Charles. Is it upon this principle that water-levels are constructed?

Father. It is the most simple kind of water-level is a long wooden trough, which being filled to a certain height with water, its surface shows the level of the place on which it stands.

Charles. I did not allude to this kind of levels, but to those smaller ones contained in glass tubes.

Father. These are, more properly speaking, air-levels. They are thus constructed (Plate 1. Fig. 2.): D is a glass tube fixed into L, a socket made generally of brass. The glass is filled with water, or some other fluid, in which is enclosed a single bubble of air. When this bubble fixes itself at the mark a, made exactly in the middle of the tube, the place on which the instrument stands is perfectly level. When it is not level, the bubble will rise to the higher end.

Emma. What is the use of these levels?

Father. They are fixed to a variety of philosophical instruments, such as quadrants,

and telescopes for surveying the heavens; and theodolites for taking the level of any part of the earth. They are also useful in the more common occurrences of life. A single instance will show their value: clocks will not keep true time unless they stand very upright; now by means of one of those levels you may easily ascertain whether the bracket, upon which the clock in the passage stands, is level.

Emma. But I remember when Mr. F

brought home your clock, he tried if the bracket was even by means of one of Charles's marbles. How did he know by this?

Father. The marble, being round, touched the board in a point only, consequently the line of direction* could not fall through that point, but the marble would roll; unless the bracket was very level; therefore, when the marble was placed in two or more

* See Vol. I. Of Mechanics. Conver. IX. VOL. II.

C

different parts of the board, and did not move to one side or the other, he might safely conclude that it was level.

Charles. Then the water-level and the rolling of the marble depend on the same principle?

Father. They do, upon the supposition that the particles of water are round. The water-level will, however, be the most accurate, because we may imagine that the parts of which water is composed are perfectly round, and, therefore, as may be geometrically proved, they will touch only in an infinitely small point; whereas, marbles made by human contrivance, touch in many such points.

We now come to another very curious principle in this branch of science, viz. that fluids press equally in all directions. All bodies, both fluid and solid, press downwards by the force of gravitation, but fluids of all kind exert a pressure upwards and sideways equal to their pressure downwards.

Emma. Can you show any experiments in proof of this?

Father, A, B, c (Plate 1. Fig. 3.), is a bended glass tube: with a small glass funnel (Plate 1. Fig. 4.) pour in the mouth a, a quantity of sand. You will find that, when the bottom part is filled, whatever is poured in afterwards will stand in the side of the tube A B, and not rise in the other side B C.

Charles. The reason of this is, that by the attraction of gravitation all bodies have a tendency to the earth; that is, in this case, to the lowest part of the tube; but if the sand ascended in the side в c, its mo-' tion would be directly the reverse of this principle.

Father. You mean to say that the pressure would be upwards, or from the centre of the earth.

Charles. It certainly would.

* See Vol. I. Of Mechanics. Conver. V.

Father. Well, we will pour away the sand and put water in its place: what do you say to this?

Emma. The water is level in both sides of the tube.

Father. This then proves, that with respect to fluids there is a pressure upwards at the point B as well as downwards. I will show you another experiment.

A B (Plate 1. Fig. 5.) is a large tube or jar having a flat bottom: a b is a smaller tube open at both ends. While I fill the jar with water, I take care to hold the small tube so close to the bottom of the jar as to prevent any water from getting into the tube. I then raise it a little, and you see it is instantly filled with water from the jar.

Charles. It is: and the water is level in the jar and the tube.

Father. The tube, you saw, was filled by means of the pressure upwards, contrary to its natural gravity.

Take out the tube; now the water hav