springs for moving machinery, as well as for easing the motion of carriages over rough roads. It is the elasticity of cords that renders them applicable to musical instruments. It is the elasticity of air that renders it a fit vehicle for transmitting sound. It is the elasticity of the etherial medium pervading space which renders it capable of transmitting light.

II. MECHANICAL PRINCIPLES

Definition of Mechanics.

14. MECHANICS is that branch of Physics which treats of the laws of rest and motion. It also treats of the action of forces upon bodies.

Rest and Motion.

15. A body is at REST when it retains its position in space. It is in MOTION when it continually changes its position in space.

A body is at rest with respect to surrounding bodies, when it retains the same relative position with respect to them, and it is in motion with respect to surrounding objects when it continually changes its relative position with respect to them. These states of rest and motion are called Relative Rest and Relative Motion, to distinguish them from Absolute Rest and Absolute Motion.

When a body remains fixed on the deck of a moving vessel or boat, it is at rest with respect to the parts of the vessel, although it partakes with them in the common motion of the vessel. When a man walks about the deck of a vessel, he is in motion with respect to the parts of the vessel, but he may be at rest with respect to objects on shore; this will be the case when he travels as fast as the vessel sails, but in an opposite direction. In consequence of the earth's motion around its axis and about the sun, together with the motion

Stringed instruments. Transmission of light. (14.) What is (15.) When is a body at rest? When in motion? Explain relati rest and motion. Illustrate by examples.

city's 193 feet. Instead of taking a sem.

ffime, night adopt a minute, or an hour.

efore ve might say, that the velocity ce

is me tard a mile per minute, or twenty miser nur

Varied Motion-Accelerated and Retarded Motion 1. En Monroy is that in which a body passes wer necu maces in equal times. If the spaces passed ore e times go on increasing, the motion is acceler

ated; men's the motion fram fara vies starting. or that of a body filling towards the fes

If the ces passed over ing

retarded; such is the motion of a train of cars when coming tò rest, or that of a body thrown vertically upwards.

When the spaces passed over in equal times are continu ally increased or decreased by the same quantity, the motion is uniformly accelerated, or uniformly retarded. The mo tion of a body falling in a vacuum, is uniformly acceler. ated; that of a body shot vertically upwards in a vacuum, is uniformly retarded.

The velocity of a body having varied motion at any time, is the rate of the body's motion at that time. In varied motion the velocity is continually changing.

Forces, Powers, and Resistances.

19. If a body is at rest, any cause which tends to set it in motion, is called a Force; if a body is in motion, any cause which tends to make it move faster, or slower, or to change its direction, is called a Force.

A Force, then, is any cause which tends to change the state of a body, with respect to rest or motion.

The attractions and repulsions between the molecules of bodies are forces; the muscular efforts of men or animals, employed in accomplishing any kind of work, are forces; the elastic efforts of. gases and vapors are forces.

Forces which act to produce motion are called Powers; those which act to prevent or destroy motion are called Resistances. The effort of steam employed in moving a train of cars is a power, whilst friction and the inertia of the air, which tend to retard the motion, are resistances. Powers tend to accelerate motion, and are for that reason called Accelerating Forces. Resistances, on the contrary, tend to retard motion, and are for that reason called Retarding Forces.

Examples. Define uniformly accelerated and uniformly retarded motion. Examples. (19.) What is a Force? Examples. Define Powers and Resistances. Examples. By what other names may they be called?

Distinctive Characteristics of Fronces.

20. In order the the effer faire pletely understood tire ting BS 1-1.5.

The point of aggdemia da

exerts its action Thu in Fram

drawing a wage de fr xened it point of applicatite u 1

T

The intensity of a free is the evil vi i w: thus, in the sume example as edes tie fraser of the force exerted is the centre 0 e 11 coming the resistive of the L

The intensity of a fem à mestre in poml: five a force of fifty pafox2 1 1 2

of fifty pounds. Thong dafnemerecend by a distance way of the

(20) What three clemens deserome & heat elpu put of maden. E The line of directia. The ateity

зented? Example

retarded; such is the motion of a train of cars when coming to rest, or that of a body thrown vertically upwards.

When the spaces passed over in equal times are continu ally increased or decreased by the same quantity, the motion is uniformly accelerated, or uniformly retarded. The motion of a body falling in a vacuum, is uniformly acceler ated; that of a body shot vertically upwards in a vacuum, is uniformly retarded.

The velocity of a body having varied motion at any time, is the rate of the body's motion at that time. In varied motion the velocity is continually changing.

Forces, Powers, and Resistances.

19. If a body is at rest, any cause which tends to set it in motion, is called a Force; if a body is in motion, any cause which tends to make it move faster, or slower, or to change its direction, is called a Force.

A Force, then, is any cause which tends to change the state of a body, with respect to rest or motion.

The attractions and repulsions between the molecules of bodies are forces; the muscular efforts of men or animals, employed in accomplishing any kind of work, are forces; the elastic efforts of. gases and vapors are forces.

Forces which act to produce motion are called Powers; those which act to prevent or destroy motion are called Resistances. The effort of steam employed in moving a train of cars is a power, whilst friction and the inertia of the air, which tend to retard the motion, are resistances. Powers tend to accelerate motion, and are for that reason called Accelerating Forces. Resistances, on the contrary, tend to retard motion, and are for that reason called Retarding Forces.

Examples. Define uniformly accelerated and uniformly retarded motion. Examples. (19.) What is a Force? Examples. Define Powers and Resistances. Examples. By what other names may they be called?