APPLICATION OF A PRINCIPLE IN HYDROSTATICS.

The acquisition of knowledge and the application of it to the practieal business of life are very different things. The former engages our almost exclusive attention, while its results are valuable to us chiefly as they are possessed in connexion with the latter. Philosophers spend their lives in the investigation of abstract principles, while they are often taught all that is useful by the practical mechanic, who could not state his knowledge in the form of a principle at all. These two things, so distinct in all our systems of instruction, yet so inseparable for all practical purposes, ought to be learned more in connexion with each other. They would then lend mutual aid. The philosophical investigation of principles would be much facilitated, and would proceed with much surer steps, by a constant reference to the facts and phenomena from which they are derived. And the observer of facts would be much aided by knowing how to class them, as they present themselves, and where to look for those suitable to verify, restrict, or extend the application of a doubtful, vague, or limited principle. The abstract principles of science, as learned by philosophers, are generally much in advance of their practical applications in the common pursuits and business of life. Most of the splendid discoveries which characterize our age, are but combinations and applications of principles, which have long been understood. We are much more indebted to those, who reduce to practice and promulgate a useful discovery, than to those who rest satisfied with having made it. Because, however the individual may enjoy in private his own inventions, the world of mankind are not made glad by them, till they are turned to some practical account, which affects their condition and happiness.

It would be of incalculable utility both to the philosopher and the artisan in their different pursuits, to bring science and the arts together -to learn theory and practice at the same time--and to observe phenomena, and trace the laws, which govern them, as parts of the same process in acquiring knowledge. It is well, therefore, occasionally to apply to phenomena the principles by which they are explained; and as locks and canals are so much the order of the day, a principle in hydrostatics is here applied to explain the great pressure upon the gate of a canal lock, when the water is high upon it, but when there is little difference in the height on each side.

Let us suppose the gate to be ten feet long and the water to be ten feet deep on the upper side, and nine on the lower side. At first thought it would seem that the pressure of the nine feet on one side, would exactly counteract the pressure of nine feet on the other; and that the gate would be pressed with only one foot, in the same manner as if there were water of but one foot deep pressing the gate on one side, and none on the other. But this is not the fact. In this case, each foot of the top of the gate would be pressed by the weight of one half of a cubic foot of water, or 314 lbs, which, on ten feet in length, would amount to 312 lbs. This pressure might be easily overcome, and the gate opened with a lever of small power and with but little strength. But this one foot of water on the top acts on all the water below, and causes a pressure on each foot twice as great as it exerts itself on the upper foot of the gate.

The pressure on the upper side of the gate, then, by the principles of hydrostatics, is

for each foot in length; and for the whole ten feet

10 x 3125 =31250 lbs;

the pressure on the lower side, by the same principles, is

The difference is 5937 lbs. which is the actual pressure upon the upper side of the gate under the supposed circumstances. As the gate is turned towards the upper lock with a lever of but small power, we may suppose one man to be able to move about 200 lbs. At this rate it would require the strength of 30 men to move it.

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