court-road, was intended as a reservoir for the New River.

Father. What appears to you, and others who pass by it, only as a mound of earth, is an exceedingly large bason capable of containing a great many thousand hogs. heads of water.

Charles. How will they get the water into it?

Father. At Islington, near the New River Head, is made a large reservoir upon some very high ground, into which, by means of a steam-engine, they will constantly throw water from the New River. This reservoir being higher than that in Tottenham-court-road, nothing more is necessary than to lay pipes from Islington to that place in order to keep it constantly full of

water.

By this contrivance the New River company will be able to extend their business to other parts of London where their present head of water cannot reach.

Charles. The weight of water in this place must be immensely great.

Father. It must; and therefore you observe what a thickness the mound of earth against the wall is towards the bottom, and that it diminishes towards the top as the pressure becomes less and less.

Emma. Would not the consequences be very serious if the water were to insinuate itself through the earth at the bottom?

Father. If such an accident were to happen when the reservoir was full of water, it would probably tear up the works and do incredible mischief. To prevent this, the vast bank of earth is sloped within, as well as without; it will then be covered with a strong coating of clay; after this it will be built up with a very thick brick-wall, which will be carefully tarrassed over; so that the whole mass will be as firm and compact as a glass bottle.

CONVERSATION X.

Of the Specific Gravities of Bodies.

EMMA. What is the reason, Papa, that some bodies as lead or iron, if thrown into the water, sink while others, as wood, will swim?

Father. Those bodies that are heavier than water will sink in it, but those that are lighter will swim.

Emma. I do not quite comprehend your meaning; a pound of wood, another of water, and another of lead, are all equally heavy. For Charles played me a trick the other day he suddenly asked which was

heavier, a pound of lead or a pound of feathers? I said the lead, and you all laughed at me, by which I was soon led to perceive, that a pound, or-16 ounces of any substance whatever, must be always equal to the same weight.

Father. You are not the first person that has been taken in by this question. It is a common trick. Although a pound of lead and another of water be equally heavy, yet they are not of equal magnitudes. Do you know how much water goes to a pound? Charles. Yes; about a pint.

Father. Do you think that if I were to fill the same pint measure with lead, that would weigh a pound only?

Charles. Oh no; that would weigh a great deal more. I do not believe that the 14 pound weight below stairs is much larthan a pint measure.

ger

Father. Yes it is, by about a fourth part: the same measure that contains one pound of water, would, however, contain about 11 pound of lead: but it would contain 14 pounds of quicksilver, which, you know, I

could as easily pour into the vessel as if it

were water.

Here are two cups of equal size; fill the one with water, and I will fill the other with quicksilver.

Emma.

Why did you not let Charles

pour out the quicksilver?

Father. The loss of water is a matter of little consequence; but if, by chance, he had thrown down the quicksilver, the accident might have occasioned the loss of sixpence, or a shilling; and economy is right in all the affairs of life. Take the cups in your hand: which is the heavier ?

Charles. The quicksilver by much.
Father. But the two cups are of equal

size.

Emma. Then there must be equal quantities of water and quicksilver.

Father. They are equal in bulk.

Charles. But very unequal in weight: shall I try how much heavier the one is than the other.

Father. If you please. In what manner will you ascertain the matter?