the vacuum, till the weight of the air and that of the fluid in the tube balance each other; the fluid in the tube will then represent a column of air of the height of the atmosphere, and the diameter of the tube. Water, under the weight of air, will rise in a vacuum to a height of 34 feet. But mercury being 13 times heavier than water, will only rise part of that quan.

only be interesting to all, but especially instructive and useful to those who go down to the sea in ships. He has fairly

"Laid his hand upon the Ocean's mane,

And play'd familiar with his hoary locks."