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must remember that any motion communicated to wa ter will continue for some time after the moving force ceases to act; as the waves of the sea raised by a storm continue after the storm is over. And if at the sizygy both should cease to act, the tide would not then instantly cease, but the water, subsiding by its own gra. vity, would acquire such a velocity as would carry it below its proper level, and thereby cause it to rise again above it; and the force of the sun and moon, in their subsequent returns to the meridian, conspiring with this tendency of the water to rise, will occasion a greater swell in the second or third tide after the syzygy. Hence, the greatest tide will be some time after the conjunction and opposition; but it cannot be long after, because the moon passing towards the quadratures begins to coun teract the force of the sun, and thereby to lessen the tides.

Two very great spring tides never follow each other at the conjunction and subsequent opposition. For should the moon be in her perigee or nearest distance to the earth at any conjunction with the sun, when her force to raise the water is greatest, at her next opposi tion she will be at her apogee, where her influence is least; and therefore the spring tide at that opposition will not be so high as at the preceding conjunction.

In winter, the spring tides are greater, and the neap tides are less, than in summer. Because the sun is nearer to the earth in winter than in summer, and his influence to raise the tides is greater in proportion to his nearness to the earth. Now, as the spring tides are produced by the joint influence of the sun and moon, they must be greatest, when the force of the sun is greatest, the moon's force being supposed to be the same. And, as the neap tides are produced by the difference of their

forces, they must be less, as the force of the sun in

creases.

The force of the sun or moon to raise a tide is greatest, when they are in the equator, and decreases gradually as they approach towards the poles, where neither of them could produce any tide at all.

The spring tides are the greatest and the neap tides are the least about the equinoxes. At the equinoxes, the sun is in the equator, where his force is greatest, and the moon either in it or not far from it, as she is never more than about 5 degrees from the ecliptic; therefore the spring tides are then greatest. But the neap tides being produced by the difference of their forces at the quadratures, when the sun is in the equator the moon will be at or near to one of the tropics, where her influence is least. Therefore the neap tides at this time of the year ought to be least, especially when we reflect that the moon has much the greatest influence in producing the tides.

The highest spring tides and the lowest neap tides are not precisely at the equinoxes, but sometime before the vernal equinox, and sometime after the autumnal equinox. They ought to be highest about the winter solstice; as the sun's approach towards the earth makes his force to raise the tides greater at that time than at any other, so far as it depends upon his distance from the earth. But they also ought to be highest, when both luminaries are in or near to the equator, at the time of the equinoxes. But as they cannot be highest at both these times, they must attain to their greatest altitude at some intermediate time between the winter solstice and the equinox; that is, before the vernal, and after the autumnal equinox. And for the same reason, the neap tides, which accompany these tides of flood, will also

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be lowest about these times; because they are produced by the difference of force in the sun and moon; and as the sun's force, which is the lesser of the two, increases at these times, both on account of his nearness to the earth and his being in the equator, the difference between their forces will then be least, and so produce the least neap tides.

The tide of flood, which is raised under the sun or moon, will be something greater than that which is raised at the same time on the opposite side of the earth, on account of the greater attraction of the water nearest to these luminaries.

This is the general theory of the tides in the open ocean; but the situation of lands and rivers will make very considerable alterations, both in the times and altitudes of the tides at particular places. As when the tide has to pass along a large river, which receives a great quantity of water at its mouth, but afterwards grows narrow and confined towards its source; it will rise to a greater altitude than in the open sea, and the time of high water will be later; as in the bay of Funday and the Bristol channel, where the tide is said to rise 70 feet, and the time of high water will be later as the length of the river or bay is greater.

Should two tides come from different seas, to any port, at the distance of six hours after one another; when it should be high water by one, it will be low by the other. There will be therefore no rising or sinking of the surface of the water, because one tide will supply the water as fast as the other carries it off. But as there will be more water brought, when the moon is above the horizon, than when she is under it, there will be a difference of altitude in the water every twelve hours, which will make one tide of flood and one of ebb, every

twenty-four hours. It is upon the same principles that we account for the tides and half tides, as they are called, in different places, where a tide from one part of the sea is brought to any place, when the tide from another part is half spent, arising from the distance from which the water must come to raise a small tide in the middle of the ebb.

MOTION OF ELASTIC FLUIDS.

WHEN water is converted into steam by heat it becomes extremely elastic, and if confined will exert a prodigious force, as is evident from the colopile. This is a small vessel made with an incurvated neck, that it may direct the steam, issuing from it through a narrow orifice, to the fire which converts the water it contains into vapour, and blows it like a pair of bellows. In using this instrument, great care must be taken to keep the orifice open, for should it happen to be closed, the vessel would burst with a loud explosion, and possibly prove fatal.

We have already taken notice of the advantage that may be derived from this elastic fluid, in the working of fire or steam-engines, for raising water from mines, &c. and future experience may derive many other advantages from it.

Air is also an elastic fluid much more beneficial to mankind, whose motion exhibits various phenomena, the principal of which we must now explain.

Upon the elasticity of the air depends the operation of the air-pump, by which it may be almost totally extracted from any vessel. Its construction is on the same principles with those of a common pump, which we have already explained.

If the barrel of the air pump communicate with a close vessel, when the piston is raised up from the lower valve, the elastic force of the air in the vessel will open this valve, and part escaping through it will follow the piston and fill the barrel of the pump; and as it occupies a much larger space than before, it must be considerably rarefied. As the piston descends, it condenses the air in the barrel, and thereby shuts the lower valve, which prevents its return to the vessel, and when it becomes more dense than the external air, it will open the valve of the piston and escape through it, and mix with the common air. Thus by the working of the pump any assigned degree of rarefaction may be produced in the air in the vessel, which is commonly called the receiver. But the whole of the air in the receiver can never be extracted, so as to constitute an absolute vacuum; because the expulsion of the air, at every stroke of the pump, is effected by the spring or elasticity of what remains in the receiver.

If the valves of the air pump be made to open the contrary way, it becomes the condenser, by which an additional quantity of air may be pressed into any vessel, until the increased spring of the condensed air be at last sufficient to burst the vessel. In this instrument, the ascent of the piston closes the lower valve, and prevents the air in the vessel from following the piston, while the weight of the atmosphere opens the valve of the piston, and presses the air through it into the barrel of the condenser, which by the descent of the piston is pressed into the vessel.

If air be sufficiently condensed by such an instrument in a tight vessel, which may communicate at pleasure with the barrel of a gun, by means of a trig ger, it will, by its elastic force, when at liberty to ex

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