Of the Mechanical Powers.

the power, but then the number of revolutions made by the handle D, will also be proportionally increased—so that we return to the general principle—what is gained in power is lost in time. The power of the screw may also be increased by lengthening the lever attached to the nut.

The screw is used for compression, and to raise heavy weights. It is used in cider and wine presses, in coining, and for a variety of other purposes.

GENERAL REMARKS.-All machines are composed of one or more of the six machines which we have described. We should remember, that friction diminishes very considerably the power of machines.

There are no surfaces in nature which are perfectly smooth. Polished metals, although they appear smooth, are yet far from being so. If, therefore, the surfaces of two bodies come into contact, the projections of the one will fall into the hollow parts of the other, and occasion more or less resistance to motion. In proportion as the surfaces of bodies are polished, the friction is diminished, but it is always very considerable, and it is computed that it generally destroys one-third the power of the machine.

Oil, or grease, is generally used to lessen the friction. It fills up the cavities of the rubbing surfaces, and thus makes them slide more easily over each other.

Of Specific Gravity.

SECTION IV.

OF SPECIFIC GRAVITY.

1. The specific gravity of a body is the relation which the weight of a given magnitude of that body bears to the weight of an equal magnitude of a body of another kind.

2. If two bodies are of the same bulk, the one which weighs the most is said to be specifically heavier than the other. On the contrary, one body is said to be specifically lighter than another when a certain bulk or volume of it, weighs less than an equal bulk of that other.

Thus, if we have two equal spheres, each one foot in diameter, the one of lead and the other of wood, the leaden one will be found to be heavier than the wooden one; and hence, its specific gravity is greater. On the contrary, the wooden sphere being lighter than the leaden one, its specific gravity is less.

3. The greater specific gravity of a body indicates a greater quantity of matter in a given bulk, and consequently the matter must be more compact, or the particles nearer together. This closeness of the particles is called density. Hence, if two bodies are of equal bulk or volume, their weights or specific gravities will be proportional to their densities.

4. If two bodies are of the same specific gravity, or density, their weights will be proportional to their bulks

Of Specific Gravities.

5. A body specifically heavier than a fluid will sink on being immersed in it. It will, however, descend less rapidly through the fluid than through the air, and less power will be required to sustain the body in the fluid than out of it. Indeed, it will lose as much of its weight as is equal to the weight of a quantity of fluid of the same bulk. If a body is of the same specific gravity with the fluid, it loses all its weight, and requires no force but the fluid to sustain it. If it be lighter, it will be but partially immersed, and a part of the body will remain above the surface of the fluid.

6. We may conclude, from what has been said in the last article,

1st. That when a heavy body is weighed in a fluid, its weight will express the difference between its true weight and that of an equal bulk of the fluid.

2d. If the body have the same specific gravity with the fluid, its weight will be nothing.

3d. If the body be lighter than the fluid, it will require a force equal to the difference between its own weight, and that of an equal bulk of the fluid, to keep it entirely immersed, that is, to overcome its tendency to rise.

7. In comparing the weights of bodies, it is necessary to take some one as a standard, with which to compare all others. Rain water is generally taken as this standard.

Of Specific Gravities.

A cubic foot of rain water is found, by repeated experiments, to weigh 62 pounds, avoirdupois, or 1000 ounces. Now, since a cubic foot contains 1728 cubic inches, it follows that 1 cubic inch weighs ,03616898148 of a pound. Therefore, if the specific gravity of any body be multiplied by,03616898148, the product will be the weight of a cubic inch of that body in pounds avoirdupois. And if this weight be then multiplied by 175, and the product divided by 144, the quotient will be the weight of a cubic inch in pounds Troy-since 144 lbs. Avoirdupois is just equal to 175 lbs. Troy.

8. Since the specific gravities of bodies are as the weights of equal bulks, the specific gravity of a body will be to the specific gravity of a fluid in which it is immersed, as the true weight of the body, to the weight lost in weighing it in the fluid. Hence, the specific gravities of different fluids are to each other as the weights lost by the same solid immersed in them.

PROBLEM 1.

To find the specific gravity of a body, when the body is heavier than water.

RULE.

1st. Weigh the body first in air and then in rain water, and take the difference of the weights, which is the weight lost.

2d. Then say, as the weight lost is to the true weight,

Of Specific Gravities.

so is the specific gravity of the water to the specific gravity of the body.

EXAMPLES.

1. A piece of platina weighs 70,5588 lbs. in the air, and in water only 66,9404 lbs. what is its specific gravity, that of water being taken at 1000?

First, 70,5588-66,9404-3,6184 lost in water.

Then, 3,6184

70,5588 1000 19500, which is the

specific gravity, or weight of a cubic foot of platina.

2. A piece of stone weighs 10 lbs. in air, but in water only 63 lbs: what is its specific gravity?

PROBLEM II.

Ans. 3077.

To find the specific gravity of a body when it is light

er than water.

RULE.

1st. Attach another body to it of such specific gravity that both may sink in the water, as a compound mass. 2d. Weigh the heavier body and the compound mass separately, both in water and in open air, and find how much each loses by being weighed in water. 3d. Then say, as the difference of these losses is to the weight of the lighter body in the air, so is the specific gravity of water to the specific gravity of the lighter body.