and whose surface is one square inch, weighs also 14 or 15 pounds.

Charles. Then the weight of the atmosphere would balance or keep in equilibrio only a column of water of 33 feet high, and consequently could not support a greater column of water, much less have power to raise it up.

Emma. A pump, then, would be of no use in the deep wells which we saw near the coast in Kent.

Father. None at all: the piston of a pump should never be sat to work more than 28 feet above the water, because at some period the pressure of the atmosphere is so much less than at others, that a column of water, something more than 28 feet will be equal to the weight of air.

You cannot better fix in your mind the principle and action of the pump, than by committing to your memory Dr. Darwin's beautiful description of it:

NYMPHS! You first taught to pierce the secret caves Of humid earth, and lift her pond'rous waves;

Bade with quick stroke the sliding piston bear
The viewless columns of incumbent air;-
Press'd by the incumbent air, the floods below
Through opening valves in foaming torrents flow;
Foot after foot with lessen'd impulse move,
And, rising, seek the vacancy above.

BOTANIC GARDEN.

CONVERSATION XXII.

Of the Forcing-pump-Fire engine-Rope-pump-and

Water-press.

CHARLES. Why is this called the forcing-pump? (Plate iv. Fig. 35.)

Father. Because it not only raises the water into the barrel like the common pump, but afterwards forces it up into the reser

voir K K.

Emma How is that operation performed, Papa?

Father. The pipe and barrel are the same as in the other pump, but the piston G has no valve; it is solid and heavy, and made air-tight, so that no water can get above it.

Charles. Does the water come up through the valve a, as it did in the last?

Father. By raising up the piston, or as, it is generally called, the plunger G, a vacuum is made in the lower part of the barrel, into which, by the pressure of the air, the water rushes from the well, as you shall see.

Emma. And the valve is shut down.

Father. The water not being able to go back again, and being a fluid that is nearly incompressible, when the plunger is forced down, it escapes along the pipe M, and through the valve 6 into the ves sel K.

Charles. Though the water stands no higher than h, yet it flows through the pipe F to some height.

Father. The pipe ri is fixed into the top of the vessel, and is made air-tight, so that no air can escape out of it after the water is higher than i, the edge of the pipe.

Emma. Then the whole quantity of air which occupied the space Fb is compressed into the smaller space h F.

Father. You are right, and therefore the extra pressure on the water in the vessel forces it through the pipe, as you

see.

Charles. And the greater the conden sation, that is, the more water you force into the vessel x, the higher the steam will

mount.

Father. Certainly for the forcingpump differs from the last in this respect, that there is no limit to the altitude to which water may be thrown, since the air may be condensed to almost any degree.

The water-works at London-bridge, alluded to p. 82, exhibit a most curious engine, constructed upon the principle of the forcing-pump: the wheel-work is so contrived as to move either way as the water runs by these works, one hundred and forty thousand hogsheads of water are raised every day.