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and condenses the air on its surface. The pistons are fixed to opposite ends of a lever, worked by men, so that while one is ascending the other is descending, and the water is continually pressed into the air-barrel. Through the top of the air-vessel a pipe passes down to the water in it, and the water by the spring of the air on its surface, is forced up through this pipe, and by means of proper joints in it, is thrown in any direction, at pleasure.

There is also another fire-engine, so called from its being worked by fire, for the purpose of working a number of common pumps, to raise water out of mines, &c. The immediate cause of motion in this machine is the spring of elastic vapour, and the weight of the atmosphere, acting alternately. The machine is thus constructed. Over the mouth of a large copper boiler is fixed a large hollow cylinder, in which a piston plays, without any valve. This piston is fixed to one end of a large lever, while a number of pistons of common pumps is fixed at the other end of it. The piston over the boiler is raised by the force of the elastic steam rising from the boiler, as soon as its elasticity is more than sufficient to counterbalance the weight of the atmosphere above it, and the pistons of the pumps descend. But when the piston over the boiler rises to a sufficient height, it opens a cock to let into the cylinder a small jet of cold water, to convert the vapour into water again; and then the weight of the atmosphere presses the piston over the boiler down again, while the other end of the lever raises the pistons of the pumps.*

* Many important improvements have of late years been made in the fire or steam-engine; particularly by Messrs. Boulton and

OF THE TIDES.

ANOTHER Cause of motion in water is the attractive force of the sun and moon, producing that alternate swell and depression of the surface of the sea and rivers, which we call the tides of flood and of ebb, and which are next to be explained.

The tides principally depend on the attraction of the moon, though in some measure upon the attraction of the sun. The waters of the ocean being attracted by the moon, with a force, which is inversely as the squares of the distances from the moon, they must have their gravity towards the earth in some measure diminished on the side next to the moon; and as this diminution of gravity must be greatest on the side next to the moon, the waters must stand highest from the center of the earth on that side, and thereby make what is called a tide of flood, or high water: and at 90° distance all round this protuberance, the water will be lowest, and thereby constitute the tide of ebb, or low water, because there the attraction of the moon in some measure conspires with the attraction of the earth, and thereby brings them nearer to the earth's center. On the opposite side

Watts in England. Their principal improvement, consists in condensing the steam in a vessel appropriated for that purpose, into which the steam passes, by means of a tube, from the principal cylinder. This condensing vessel is furnished with two small pumps; one for the purpose of extracting the air, and the other for that of extracting the water produced from the condensed steam. In some of their steam-engines, the steam acts only on the under side of the piston, the upper side being exposed to the counteraction of the atmosphere; in others, the steam acts, alternately, on both sides of the piston, the action of the atmosphere being entirely excluded by an air-tight stuffing round the pistonrod.-Ed.

of the earth, there is also a tide of flood, at the same time. As the whole globe of earth and water is attracted by the moon with different forces, decreasing as the squares of the distances increase, the water on the side of the earth opposite to the moon, will be less attracted than the center of the earth, on account of its greater distance, and therefore may be considered as left behind, to stand highest from the center; and this occasions another tide of flood opposite to the former. By this means, the waters, which surround the earth, will put on the figure of a prolate spheriod, whose greater axis points towards the moon; and as the earth revolves round her axis, so as to bring any place under the moon once in every 24 hours, it follows, that we must have two tides of flood and two of ebb in that space of time.

The greatest height of the water is not exactly when the moon is on the meridian of a place, but about three hours after; as the longer axis of the spheroidal figure of the water does not point exactly to the moon, but to the eastward of it. The reason is this: although the force of the moon to raise the water is greatest when she is in the zenith or nadir; yet her force begins to act before she comes to these points, and continues to act after she has passed them; and as long as this force continues, the water will continue to rise, which will be about three hours afterwards. And for the same reason, the tide of ebb, which is six hours after the flood, will happen when the moon has been set about three hours. As the moon comes to the meridian of any place, about three quarters of an hour later on any day, than on the preceding day, the tides must be so much later also.

The greatest swell of water following the path of the moon, when she is in the northern signs, the tide of

flood, which comes with the moon to the meridian of any place in northern latitudes, will be greater than the tide which comes to the same place 12 hours afterwards: because the greatest altitude of this latter tide will be in south latitude, directly opposite to the moon. The contrary happens when she is in any of the southern signs, with respect to any places that are north of the equator; for the latter tide will then be the greatest. This is evident from a bare inspection of a globe.

If to the force of the moon we add that of the sun, the tides must be greatest at the conjunctions and oppositions of the sun and moon, as their attractions conspire at that time to raise the water, and these are called the spring tides; but when the sun and moon are in the quadratures, the tides are least, as their forces are then contrary to each other, the effect of the one being to raise, while that of the other depresses the water. These are called neap tides.

The spring tides are therefore produced by the sum of the attractive forces of the sun and moon, but the neap tides by the difference of their attractions. Were their forces equal, there would be no neap tides, because the sun would depress the waters, as much as they are raised by the action of the moon. But we find that the neap tides rise in the open ocean about 7 feet, while the spring tides rise about 11 feet: consequently, their forces to raise the tides are to one another as two numbers, whose sum is 11, and whose difference is 7; and these two numbers are 9 and 2; and as all the tides follow the moon, its power must be greater than that of the sun in the proportions of 9 to 2.

Although both the sun and moon raise the water by their attractive force, they will not produce four separate tides of flood in each day, but only two; for at the

conjunctions and oppositions they would raise the water in the same place, and at the same time; and the same will be the effect of their contrary attractions at the quadratures, where the sun's attraction would make low water, while the moon's would make a tide of flood in the same place, and therefore the sun only prevents the flood from being as high as it would otherwise be by the single attraction of the moon. Nor will they produce separate tides at any intermediate distance between the syzygies and quadratures; for although they would by their separate forces raise tides in different places at the same time; yet their joint effect will be only to hasten or retard the time of high water, according as the sun or moon passes the meridian first, or by opposite meridians, in the following manner.

When the moon is passing from conjunction or opposition to the quadrature, she passes the meridian after the sun, and the time of high water will be sooner than three hours after the moon's passage over the meridian: but when she is passing from the quadrature to the conjunction or opposition she passes the meridian before the sun, and the time of high water will be retarded, and be more than three hours after her passing the meridian. Their joint effect will produce one common tide between the places, where each of them would produce high water by their separate forces, which occasions the acceleration and retardation of the times of high water, bringing on the tides of flood sometimes sooner, and sometimes later, than three hours after the moon's passage over the meridian. The greatest tides are not at the time of the conjunction or opposition, but about three tides afterwards. Although the sun and moon act with the greatest force at the syzygies, and therefore should raise the highest tides at these times; yet we

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