enters into chemical combination with the atmosphere, we are compelled to admit (from its greater specific gravity) that the holds and store-rooms, &c. will become the receptacles of this pestiferous gas. But the evil does not end here; for fixed air furnishes a natural and luxurious nourishment for fungi; carbon being part of the food and the base of all the vegetable kingdom. Indeed, this is a process which Nature employs for the double purpose of norishing plants and renovating our atmosphere.1

The method I would recommend for ventilating ships is as follows. During the continuation of wind, and while there is considerable motion in a ship, the windsail now in use and Mr. Perkins' ventilator will answer the purpose most completely: but when neither wind nor motion is present, we must have recourse to manual strength and machinery to pump out the vitiated air; and the atmospheric pressure will supply its place with that of a cold and pure quality.

The windsail and Mr. Perkins' ventilator will be best suited to sea-going ships; while an engine for pumping out the foul air, (after the manner of ventilating the Session House at the Old Bailey,) will be better adapted for ships in ordinary and guard ships, where the rolling is inconsiderable.

Mr. Perkins' method of ventilating the hold of a ship will at once be understood, by referring to the plate-the Frontispiece.

A SECTION OF THE VENTILATOR.

a a, tanks, or water butts.

bb, hose for conducting the foul air into the tanks. cc, hose for conducting the foul air from the tanks. d, connecting water pipe.

e e, valves for admitting the foul air into the tanks. ff, valves for allowing the foul air to escape.

The operation of this self-acting ventilator is as follows: Each tank or butt is half filled with water, which flows freely from one to the other through the pipe d. The quantity of water running alternately from each depends upon the motion of the ship. When one of the tanks is elevated by the ship's motion, the water will run through the pipe d into the depressed tank, and thereby discharge as much foul air through the valve ƒ as the additional water displaces. The elevated tank at the same time is receiving

I once saw the hold of a ship so filled with fixed air, that a candle would not burn in it; and a dog, which was thrown in, instantly became convulsed. The hold of a ship may thus be rendered as hostile to the canine tribe as the famous " Grotto del Cane" in Italy.

the foul air through valve e, from the hold of the ship, to supply the vacuum that would otherwise be made by the escape of the water. If the tanks are fixed at right angles with the keel of the ship, the ventilator will operate only with the roll of it; but, if placed diagonally, both the pitch and roll of the vessel will discharge the foul air. It would be most economical to fill the tanks at the beginning of the voyage. The first water for the ship's use should be taken from the ventilating tanks, leaving, however, half of it behind for operation. If the remaining water should ever be wanted for the ship's use, it can be drawn off, and replaced by salt water. It will be seen that, by this mode of ventilating, nothing but the hose and valves are to be added to what must necessarily be on board every ship.

Any improvement in the arts generally becomes valuable in exact proportion to its strength and simplicity; for when an implement is complicated or easily deranged, it is only useful in the hands of the inventor and the scientific. Mr. Perkins' ventilator not only combines those properties in a most eminent degree, but, from its principle, works hardest when most wanted. In a gale of wind, when the hatchways are on, and when there is much straining and rolling in a ship, the noxious gases are then generated to the greatest extent; then, also, the operations of the ventilator become most powerful, both in admitting fresh, and expelling foul, air. If another ventilator be placed parallel with the keel of a ship, it would, on many occasions, be useful in forcing out the gases, when she is pitching deep at anchor. I will venture to say, if ships were built of well-seasoned timber, protected from the excess of washing decks, and ventilated after the principle here laid down, their decay would never be premature.

The application of heat, by means of stoves, to dry the holds and under-works of a ship, is as useless as kindling a fire in a parlor to dry the cellar. Besides, combustible heat is altogether a very doubtful remedy; for when a ship shall happen to have been built of wood containing part of its natural sap and moisture, heat, applied short of expelling them completely, will unquestionably do harm, by calling into vegetation the seeds of the fungi, which, under a lower range of temperature, would have remained in a quiescent state.

As the juices of the oak are known to abound with tannin and gallic acid, those individuals who imagine vegetation proceeds from the sap of the timber, have recommended it to be drenched in solutions of the alkalies and alkaline earths, in order to decompose them; and speak of the union of the acid with the alkali in the ligneous fibre with the same confidence as if it were in the mortar of an apothecary. For my own part, I would as soon believe soda

capable of absorbing the phosphoric acid from the human bones, on applying it to the skin, as expect it would neutralise the gallic acid in the heart of oak.

Farther, alkaline solutions, owing to their great affinity for water, would hasten that decay in timber which they are intended to prevent; moreover, men could not live in a ship so saturated.

All the benefit, therefore, to be derived from lime-water, solutions of glue, common salt, oil, &c. is to be obtained from common paint alone; so far as they are capable of closing the pores, and rendering the wood less pervious to heat and humidity, are they useful, and no farther.'

Of the Growth and Properties of Moss in preserving Timber.

Moss is produced by an accumulation of dead vegetables, preserved in a partially decayed state, by a steady range of low temperature. Upwards of three hundred different species of Moss have been enumerated by naturalists; but as flow-moss only is capable of preserving timber from decay, I shall confine myself chiefly to that species.

Flow-mosses are to be found, in the greatest perfection, in flat situations, at considerable altitudes from the sea, and where water cannot easily make its escape. In Great Britain they thrive best when exposed to, and fed by, the moist winds of the western ocean. Hence it is that more extensive mosses are to be found on the west than on the eastern coast of Scotland; for, when these occidental winds, loaded with humidity, come in contact with the cold mountainous districts of the west, but more particularly with the flow-mosses there, condensation of moisture instantly takes place; and, owing to the prevalence of such winds, the most abundant supply of food is thus furnished to the different species of plants indigenous to moss. After the death of one race of flow-moss plants, the medium, (moss) in which they had vegetated, maintains so low a range of temperature, that their elementary parts are not suffered to be dissipated in the air, by

• Moisture has another destructive influence on timber, viz. that of causing it to throw off its coat of paint. If a piece of timber is painted, after having been previously saturated with water, on the arrival of the first frost, the paint will be observed to scale off. This is owing to water obeying different laws from most other substances, or occupying a greater space in the frozen, than it did in the aqueous, state.

repulsive caloric, after the usual manner, but are arrested on the site of their germination: and thus, in process of time, mosses have accumulated from five to fifty feet in thickness.'

"Captain Duff, R. N. in a paper lately read before the Royal Society of London, after stating the well-known effects of peat moss in preserving wood for ages unaltered, suggests, that a series of experiments should be made to ascertain the effects of impregnating timber, both sound and already partially decayed by the dry rot, with the water from peat mosses, with a view to determine whether it possesses any power in preventing or suspending the insidious operation of that destructive agent."

The experiments here recommended to be tried by Captain Duff have been most extensively tried by Nature, in all the different varieties of moss, and have uniformly failed, except in deep flow mosses. For, neither in hill-moss, bent-moss, nor even in the edges of flow-mosses, (where it is of little depth) is timber ever found in a preserved state. But in deep flows, I have frequently seen the remains of stately trunks of fir trees in so high a state of preservation that part of them is often split by the country people, and made into a kind of rope.

The antiseptic qualities of flow-moss, therefore, is solely to be attributed to its maintaining an uniformly low temperature, and to the exclusion of atmospheric air by the interposition of several feet of wet moss, to the bottom of which solar heat never pene

A flow-moss, situated in the middle of a country, of perhaps thirty feet thick, ten miles long, and three or four miles in breadth, (there are several larger than this in Scotland and Ireland) is like an immense ice-berg floating in the ocean; it maintains a low temperature itself, and diminishes that of all surrounding bodies. During the continuation of westerly and southerly winds, such a huge mass of cold matter generally acts as a condenser; hence, we observe mist precipitating on the "mountain's brow," and fogs descending into the bosom of flows in such weather.

Under ordinary circumstances, condensation and evaporation are nearly equal on a given surface; but moss is of a more greedy nature; it takes more than it gives, and, in a great measure, preserves that which it had taken.

The action of moss on Boreal wind, on the other hand, is of a more negative kind, from such wind containing less moisture, and being nearer the heat of the flow, it seldom yields up much of its humidity.

On other occasions, when the atmosphere has a great capacity for moisture, the most exuberant and pernicious exhalations are carried off from these immense arsenals of humidity, chilling and deteriorating the ambient air to a great extent around. Indeed, when we consider the great quantity of water carried off from the equatorial regions, (where evaporation is excessive) to be eternally fixed near the Poles by congelation, as well as the immense reservoirs of water, arrested in all our accumulating mosses, we are enabled, in some measure, by those increasing receptacles, to account for the receding of the ocean.

trates, and where a thermometer, when buried, oscillates only from 35% of Fahr. in winter, to a few degrees above 40° in summer.' In such mosses the bodies of certain human beings shot by the military in the reign of Charles II. in the struggle to establish episcopacy in Scotland, have lately been found in a state of high preservation.

Some individuals have attributed the preservation of fir-trees to a large quantity of resin and turpentine found in their composition; but these cannot be the antiseptic agents, otherwise, timber would be found equally fresh in all the different varieties of moss, which is never the case.

I have observed, in some wretched hovels in the Highlands of Scotland, where the smoke was suffered to find its way out of the house by different apertures, that the wood which supported the heath or thatch covering, had resisted the efforts of all-destroying Time, accompanied by heat and humidity, to an amazing degree. The timbers, in such houses, are covered with a black oleaginous substance, arising from the diffusion of the smoke of the wet peat fuel. Now, as an impure pyroligneous acid, and empyreumatic oil, may be obtained by destructive distillation from peat fuel, as well as from the beech and birch, there is no doubt it is through the agency of these substances the wood is preserved.

Smoking timber, therefore, deserves a trial, and bids fairer than any of the topical applications to preserve it to a good old

age.

In these times of peace, when the growth of timber must be considerably more than its expenditure, it would be interesting to deposit a certain number of trees, annually, in some of our deep flow-mosses, for the double purpose of preserving them, and of ascertaining if wood, so saturated, possessed more durability than other timber afterwards.

On the principles laid down in this Essay, the driest and coldest haven ought to be selected for laying ships up in ordinary, in preference to a warm and moist one. Plymouth harbour, on account of the heat and humidity of the atmosphere, is undoubtedly the worst suited for this purpose of any in England.

To recapitulate-I feel convinced that I have made good the

' In chemical language, the cohesive attraction of all organised substances must first be overcome by caloric or insensible repulsion, before their constituent parts are suffered to enter into chemical affinities, or form new combinations. Now, owing to the great cold at the bottom of flow-mosses, the cohesive attraction of an animal or vegetable substance is never overcome there by heat; consequently, no putrefaction nor decomposition can rapidly follow.