SEC. 134. Hydrometers should be immersed in water also; par, ticularly Baume's Areometer.

Manner of using BAUME'S GLASS AREOMETER* in ascertaining the specific gravity of liquids.

In constructing this instrument two stationary points are assumed; and if you have none at hand, these points may be found as follows. Take a slender glass tube, with a hollow bulb at the bottom. Put into the bulb mercury or fine shot, until you sink it in pure water almost to the top. Mark the zero point at the surface of the water. Then weigh 85 parts of water and 15 parts of table salt (muriate of soda.) After the salt is perfectly dissolved in the water, bring the temperature to 57° of Fah. Immerse the tube in this solution, and mark the point at the surface of the water, for the lower termination of 15 degrees. Being equally divided into 15 parts, these parts may be assumed as standard measures for any series of tubes (one ending where another begins,) for taking the relative specific gravities of liquids from the heaviest sulphuric acid to the lightest ether.

WATER USED IN TAKING SPECIFIC GRAVITY OF SOLIDS.

SEC. 135. To be familiar with taking the specific gravities of materials for construction, is often of great use to persons in all other situations in life, as well as to engineers.

Illustration. Tic a strong silk thread or silk twine around a piece of marble weighing seven or eight pounds. Weigh it carefully, using balancing quarter-ounce or half quarter-ounce weights; so as to bring it to an even ounce weight on the steelyard bar. Then weigh it in water, sinking it so as to be wholly about half an inch below the surface of the water. Next subtract its weight in water from its weight in air-take this remainder for a divisor, and its weight in air for a dividend; and the quotient will be its true specific gravity. As the weight in air is 8 ib. 74 oz.; weight in water 5 . ‡ oz,

*Araios, Greek, slender or delicate; and metron, a measure.

3.437)8.453(2.459 spe. grav.; that is twice and

about 4 heavier than an
10%
equal bulk of water.

6.874

15790

13748

20420

17185

32350

30933

In this manner the solidity of materials for construction may be readily obtained—and it is preferable to the usual practice with grain-weights, for coarse materials.

HYDROSTATICS.

SEC. 136. Make a cylindrical bellows, by cutting two circles of thick board 10 inches in diameter, and nailing to the outside rim of each, with broad headed tacks, a hollow cylinder of leather. When finished it will present a leathern cylinder of strong calf skin, 10 inches in diameter and 8 inches long. Set in the middle of the top board a leaden tube of about the fourth of an inch in calibre, and 3 or 4 feet high. Let the top fit into a glass tube, 5 to 10 inches long, by a bandage of tow. When used, the leather needs to have been soaked in water several hours. Fill the cylinder with water through a plug-hole in the top board. Lay a weight on the top board, or let a student of suitable size stand on it, so that the water may rise into the glass tube. On measuring the height to which the water rises in the glass tube from the top board, and making the proper calculation, this result will be found the weight set on will precisely equal the weight of a cylinder of water, 10 inches in diameter, of the height of the water in the tube. Hence it follows, that water presses according to its height; not according to its quantity by measure or weight. Therefore were it not for the impossibility of

:

maintaining the perpetual supply of water, a tube of an inch calibre would be sufficient for moving the machinery of an extensive factory, under a hundred feet head, supplied from a small reservoir or tub.

HYDRODYNAMICS.

SEC. 137. After this is demonstrated, that water pressure depends on its height, and the weight of a standard measure of water is ascertained, we must determine by trial, what measured velocity will be given to water by a measured head. It was before stated, that trial has shown that a measured pint of pure water weighs a pound.

SEC. 138. Trial has also shown, that under one foot pressure, water will be forced through a lateral aperture with a velocity of 8 feet and a tenth, per second, in a vacuum-probably it will be correct in practice to say 8 feet per second in the atmosphere, at the precise point of effusion. This trial prepares us for the universal rule which governs in all cases of motion by gravitation.

SEC. 139. The increased velocity of water effused, and of falling solids, is as the square root of the head of water, and as the square root of the distance through which solids fall. Taking 8 feet of lateral effusion per second for the first foot, and the increase from that zero (if it may be so called,) as the increase of the square root of the head, and the increase of the distance fallen through in the case of solids, we arrive at results of vast importance in engineering.

SEC. 140. Illustration. A spacious flume has 25 feet of water in depth, with five apertures or gate-holes, of one square foot each. The centre of the first gate-hole is one foot below the top of the water—the second 4 feet—the third 9 feet—the fourth 16 feet—the fifth 25 feet. The square root of one is one, and the lateral effusion of water is 8 feet per second, as demonstrated by trial. The square root of 4 is 2, and the lateral effusion of water is 8x2=16. The square root of 9 is 3, and the lateral effusion of water is 8x3=24. The square root of 16 is 4, and the lateral effusion of water 8 x 4= 32. The square root of 25 is 5, and the lateral effusion of water is 8x5=40. All intermediate heights of head may be calculated in the same manner. That is, extract the root of the height given, in feet and decimals of feet. Multiply that root by 8, the velocity of the first second of pressure.

See the annexed diagram.

Remark. Though the distinction between Statics and Dynamics is truly philosophical, it is inconvenient in its application to the Mathematical Arts—more especially so in a concise plain treatise, wholly devoted to practice. Olmsted's Compendium of Natural Philosophy is particularly recommended to students in Engineering, who have time to discipline their minds for a more systematic view of Mechanical Science.

WATER UNDER THE INFLUENCE OF ATMOSPHERIC PRESSURE.

SEC. 141. The atmosphere presses upon all bodies on the earth, at tide-water level, at an average of about 15 s. on every square foot. Water is held down with a force, very unexpected by the student, until he makes the common trial, as follows: Boil water in the open air; and introduce the bulb of a thermometer at the moment of boiling. The mercury will rise to about 212° of Fah. Take off the atmospheric pressure perfectly, and it will boil at 67° As such accurate apparatus as this experiment requires is not common, an approximation to it must be used, which will satisfy every student. Put a gill of water in an oil-flask (I mean those Florence flasks, with a flag covering woven on them.) Cork it perfectly tight; and let one inch of the top of the neck be well wound with waxed thread, to prevent splitting by suddenly forcing in the cork. Before the cork is put in, boil the water for about one minute. This will force out the air, mostly. Thrust in a compact, soft, vel. vet cork (as the best are called,) while the water is boiling. Now let the water cool down to blood-heat; which may be known by applying the hand of a healthy person. If he can scarcely perceive any warmth to the hand on applying it to the bottom of the flask, the temperature is about 98 degrees, that is, 66 degrees above freezing. This is 114 degrees below boiling. As it is but 180 dcgrees from freezing to boiling, and as 114 is 66 above freezing; even this imperfect experiment proves, that almost half of the heat required for boiling water, is applied in resisting atmospheric pressure. And accurate experiments prove that more than two thirds of the heat is employed in counteracting the pressure of the atmosphere.

SEC. 142. Atmospheric pressure is most perfectly exhibited, by a tin tube about 34 feet in length. Let this be closed at one end,