will see that when I open the cocks, the water will cut the curve line nearly, but not accurately, in those parts to which the horizontal lines were drawn.

Charles. That which spouts from the centre, is thrown to the point м,

as it was

The two

from the centre horizontal pipe. others fall on the point K, on which the upper and lower horizontal pipes ejected

the stream.

Emma. I thought the water from the upper cock did not reach so high as the

mark.

Father. It did not. The reason is, that it had to pass through a larger body of air, and the resistance from that retarded the water and prevented it from ascending to the point to which it would have ascended if the air had been taken away.

While we are on this subject, I will just mention, that as you see the water spouts the farthest when the pipe is elevated to an angle of 45°, so a gun, cannon, &c. will project a bullet the farthest if it be elevated to an angle of 45o.

Charles. Will a cannon or mortar carry a ball equally distant if it be elevated at angles equally distant from 45°, the one above and the other below?

Father. It will in theory: but owing to the great resistance which very swift motions meet with from the air, there must be allowances made for some considerable variation between theory and practice.

A regard to this will explain the reason why water will not rise so high in a jet as it does in a tube.

Emma. I do not know what this means. Father. You have seen a fountain. Emma. Yes, I have often been amused with that in the Temple.

Father. All fountains are called jets, or jets d'eau. Now if the water of that in the Temple ascended in a pipe, it would rise higher than it does in the open air. Turn to Fig. 10, the water in the small tube rises to a level with that in the larger one; now if the tube HG were broken off at t, the water would spout up like a fountain, but not so high as it stands in the tube, perhaps no higher than to d.

Charles. Is that owing wholly to the resistance of the air?

Father. It is to be ascribed to the resistance which the water meets with from the air and to the force of gravity, which has a tendency to retard the motion of the

steam.

Emma. Why does the fountain in the Temple sometimes play higher and sometimes lower?

Father. Near the Temple-hall there is a reservoir of water, from which a pipe communicates with the jet in the fountain: and according as the water in the reservoir is higher or lower, the height to which the fountain plays is regulated.

Charles. By turning a cock near the pump, the fountain is instantly lowered.

Father. That cock is likewise connected with the reservoir, and therefore taking water from it must have the effect of lowering the stream at the fountain, as well as that in the reservoir.

Emma. It soon recovers its force again. Father. Because there is a constant supply of water to the reservoir, which, how

ever, does not come in so quick as the cock lets it out, or the fountain would always play to the same height.

From what you have already learnt on this subject, you will be able to know howLondon and other places are supplied with

water.

Charles. London is, I believe, supplied from the New River, but I do not know in what manner.

Father. The New River is a stream of water that comes from Ware in Hertfordshire; it runs into a reservoir situated on the high ground near Islington. From this reservoir pipes are laid into those parts of town that have their water from the New River, and through these pipes the water flows into cisterns belonging to different houses.

Emma. Then the reservoir at Islington must be higher than the cisterns in London.

Father. Certainly, because water will not rise above its level. On this account some of the higher parts of town have hitherto

been supplied from the ponds at Hampstead; and others are supplied from the Thames, by means of the water works at London-bridge.

Gharles. Are pipes laid all the way from Hampstead to town?

Father. They are; but these supply the intermediate villages, as well as London: and Hampstead standing so high, the water is carried up into the first and second stories in some houses. Thus you see that water may be carried to any distance, and houses on different sides of a deep valley may be supplied by water from the same springhead. You must remember that if the val

leys are very deep, the pipes must be exceedingly strong near the bottom, because the pressure increases in the rapid proportion of the odd numbers 1, 3, 5, 7, &c. and therefore, unless the strength of the wood or iron be increased in the same proportion the pipes will be continually bursting.

Emma. You told me the other day, that the large mound of earth, for it appears nothing else, near the end of Tottenham