air rises, and forms those two upper currents which rush towards the poles, above and counter to the Trade-winds. We are told that the so-called African dust is really south American, and that much more rain falls in the Northern than in the Southern hemisphere, from which it is argued that the air which formed the S.E. Trades, having traversed more sea and picked up more moisture, rises in these doldrums, and travels to the N.E. above the N.E. Trades; and, vice versâ, that the N.E. Trades travel to the S.E. above the S.E. Trades. Maury does not say how they pass each other, neither can we, but we have strong evidence of a current of air travelling above and in opposite direction to the Trade-winds, because we generally see the high clouds travelling in that direction.

We have, however, as it were, even seen the air ascending; for on the 15th of March, 1865, in 4° 18′ N. lat. and 20° 33′ W. long., when we had light fleecy clouds passing over us from the N.E., and we lay becalmed and roasting, longing for the Trades, my chief officer came and reported to me with a hopeful countenance that he had seen these light fleecy clouds travelling from the N.E. None but those who have experienced these calms can imagine how anxiously wind is looked for: to the N.E. of us there was an arch of clouds in the sky extending from the S.E. to the N.W. points of the horizon, with a calm and low barometer on the south side of it where we were, and (as we found afterwards) the N.E. Trade and a higher barometer on its North side; therefore the arch of clouds was probably formed by the condensation of moisture as the air rose, while we lay becalmed at the foot of the inclined plane of still air, up which the N.E. Trade was just commencing its ascent.

Travelling South across the equator, the barometer rises uniformly until we arrive at the Southern limit of the S.E. Trades; but in February the barometer ranged much lower than in August. The homeward route through the Atlantic differing much from the outward, does this difference of barometer arise from difference of seasons or difference of longitude? By comparing our routes near the equator, where they come very close to each other, and where the difference of height in the quicksilver is as great as in any other part, I am led to think that it depends upon the different seasons.

Whilst speaking of the homeward route, it is interesting to remark how on leaving the Cape of Good Hope we invariably bad a valley, as it were, in the atmosphere, which quickly rose as we sailed to the N.W., even though we may have started in a South-easter, which is the high-barometer wind in those latitudes. I had noticed that after rounding Cape Agulhas with a South-easter and high barometer, the column fell suddenly after rounding the Cape of Good Hope, though the S.E. wind continued; and I suppose that the fall is caused by the ascending air as it comes in contact with the high land.

At both seasons of the year there is a heaping-up of the air at the polar end of each Trade, in the place where Maury tells us that two upper currents come to the surface of the earth. That which we have already alluded to, as coming from the equator towards the pole,

moving above the Trades, the clouds, proving its existence; the other, Maury tells us, rises at the pole and travels as an upper current, above the strong westerly winds which prevail in high latitudes, towards the equator; it can hardly be expected to have many clouds, he says, as its moisture must have been condensed by cold before rising at the pole, so that it becomes cold dry air.

We may ask what evidence can the sailor give for this theory to be deduced from his observation.

First, then, from these heaps of air he finds two surface winds blowing in opposite directions: the one moving towards the equator is cool, dry, and heavy, the other moving towards the pole is warm, damp, and light. He may well say, if two surface winds blow in opposite directions from this heap of air, there must be air brought to it by an upper current or currents to keep up the heaping; but he may naturally ask, how do I know that an upper current comes from the pole? First, because the prevailing surface winds in high latitudes blow towards the pole, which air must return; and, secondly, because the Trade-winds are composed of cool dry air, which could not have come from the equator: here there is pretty good evidence that two upper currents come to the surface of the earth in these zones where the air is heaped up; and again, that in dipping to the surface by some unknown means they cross each other, as Maury conjectured.

A few words now as to the manner in which the westerly winds which blow in high latitudes appear to draw the air from the heaps above mentioned. Here we will refer to our experience in 40° S. lat., where the normal circulation of the air is less interferred with by the land.

This parallel of latitude is subject to a series of gales which commence at North and end at N.W. or West. As the North wind sets in, the barometer falls, the air becomes warm, damp, and cloudy; the wind gradually draws round to the N.W., after a time rain accompanies the wind, the barometer continues to fall, often fast, until in a heavy shower of rain the wind shifts to the West, when the barometer immediately rises, generally followed by a strong breeze from the westward, which decreases as the quicksilver rises, very often settling down into a calm. After a few hours the North wind sets in again, with a falling barometer, and a repetition of the whole series takes place.

One is naturally led to ask why the Trade wind draws air from this heap in a regular continuous stream, when these gales are fitful. May it not be because in the direction in which the Trade moves the meridians diverge and give plenty of room for the flow, whereas the westerly winds have converging meridians which seem to check the progress of the air. These fitful gales have always led me to think that the air was checked in its course. If further South, say in 50° S. lat., the wind continues steady from the West, then this zone of 40° seems to act as a reservoir for the westerly winds, being constantly refilled and steadily drawn off, only the stream into the reser voir is freer than that which runs out.

NO. 1.-VOL. XXXV.

E

Now if we consider that these gales are composed of the warm damp air which comes to this heap from the equator above the S.E. Trades descending to the surface of the earth and travelling towards the pole, their Westing is accounted for by the change in the diameter of the circular route which the air has to describe in accompanying the earth in its revolution. These gales changing from North to N.W. and West have been treated as the N.E. quarters of Southern hemisphere cyclones; and we read in the Nautical Magazine of a ship having hove to to allow one of them to pass; but if, as we suppose, they form part of the normal circulation of the air, it seems useless to heave to to avoid them. The source of these gales being to the north of them is a sufficient reason why the wind does not change to South of West. The polar-wind gales which are experienced in these high latitudes, seem to derive their air from that upper current returning from the pole, part of which sometimes makes its downward way to the surface in high latitudes, especially in spring.

The gales of the Southern hemisphere, have their exact counterpart in the high latitudes of the Northern hemisphere, though I have not noticed them to be so constant, perhaps on account of there being much more land in the Northern hemisphere. Still all seamen know how, after getting North of the N.E. Trades, we look for the wind to come from South, S. W. and West, with warm air and rain.

The arguments deduced from my observations, seem to favour Maury's theory of the circulation of the air. Where he supposes two rising currents we have a low barometer, and where he supposes two descending currents we find a high barometer. But they are also suggestive. A series made with standard instruments for each month in the year might lead to most useful discoveries as to the normal circulation, and its disturbance by the effect of land.

My observations prove the uniform state of the atmosphere in those parts of the Atlantic between the Trades, at the same seasons of the year! especially in contrast with their sudden distortions on the polar side of the Trades, where their irregularities resemble the waves of the sea in the same latitudes, which may in fact be called the resultants of these distortions. Similar remarks outward and homeward, deduced from the same logs, between the latitudes of 40° South to 20° North, in the Indian Ocean and Bay of Bengal, would, I think, give interesting results, and I hope some day to work at them.

ANTI-FOULING COMPOSITIONS FOR SHIP'S BOTTOMS.

The protection of ships from the effects of fouling, in fact of keeping the part immersed in the sea free from accumulations of weed, would seem to be still a desideratum were we to form our opinion from cer

tain reports by dockyard authorities from their experience of these matters, to which we have given place lately in these pages. But if we look about us for further experience on these matters, we find after all that the subject has been set at rest, as far as concerns the obtaining a surface that does resist the weed, but which, owing to that severe friction, as well as deteriorating effects to which it is always exposed, requires a periodical renewal extending only over a year. Whether the bottom of the ship be coated with copper or whether it be merely iron, the effect of the salt water on it, and especially in warm climates, is such as to demand this periodical examination and renewing. The copper, it is well known, becomes gradually eaten away, and the iron goes even faster, accumulating weed that is compared even with long grass. But to prevent this injurious condition of affairs several remedies have been proposed, and the last we have heard of is by Captain Coles, R.N., who contrives to obtain a facing of absolute stone over the bottom of a ship, and the mode in which this is effected is thus described in one of our daily journals:

"One of the iron mortar boats in the Portsmouth reserve has been lying afloat in that harbour for some months, coated below her water-line with preservative and anti-fouling compositions, the preparations of various persons who have come forward to assist the Admiralty in devising a safe method of preserving and keeping clean the bottoms of iron ships. This mortar boat has now been brought down Portsmouth harbour and placed in dock, where the different compositions have been surveyed by the officials of the yard and reported upon.

"Some of them appear to have stood very well, as they generally do where no abrasion has taken place, but the anti-fouling pigments, with one exception, have utterly failed in preventing the growth of marine matter. The one instance alluded to is a preparation of mercury.

"There is one preservative composition, however, that from its novelty and the well known name and reputation of the officer who has suggested its use deserves more than the passing notice we have given to the others. This is nothing more than a coating of Portland cement, the use of which and the mode of its application were suggested to the Admiralty by Captain Cowper P. Coles, R.N. The patch of cement has been on the mortar boat's bottom upwards of twelve months, and has been subjected to considerable rough usage, yet it retains its hold on the iron, except round the edges of the frame, as firmly as ever, and remains as hard as a block of granite. Whether a coating of cement half an inch in thickness over a vessel's bottom could be applied without producing a damaging effect upon her speed, by increasing the displacement, is a distinct question, which must be judged by itself, our present purpose being to explain how Captain Coles proposes to preserve the bottoms of our iron ships by turning them into stone.

"Thin iron plates, the thickness of an ordinary sheet of copper, are taken, and a series of cuts punched in them, each cut forming three sides of a parallelogram about half an inch in length and two inches apart. These plates are next screwed on the ship's bottom by common iron screws tapped in threeeighths of an inch, the cuts in the plates being then bent outwards by a chisel to an angle of about forty-five degrees. Covered with these plates the bottom of the ship has reached its "hedge-hog back" form, and to enter on its final stone state the Roman cement has merely to be applied over all in the manner followed in buildings on shore.

"It is not attempted by Captain Coles to prevent fouling on the outer sur

face of the cement; that he leaves to the care of the chymists. What he professes to do is to preserve the iron of the ship's bottom from oxidation and make the cement coating serve as an intermediary between the ship's bottom and any sheet-copper or anti-fouling mixtures that might be applied to keep the ship under the water-line free from marine adhesions of any kind that can affect the speed of the ship.

The advantages that may be quoted in favour of the cement are-1, rapidity of application when a ship's bottom has once been prepared; 2, the possibility of replastering when a ship's bottom has once been cemented over any portion damaged, by striking the ground, or by shot, without displacing any other portion; 3, the rapidity with which the cement sets after being applied; 4, the fact that the cement can be "hogged," scraped, or otherwise cleaned without fear of its being rubbed off; 5, its efficacy in protecting the armourplates on wooden ships from galvanic action, by its being a perfect insulator between the iron plating of the ship and the copper or Muntz metal on her bottom; and 6, increased strength to the bottom of the ship and protection to it under certain conditions against the action of torpedoes. Against the advantages may be placed the disadvantages of increased displacement and weight of hull, and the possible effect this would produce upon the ship's speed.

This process may be looked on then as another answer in favour of the effects of fouling being absolutely prevented. The arguments of additional weight, durability, facility of renewal, and the question of economy will be only decided with the assistance of experience. But Captain Coles has contributed his views towards removing the difficulty as he has already done to the snbject of using the heaviest cannon that can be employed in ships afloat by means of his turret principle turned to account elsewhere in the Monitor system.

Turning to other modes which have been in course of application, we find the subject has been frequently discussed at the meetings of thr United Service Institution in London, where Admiral Sir Edward Belcher showed that the smoothest possible surface was no protection against fouling, and hence the proposal in the way of glass, alluded to in a former number of this journal, was considered as hopeless. But we find in a Brochure, published by Messrs. Peacock and Buchan, the following experiments were made a few years ago :—

"No sooner were the voyages of iron stramers extended to latitudes where the fouling of the bottom takes place so rapidly as to give rise to serious inconvenience, than the public ingenuity was stimulated to the device of measures calculated to remedy the evil. Most of the resulting projects, as usually happens in such cases, were failures; but to test the relative merits of the various compositions which had been suggested, the Peninsular and Oriental Steam Company directed one of its steamers, the Ripon, to be painted over in spots with the whole of the different compositions which had been suggested as a remedy, with a view of affording a fair comparison of their respective qualities. The following are the names of the parties who applied their compositions to the bottom of the Ripon, on the 27th of January, 1848:-Lees, Moresby, Clark, Ince and North, Hayes, Chanter or Weddersted, Grantham, and Parker. After running the usual time, the vessel was taken into dock and examined, when the following appearances were presented:

"Lees' composition: as foul as red lead, hard when scraped, but there was corrosion in spots.