from Vesuvius, twelve feet long, and forty-five feet in circumference.

Volcanoes seem to become quite extinct, and after remaining for ages in that quiescent state, to be rekindled. Vesusius had been so long at rest, that pools of water had collected in the crater, and woods were growing in it, and on the sides of the mountain, when its fires revived, and burnt during a thousand years. Again, it was quiescent from 1136 to 1506; and it has now continued in action for three centuries.

Volcanoes are known to exist even at the bottom of the ocean; and these are called submarine volcanoes, and are few in number. Those which are certainly known are near the Azores and Iceland. By the force of volcanic eruption, islands have been raised from the bottom of the sea, and, after having been for some time visible, have disappeared.

Volcanoes have been ascribed to the effect produced by water bursting in suddenly upon some vast collection of fused or burning matter, to the action of central fires; and to the decomposition of different substances, by which heat and inflammation are produced.

Volcanoes, at astonishing distances, appear to be connected with one another. Steam is supposed to be one of the most powerful agents in volcanic operations.

Volcanoes have been observed in the moon by Herschel, Kater, and others.

QUESTIONS.

What are earthquakes, and what are the effects produced by them? To what causes are earthquakes principally at

tributed? In what regions of the earth, do earthquakes take place most frequently? What phenomena usually precede and accompany earthquakes? What are volcanoes? What number of volcanoes are known to exist and in what parts of the world are they found? What are the general effects of volcanoes? In what situation with respect to the sea do volcanoes commonly exist? What substances do volcanoes usually throw out? To what causes are volcanoes chiefly referred? What is the general process of a volcanic eruption? Is there supposed to be any connection between distant volcanoes? Have any volcanoes ever been discerned in the moon?

SOLIDS

CHAP XXXIV.

STATICS

DYNAMICS.

A SOLID is a body, whose parts are so firmly connected together, as not easily to give way, or slip from one another. In this view, solid stands opposed to fluid.

Solids are commonly divided into regular and irregular.

Regular solids are those which are terminated by regular and equal planes, and are reckoned as being only five in number; namely, the tetrahedron, which consists of four equal triangles; the cube, or hexahedron of six equal squares; the octohedron, of eight equal triangles; the dodecahedron of twelve equal triangles; and icosahedron, which is composed of twenty equal triangles.

The irregular solids are almost innumerable; such as the sphere; the cylinder; the cone; the parallelogram; the prism; the parallelopiped, &c.

Statics, a term derived from the Greek word, signifying to weigh, is applied to that science which contemplates solids as at rest; considering their equilibrum, their weight, their pressure; while to the science which treats of the motions of solids, is given the name of dynamics, borrowed from the Greek word, signifying

power.

Matter denotes the general substance of which all bodies are formed. Volume, or bulk, or magnitude, denotes the size of a body with regard to the space it occupies.

Density means the proportional quantity of matter which is contained within a given extension; and this quantity of matter is estimated by its weight.

Mass signifies both the bulk and density of a body; that is, the product of the bulk multiplied by the density.

The science called statics, comprehends all the doctrines of the excitement and propagation of pressure, through the parts of solid bodies, by which the energies of machines are produced.

It comprehends every circumstance which influences the stability of heavy bodies; the investigation and properties of the centre of gravity; the theory of the construction of arches, vaults, and domes, and the attitudes of animals.

It considers the strength of materials, and the principles of construction, so as to make the proper adjustment of strength and strain, in every part of a machine, edifice, or structure of any kind. It furnishes, consequently, the theory

of carpentry, and gives instructions for framing floors, roofs, centres, &c.

The science of statics comprehends, also, the whole doctrine of the pressure of fluids, whether liquid (in which case it receives the name of hydrostatics) or aëriform, when it is called aërostatics. Hence is derived the knowledge of the stability of ships, or their power of maintaining themselves in a position nearly upright, in opposition to the action of the wind on their sails.

Motion is a continual and successive change of place.

Motion is generated by certain causes; by particles of matter, which possess peculiar properties, stimulating or exciting nerves and organs; or decomposing bodies; by the operation of animal power; by the force of gravitation, or of other natural agents, such as wind, water, steam. The original source of all motion is the great First Cause, the glorious Creator of all things.

Equable motion is that movement of a body which passes over equal portions of extension, or of space, in equal portions of time.

Accelerated and retarded motion is, when equal portions of extension are passed over in portions of time, either successively smaller, or successively larger; or accelerated motion is that which, by some constantly operating cause, is continually increasing, as is exemplified in a body falling from some considerable height, whose motion is augmented in regularly increasing ratio,during every instant of its fall. This ratio

is, as the numbers 1, 3, 5, 7, 9, &c. For instance, if a body fell eight feet in the first second of its fall, it would fall three times eight in the next; five times eight in the third second; seven times eight in the fourth second, and so on.

Retarded motion is that which, by some constantly operating cause, is continually decreasing. For instance, if an arrow were shot straight upward, its motion would be diminished every instant of its ascent, till, its direction being changed, it would descend with accelerated motion, in the same time as its ascent took.

The whole spaces through which a falling body passes, from the commencement of its motion, will be as the squares of the times. That is, if it fall eight feet in the first second, it will fall four times eight in two seconds; nine times eight in three seconds; sixteen times eight in four seconds; and thus on; since, 4, 9, 16, are the squares of 2, 3, 4.

Velocity is the ratio of the quantity of lineal extension that has been run over in a certain given portion of time.

A force is that which causes a change in the state of a body, whether of rest or of motion.

The momentum, or quantity of motion, is the force of a body in motion; and is equivalent to the impression which it would make on another body, that should be placed, at rest, just before it.

A pressure is a force which acts upon a body so as to put it in motion, and, by the continuance of the action, accelerates that motion. Of this