the same in both cases.

Take this shilling and a feather: let them both drop from your hand at the same instant.

Charles. The shilling is soon at rest at my feet, but the feather conti nues floating about. Is the feather specifically lighter than air?

Father. No: for if it were, it would ascend till it found the air no heavier than itself; whereas, in a minute or two, you will see the feather on the floor as well as the shilling: it is however so light, and presents so large a surface to the air, in comparison to its weight, that it is considerably longer in falling to the ground than heavier bodies, such as a shilling or a guinea. Take away the resisting medium, and they will both reach the bottom at once.

Emma. How will you do that?

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, unless the bracket was very level; therefore, when the marble was placed in two or more different parts of the board, and did not move to one side or the other, he might safely conclude that it was a level..

Charles. Then the water-level and

* See Vol. I, of Mechanics, Conver. IX.

the rolling of the marble depend on the

same principle?

Father. They do, upon the suppositon 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 kinds exert a pressure upwards and sideways equal to their pressure downwards.

it as often as you please with hardly any noise.

Charles. Perhaps the air breaks the fall of the water by dividing its particles.

Father. It acts, with respect to water, as water acts with regard to the fall of any other substance thrown into it it impedes the velocity of the falling body.

BO

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

Charles. It certainly would.

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 is a large tube or jar having a flat bottom: a b is a similar 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