been successful. To adapt such suggestions as these supply, and to combine them with the structural schemes which ingenuity or necessity furnished, seemed to satisfy all the wants of men until the steam-engine enlarged its usefulness by giving a rotary motion in addition to a reciprocating rectilineal one. Watt did this in 1770, and from that time we began to date our mechanical progress.

Extract from a lecture on Mechanics by the
REV. A. RIGG, M.A.

1. MENAI STRAITS, straits between Carnarvon and Anglesea in Wales. The straits are crossed by two famous bridges, a suspension one built by Telford, and a tubular railway-bridge. The latter formed of hollow iron tubes fitting one into another.

2. CAMERA OBSCURA, a dark chamber or compartment in which are exhibited the images of external objects on a white surface placed within it. These images are depicted by the rays of the sun entering through a small hole, and are perfect miniature copies.

3. VICE, an instrument consisting of two jaws, closing by a screw, for holding anything tightly, used by smiths, carpenters, &c.

4. STATICS, the science which treats of the properties of bodies when standing still, or at rest.

5. THEODOLITES, instruments used for surveying, observing, and measuring heights and distances.

6. LABORATORY, a place for labouring or working in ; a chemist's workroom. (Lat. labor, labour, toil.) 7. RECTILINEAR, in straight lines. linea, a line.)

(Lat. rectus, straight, and

THE CENTRE OF GRAVITY.

PART I.

EVERY atom of matter is equally attracted to the earth. When the atoms form a solid, they cannot separately act, but, as it were, concentrate the whole weight of the body at a point which, if supported or suspended, will balance, hold in equilibrium, or keep in a state of

rest the entire mass: this point is called the centre of gravity.

If a stick be laid across a finger, one particular spot will be found, where it will balance and remain at rest; that part is the centre of gravity in the stick. The bulk and density on both sides of this point of the stick will be equal; and thus, by a sufficient support at this part, the attraction of the earth is successfully resisted, for in any other position the stick would fall to the ground. The centre of gravity appears, then, to be the point which seeks the lowest level, and exists in everything, of whatever shape it may be, in the universe.

By lifting a solid body at this point, the whole is lifted; or by preventing this part being moved, the mass is kept at rest.

A rod of iron having equal quantities of matter throughout its length, will have its centre of gravity exactly in the middle. If a piece of wood, or any other substance, in the shape of a triangle, be suspended from

Fig. 1.

each of its angular points, so as to swing freely, and a string with a plummet attached, it will exactly cut the triangle in two from each point; and in the centre, where the lines cross each other, will be the point of gravity. By marking the lines with a pencil or piece of

chalk, the exact spot can be found. A line drawn from the centre of gravity direct to the earth is called the line of direction. This is only an imaginary line, but one of great importance in the concerns of life; for if a square or angular figure be placed upon the ground, and this line does not fall within its base, it will fall over. This is most clearly illustrated

in the case of a loaded cart. Suppose a cart (see figs. 2

and 3), having lead or iron in it, passes along an uneven

other, and the centre of gravity be at the part marked at the back of the cart, then the line of direction falls within the wheel, and the cart will not upset; but if it be so loaded as is done with hops and wool, then the centre of gravity is moved much higher up, and the line of direction falling outside the wheel, which in this case is the base, the cart will turn over. Thus, then, the nearer the centre of gravity is to the base, and the line of direction in its middle, the steadier an object is. This is what gives such prominence to those colossal erections of the ancient Egyptians, the Pyramids.

A ship at sea will upset in a squall from not having sufficient ballast,1 so that the centre of gravity must be low down to counterbalance the weight of masts and rigging; while if the vessel be loaded only close to the keel, it rolls about most unpleasantly.

In a steamboat having the deck laden with passengers, as seen on the Thames and elsewhere, the greatest cause to anticipate an accident lies in some event occurring, from which the people might start upon their feet and rush to one side of the vessel, when the centre of gravity being changed, it would capsize, and the pleasure-seeking crowd be launched into the stream.

Accidents in boats are of continual occurrence, in consequence of people, instead of sitting or standing steadily, or lying at the bottom of a little bark, rushing to one side on the slightest cause for fear; and thus creating the very mischief which they desire to avoid.

In a round ball equally made, the centre of gravity is in the middle, but the base of all round bodies is a mere point. This is what enables them to move with little force, so that even a gust of wind whirls them along a smooth surface.

The celebrated belfry or round tower at Pisa, in Italy,

encased with marble, is 190 feet high, and leans to one side twelve feet beyond the foundation; but the line of direction is within the base, and therefore it safely stands, to astonish and frighten the gazing traveller.

At Bologna there is a tower 165 feet high, having an inclination of seven or eight feet from the perpendicular, yet it stands firmly, for the same reason that applies to the Pisa Tower.

PART II.

In scale-beams 2 the centre of gravity is made in the same place as the centre of motion, from which arises the utility of the machine, as it will rest according as it may be adjusted in the placing of weights and materials upon each side.

Rope-dancers, that they may balance themselves on a narrow foundation, use a long pole loaded at each end with lead; this they hold across the rope, and fixing their eyes upon some object, perceive when their centre of gravity tends either to one side or the other, which they recover by a movement of the pole, and thus keep the centre of gravity over the base.

Among other devices of itinerants3 to gain a precarious livelihood is that of dancing upon stilts, imitating the unstable rollings of a drunken man, and hopping upon one long leg or pole. This is all cleverly performed by a due attention to the preservation of the centre of gravity.

When a person stands on one leg, the other leg is held up behind to adjust the centre of gravity. This is beautifully shown in the famous statue of Mercury, who is poised on the toes of one foot, while the other is elevated behind. Opera-dancers while raised on one