is managed, without shuddering at the idea of the great risk to which all on board are at every moment exposed.

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Explosions are of very frequent occurrence; and, with a view to cure this evil, several attempts have, at different periods, been made to introduce low-pressure engines on the Western waters, but the cheapness of high-pressure engines, and the great simplicity of their parts, which require comparatively little fine finishing and good fitting, certainly afford reasons for preferring them to low-pressure engines, in a part of the country where good workmen are scarce, and where the value of labor and materials is very great. It must also be recollected, that a condensing or lowpressure engine takes up a great deal more space than one constructed on the high-pressure principle. I do not apprehend, however, that the number of accidents would be diminished by the simple adoption of low-pressure boilers, without the strict enforcement of judicious regulations; and if those regulations were properly applied to high-pressure boilers, they would not fail to render them, perhaps, quite as safe as those boilers which are generally made for engines working on the low-pressure principle. One very obvious improvement on the present hazardous state of the Mississippi navigation, would be the enactment of a law that the pressure of the steam should in no case exceed, perhaps, fifty pounds on the square inch.

"The steamers make many stoppages to take in goods and passengers, and also supplies of wood for fuel. The liberty which they take with their vessels on these occasions is somewhat amusing, and not a little hazardous. I had a good example of this on board of a large vessel called the Ontario. She was sheered close inshore among stones and stumps of trees, where she lay for some hours taking in goods. The additional weight increased her draught of water, and caused her to heel a good deal; and when her engines were put in motion, she actually crawled into the deep water on her paddle-wheels. The steam had been got up to an enormous pressure to enable her to get off, and the volumes of steam discharged from the escapement pipe at every half stroke of the piston made a sharp sound almost like the discharge of firearms, while every timber in the vessel seemed to tremble, and the whole structure actually groaned under the shocks.

"During these stoppages, it is necessary to keep up a proper supply of water to prevent explosion; and the manner in which this is effected on the Mississippi is very simple. The paddlewheel axle is so constructed, that the portions of it projecting over the hull of the vessel to which the wheels are fixed can be thrown out of gear at pleasure by means of a clutch on each side of the

vessel, which slides on the intermediate part of the axle, and is acted on by a lever. When the vessel is stopped, the paddle. wheels are simply thrown out of gear, and the engine continues to work. The necessary supply of water is thus pumped into the boiler during the whole time that the vessel may be at rest; and when she is required to get under weigh, the wheels are again thrown into gear, and revolve with the paddle-wheel shaft. The fly-wheel is useful in regulating the motion of the engine, which otherwise might be apt to suffer damage from the increase and diminution in the resistance offered to the motion of the pistons, by suddenly throwing the paddle-wheels into and out of gear. The water for the supply of the engine is first pumped into a heater, in which its temperature is raised, and is then injected into the boiler.

"I saw several vessels on the Ohio which were propelled by one large paddle-wheel placed at the stern of the vessel, but it is doubtful whether this arrangement is advantageous, as the action of the paddle-wheel, when placed in that situation, must be im. peded by the float-boards impinging on water which has been disturbed by the passage of the vessel through it.

"The third class of vessels to which I have alluded, are those which navigate the lakes and the river St. Lawrence. They differ very materially from those I have already described, being more like the steamers of this country, both in their construction and appearance. Steamboats were first used on the St. Lawrence in 1812, and it is probable that they were also introduced on the lakes about the same time. The lake steamers are strongly built vessels, furnished with masts and sails, and propelled by powerful engines, some of which act on the high-pressure and some on the low-pressure principle."

Simple Origin of Important Discoveries.

It is certain, says Pliny, that the most valuable discoveries have found their orign in the most trivial accidents. As some merchants were carrying nitre, they stopped near a river which issues from Mount Carmel, and not happening to find stones for resting their kettles, they substituted in their place some pieces of nitre, which the fire gradually dissolving, mixed with the sand, and occasioned a transparent matter to flow, which, in fact, was nothing else but glass.

It is said that the use of telescopes was first discovered by one

Hansen, a spectacle-maker, at Middleburgh, in Holland, whose children playing in the shop, casually placed a convex and concave glass in such a manner, that, by looking through them at the weathercock, they observed it appeared much larger and nearer than usual, and, by their expressions of surprise, excited the attention of their father, who soon obtained great credit for this useful discovery.

Heylin, in his cosmography, tells us that the art of steering was discovered by a man of the name of Typhis, who took his hints for making both the rudder and helm from seeing a kite, in flying, guide her whole body by her tail.

Invention of the Safety Lamp.

This lamp, by means of which hundreds of lives have been preserved, was invented in the autumn of 1815. Sir Humphry Davy, the inventor, was led to the consideration of this subject, by an application from Dr. Gray, now Bishop of Bristol, the chairman of a society established in 1813, at Bishop-Wearmouth, to consider and promote the means of preventing accidents by fire in coal-pits. Being then in Scotland, he visited the mines on his return southward, and was supplied with specimens of fire-damp, which, on reaching London, he proceeded to examine and analyze. He soon discovered that the carburetted hydrogen gas, called fire-damp by the miners, would not explode when mixed with less than six, or more than fourteen, times its volume of air; and further, that the explosive mixture could not be fired in tubes of small diameters and proportionate lengths. Gradually diminishing these, he arrived at the conclusion that a tissue of wire in which the meshes do not exceed a certain small diameter, which may be considered as the ultimate limit of a series of such tubes, is impervious to the inflamed air; and that a lamp covered with such tissue may be used with perfect safety, even in an explosive mixture which takes fire and burns within the cage, securely cut off from the power of doing harm. Thus, when the atmosphere is so impure that the flame of a lamp itself cannot be maintained, the Davy still supplies light to the miner, and turns his worst enemy into an obedient servant. This invention, the certain source of large profit, he presented with characteristic liberality to the public. The words are preserved in which, when pressed to secure to himself the benefit of a patent, he declined to do so, in conformity with the high-minded resolution which he formed, upon acquiring independent wealth, of never

making his scientific eminence subservient to gain. "I have enough for all my views and purposes; more wealth might be troublesome, and distract my attention from those pursuits in which I delight. More wealth could not increase my fame or happiness. It might undoubtedly enable me to put four horses to my carriage; but what would it avail me to have it said that Sir Humphry drives his carriage and four?”

Like most individuals of worth, Davy was a man of true modesty and in his dress and manners very simple. Volta, to whom he was introduced at Pavia, had attired himself in full dress to receive him, but is said to have started back with astonishment, on seeing the English philosopher make his appearance in a dress of which an English artisan would have been ashamed. The following anecdote is told of him: whilst staying for the night at a small inn in North Wales, with his friend Mr. Purkis, a third traveller entered into conversation with both, and, as happened, talked very learnedly about oxygen and hydrogen, and other matters relative to chemical science. When Davy, who had listened with great composure, had retired to rest, Mr. Purkis asked the stranger, what he thought of his friend who had just left him. "He appears," coolly replied the other, "rather a clever young man, with some general scientific knowledge-pray what is his name?" "Humphry Davy, of the Royal Institution," coolly replied the other. "Good heavens !" exclaimed the stranger, was that really Davy?-how have I exposed my ignorance and presumption!"

The Thames Tunnel.

As far back as the year 1802, a project was set on foot by some enterprising gentlemen, with a view of opening an archway under the Thames, between Rotherhithe and Limehouse, not far from the line of the present tunnel. The engineer selected for this enterprise was particularly qualified for such an undertaking, being an experienced Cornish miner. Having made some borings at the Horse-ferry and on the opposite side of the river, he reported that "he was firmly persuaded the undertaking would not cost so much as had been conceived." A subscription was, in consequence, raised; and a company was formed, under the denomination of the "Thames Archway Company." Surveys, plans, and estimates were made, and an act of parliament being obtained, the work was begun. The engineer commenced opera

tions by sinking a shaft of eleven feet diameter, at three hundred and thirty feet from the line of the wharf on the Rotherhithe side But the obstacles which he encountered from the nature of the ground increased to such a degree, as he proceeded, that at the depth of forty-two feet he was obliged to desist. A subsequent report of borings, however, having proved very favorable, an enterprising proprietor engaged to complete the shaft (reduced to eight feet diameter) to seventy-six feet, at which depth it was dis covered that it would be dangerous to go deeper. At this stage of the proceedings, viz., in August, 1807, a second engineer was engaged by the company, a gentleman whose name had been coupled with very great enterprises in the mining department. Before opening the drift-way, both engineers agreed to reduce its breadth to two feet six inches at the top, and three feet at the bottom. At the depth of seventy-six feet they found the ground to consist of a firm dry sand; and there they opened the drift, which they carried forward in a gentle ascent. In November, 1807, when three hundred and ninety-four feet of the drift had been completed, the services of the first engineer were dispensed with, after four years and a half of hard labor. The directors then agreed to give the second engineer £1,000, by way of premium, if he succeeded in reaching the opposite shore. The drift was further extended to eight hundred and fourteen feet, through equally firm dry ground, with the precaution, which had been employed from the beginning, of a substantial planking all the way. One hundred and thirty-eight feet more were cut through a bed, of calcareous rock eight feet thick. But on the 21st of December, the head of the drift had hardly entered two feet into the stratum, which lay immediately over the rock, when the roof broke down in a loose state, leaving above head a cavity large enough for a man to stand in it. It is to be observed that there was no less than thirty feet of intervening ground between the drift and the river at the time this accident happened. The engineer succeeded in filling and securing the cavity; but, such was the nature of the whole ground above the rock, that, under the influence of an extraordinary high tide, (on the 26th of January, 1808,) the ground again made its way fast in a loose state into the drift, and the river soon broke through twenty-five feet of ground. This same tide caused the destruction of the Deptford and Lewisham bridges. The engineer having succeeded in filling and closing this hole, the miners re-entered the drift, which was reduced to three feet in height, for the purpose of clearing the dangerous place. The miners had therefore, to work on their knees: however, notwith