to show

that you

a very

few ounces of wa

ter will lift up and sustain a large weight. Emma. What is the instrument called?

Father. It is made like common bellows, only without valves, and writers have given it the name of the hydrostatic bellows. This small tin-pipe e o p communicates with the inside of the bellows. At present the upper and lower board are kept close to one another with the weight w. The inside of the boards are not very smooth, so that water may insinuate itself between them: pour this half pint of wa ter into the tube.

Charles. It has separated the boards, and lifted up the weight.

Father. Thus you see that seven or eight ounces of water has raised and continues to sustain a weight of 56 th. By diminishing the bore of the pipe, and increasing its length, the same or even a smaller quantity of water would raise a much larger weight.

Charles. How do you find the weight

that can be raised by this small quantity of water?

Father. Fill the bellows with water, the boards of which, when distended, are three inches asunder. I will screw in the pipe. As there is no pressure upon the bellows, the water stands in the pipe at the same level with that in the bellows at z.

Now place weights on the upper board till the water ascend exactly to the top of the pipe e these weights express the weight of a pillar or column of water, the base of which is equal to the area of the lower board of the bellows, and the height equal to the distance of that board from the top of the pipe.

Emma. Will you make the experiment? Father. Your brother shall first make the calculation.

Charles. But I must look to you for assistance.

Father. You will require very little of my help. Measure the diameter of the bellows, and the perpendicular height of the pipe from the bottom board.

Charles. The bellows are circular, and 12 inches in diameter; the height of the pipe is 36 inches.

Father. Well; you have to find the solid contents of a cylinder of these dimensions; that is, the area of the base multiplied by the height.

Charles. To find the area I multiply the square of 12 inches, that is, 144, by the decimals .7854, and the product is 113, the number of square inches in the area of the bottom board of the bellows. And 113. multiplied by 36 inches, the length of the pipe, gives 4068, the number of cubic inches in such a cylinder; this divided by 1728 (the number of cubic inches in a cubic foot,) leaves a quotient of 2.3 cubic feet, the solid contents of the cylinder. Still I have not the weight of the water.

Father. The weight of pure water is equal in all parts of the known world, and a cubical foot of it weighs 1000 ounces.

Charles, Then such a cylinder of water as we have been conversing about weighs 2300 ounces, or 144 pounds nearly.

Emma.

Let us now see if the experi

ment answers to Charles's calculation. Father. Put the weights on carefully, or you will dash the water out at the top of the pipe, and I dare say that you will find the fact agrees with the theory.

Charles. If instead of this pipe, one double the length was used, would the water sustain a double weight?

Father. It would: and a pipe three or four times the length would sustain three or four times greater weights.

Charles. Are there then no limits to this kind of experiment, except those which arise from the difficulty of acquiring length in the pipe?

Father. The bursting of the bellows would soon determine the limit of the experiment. Dr. Goldsmith says that he once saw a strong hogshead split by this means. A strong small tube made of tin, about 20 feet long, was cemented into the bung-hole, and then water was poured in to fill the cask: when it was full, and the water ha

risen to within about a foot of the top of the tube, the vessel burst with prodigious force.

Emma. It is very difficult to conceive how this pressure acts with such power. • Father. The water at o is pressed with a force proportional to the perpendicular altitude e o; this pressure is communicated horizontally in the direction o p q, and the pressure so communicated acts, as you know, equally in all directions: the pressure, therefore, downwards upon the bottom of the bellows is just the same as it would be if pqnr were a cylinder of water.

The experiment made on the bellows might, for want of such an instrument, be made by means of a bladder in a box with a moveable lid.

Emma. Has this property of hydrostatics been applied to any practical purposes?

Father. The knowledge of it is of vast importance in the concerns of life. On this principle a press of immense power has been formed, (Plate 11. Fig. 14.) which