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THE DEFENCE OF ACRE.

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LESSON LXXXIX.

MARCH THE THIRTIETH.

The Defence of Acre.

On this day, in 1799, the gallant Sir Sidney Smith repulsed Buonaparte in a violent attack on the fort of St. John d'Acre.

In order to oppose and prevent the designs of the French on Egypt, which country they were desirous of colonising, the British government entered into an alliance with the Sublime Porte, and a plan was concerted betwixt them, the chief preparations for the accomplishment of which were made in Syria, under the superintendence of the Pacha Djezzar. An army from Asia Minor was to make an attack upon the frontiers of Egypt towards Syria, while its operations were to be favoured by making a powerful diversion towards the mouths of the Nile, as well as by different assaults to be made in Upper Egypt, with the remains of Mourad Bey's army.

Sir Sidney Smith sailed from Portsmouth to direct the execution of this extensive plan, and to co-operate, as much as possible, towards its success with the maritime force under his command. Care was taken in the meantime to blockade the harbour of Alexandria with four ships of the line and five frigates, under the command of Commodore Hood, who, without the assistance of a land force sufficient to attack Alexandria, found it impracticable to burn or destroy the French fleet of transports. The report that the French vessels in the old port were burned he also found to be groundless, and he had made no use of the light vessels sent him by the combined fleet of Turks and Russians.

Buonaparte, understanding what was going on, quickly formed the design of leaving Egypt, and of marching into Syria, for the purpose of destroying the preparations of the Pacha Djezzar, and of disconcerting the plan of Sir Sidney Smith; but the result of this enterprise proved the reverse of the French hero's expectations. Jaffa, the ancient Joppa, did not surrender till it made an obstinate defence, and even then it was only to the superiority of European tactics. From Jaffa the hitherto triumphant general marched his army in three divisions against St. John d'Acre, but here he was obliged to stop; for the Pacha, encouraged and supported by Sir Sidney Smith, baffled all his attempts upon the place during a siege of about two months; and after the loss of nearly the half

of his army, he was forced to return to Egypt. Perhaps the ultimate cause of Buonaparte's mortification at Acre was the interception of his heavy artillery by the British, on their way from Damietta and Rosetta.

1. What British officer repulsed Buonaparte on this day, in 1799, at the siege of St. John d'Acre ?

2. With whom did the British government enter into an alliance; and for what object?

3. What did Sir Sidney Smith sail from Portsmouth to direct ?

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The Elements of Hydrostatics.

HYDROSTATICS is the science of weighing fluids, or of weighing solid bodies in fluids: it also treats of the nature, pressure, and motion of fluids in general.

A fluid is a body whose parts yield to any impressed force, and, in yielding, are easily moved among one another; and the less force that is required to move them, the more perfect is that fluid.

All fluids have not, like water, and many other liquids, the peculiar quality of wetting the hands when dipped into them. The atmospheric air, for instance, is a fluid, the parts of which give way to the smallest force impressed, but it does not stick to the bodies surrounded by it like water. Quicksilver is a fluid, but it will not adhere to the finger when plunged into it, though it will to tin, gold, and several other metals.

In natural philosophy there is a distinction between fluids and liquids. Air, quicksilver, and melted metals, are fluids, but not liquids; whereas, water, milk, beer, wine, oil, spirits, &c., are both fluids and liquids.

Liquids are generally known by the property of wetting the hand, or of sticking to any solid body, if immersed in it.

As the particles of which fluids are composed have never yet been discovered by the best constructed microscopes, philosophers conceive they must be exceedingly small. And they contend, that these particles are round and smooth, from the facility with which they are moved among and over one another. Now if they are round, it follows that there must be vacant spaces between them.

Imagine, then, a number of cannon-balls (48-pounders, for instance) to be placed in a large tub, or any other vessel, so as to fill it to the edge: notwithstanding the

THE ELEMENTS OF HYDROSTATICS.

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vessel would hold no more balls of this magnitude, yet a great many shot of a smaller size might be put in the vacant spaces; and in the interstices of these, bullets, still smaller, might be introduced. Into the vessel, now filled with balls, shot, and bullets of various dimensions, may be shaken a large quantity of sand; and when it will contain no more sand, several gallons of water may yet be poured into the vessel, which fluid will readily insinuate itself between the particles of sand, and into all the other vacant spaces, and thus fill the tub completely to the brim.

The air is called a fluid, because it flows like a fluid, presses in every direction like a fluid, and because light substances will float in it. The air is also known to be a fluid, by the easy conveyance which it affords to sound. In chemical laboratories a vacuum is often formed in the retorts and other glass vessels, which occasions explosions, and sometimes dreadful accidents. These are frequently produced by a torrent of cold atmospheric .air rushing into these vessels; and if the air did not possess the common properties of fluids, these effects could not take place. It is a property of fluids to press in all directions, upwards as well as downwards; so does atmospheric air.

All fluids upon which solids float are specifically heavier than those solids. This is an established law of nature: thus, a cork swims upon water, while a stone sinks in it; because the one is lighter, and the other heavier, than a portion of water of its own bulk. It is on this principle that ships and other vessels are constructed, and it is this property of fluids which enables us to float a vessel from one country to another. The weight of goods in a vessel is indicated by the depth to which the vessel sinks in the water. A ship will have a different draught in salt and fresh water, owing to the different specific gravity of the two fluids. To nautical men a correct knowledge of this is of the greatest importance, for should a captain load his vessel with a full cargo at any sea-port, his vessel would inevitably sink when brought into the Thames.

1. Of what does the science of hydrostatics treat?

2. How do you describe a fluid body?

3. What is the distinction between fluids and liquids ?

4. Why is the air called a fluid?

5. Upon what principle is it that ships and other vessels are constructed?

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ON THE CIRCULATION OF THE BLOOD.

LESSON XCI.

APRIL THE FIRST.

127

On the Circulation and other Properties of the Blood. IN Dr. Paley's Natural Theology is given a curious and interesting account of what may be termed the mechanical apparatus for elaborating the blood in the body of every human being. "There is provided," says this able writer, "in the central part of the body a hollow muscle, called the heart, which is invested with spiral tubes, running in both directions. By the contraction of these fibres, the sides of the muscular cavities are necessarily squeezed together so as to force out from them any fluid which they may at that time contain: by the relaxation of the same fibres, the cavities are in their turn dilated; and, of course, prepared to admit every fluid which may be poured into them. Into these cavities are inserted the great trunks, both of the arteries which carry out the blood, and of the veins which bring it back.

"This is a general account of the apparatus: and the simplest idea of its action is, that by each contraction a portion of blood is forced as by a syringe into the arteries; and at each dilatation an equal portion is received from the veins. This produces, at each pulse, a motion and change in the mass of blood to the amount of what the cavity contains, which in a full-grown human heart is about an ounce, or two table-spoons full. Each ventricle will at least contain one ounce of blood. The heart contracts 4000 times in one hour; from which it follows that there pass through the heart every hour 4000 ounces, or 350 lbs. of blood, troy weight. Now the whole mass of blood is about 25 pounds; so that a quantity of blood equal to the whole blood within the body passes through the heart fourteen times in one hour; which is about once every four minutes." Only consider what this is in very large animals. According to Dr. Hunter, ten or fifteen gallons of blood are thrown out of the heart of a whale at a stroke, with an immense velocity, through a tube of a foot diameter. The whole idea fills the mind with wonder.

It has been shown, by a variety of experiments, that the blood perpetually receives oxygen gas from the atmosphere, by the agency of the lungs. The blood is purple when it arrives at the lungs; but it there imbibes the vital air of the atmosphere, which changes its dark colour to a brilliant red, rendering it the spur to the action of the heart and arteries; the source of animal heat; and

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