terms, gravity, specific gravity, centre of gravity, force of gravity, &c.?

Gravity is downward pressure or weight; all bodies possess this property more or less, in proportion with their various degrees of density.

Specific gravity means the comparative density or weight that one body bears to another of equal bulk,—thus, a body of less specific gravity than water, or other fluid, will float upon that fluid; but if of greater specific gravity, will sink.

Centre of gravity is that point in a body or system of bodies, on which, if rested or suspended, the whole will remain in a state of equilibrium or rest; thus, if a wall or other structure be raised perpendicular to the base, it will remain secure while in that state; but if the foundation be not of sufficient solidity, and by degrees allow it to depart so far from its vertical position that the centre at the top exceeds the base at the bottom, the same will fall, unless restricted by cohesion of the parts of which it is composed.

Force of gravity is an accelerated velocity, which heavy bodies acquire in falling freely from a state of rest;—thus, the velocity that a body will acquire in one second of time, equal 32.1666

feet, or in five seconds 32.1666 x 5160.833 feet, &c., the velocity in feet, per second, that a body will acquire in falling through a given space, equal the square root of the product of the time multiplied by 64.3333. Hence, the velocity acquired at the end of 249 feet-64.3333 x 240 124.26 feet. Again: the space through which a body will fall in feet, in any given time, equal the product of the square of the time, multiplied by 16.0833. Hence the space a body will pass through in seven seconds 16.0833x72-788-0817 feet.

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Force of gravity is also the restrictive cause to a pendulum's motion; consequently its motion at any place is dependent upon the energy of the force of gravity at that place.

Pendulums of the same length vibrate slower the nearer they are brought to the equator, because of the earth's spheroidal form, its polar axis being about twenty-six miles shorter than its equatorial diameter; for which reason, also, gravity is lessened th part, the centrifugal force arising from the diurnal motion of the earth being greater at the equator than at the poles.

The measure of the force of gravity in feet,

per second, at any place, is equal to the product of the length of a pendulum that beats seconds; these multiplied by 9.8696; or, if any length of a pendulum be taken in feet, and the time in seconds observed between each of its oscillations, then the length of the pendulum divided by the square of the time in seconds, and the quotient multiplied by 9.8696, the product equal the number of feet, by which gravity will at that place increase the velocity of the descent of a falling body in each second of time.

The length of a pendulum to vibrate seconds, or 60,

118. The length of a seconds pendulum at a given place being known, how is the length of another pendulum obtained, to perform any other number of vibrations in the same time?

RULE.-Multiply the square root of the known length of pendulum by 60, divide the product by

the required number of vibrations, and the square of the quotient is the length required.

Ex. What must be the length of a pendulum to make 80 vibrations per minute in the latitude of Edinburgh, the length of seconds pendulum being 39.1555 inches?

119. What is the subsisting ratio between that of a seconds pendulum and gravitating force?

The space through which a heavy body will fall during the time of one vibration of a pendulum vibrating seconds, is to half the pendulum's length as the square of the circumference of a circle is to the square of its diameter, or as 4.9348 to the whole length; hence, the force of gravity in the latitude of London equal 39.1393 × 4.9348 193-1446 inches, or 16 feet 1 inch nearly.

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120. What is meant by centre of oscillation? The centre of oscillation is a certain point in a vibrating body into which all its force is collected, and to which, if an obstacle be applied, motion will instantly cease. The most simple means by which to ascertain the centre of oscillation in a

compound pendulum, is to suspend a small belly a fine thread in front of that in which the centre of oscillation is required, lengthen or shorten the thread until the vibrations of each are alike, then stop both and let them hang freely. Opposite the centre of the ball is the centre of oscillation.

121. How is gravity affected by motion on descending planes or curved surfaces?

The force which accelerates the motion of a heavy body on an inclined plane, is to the force of gravity as the sine of the inclination of the plane to the radius, or as the height of the plane to its length.

The velocity acquired by a body in falling from rest through a given height is the same, whether it fall freely or descend on a plane at whatever inclination.

The space through which a body will descend on an inclined plane, is to the space through which it would fall freely in the same time as the sine of the inclination of the plane to the radius.

The velocities which bodies acquire by descending along chords of the same circle, are as the lengths of those chords.