MECHANICAL PHYSICS.

Tuapeka Times, Volume XVI, Issue 954, 11 July 1883, Page 3

MECHANICAL PHYSICS.

Professor Shand began a course of le. tures on the above subject to teachers on Saturday last, at the University, Dunedin. A great concourse of teachers attended from various pan sof Otago.- The lecturer explained that i here are three essential attributes of matter — (1) Extension, (2) impenetrability, (3) inertia. Extension is evident and easily understood. The phenomena of chemical combination, solution, and absorption appear at first sight to conflict with the principle of impenetrability, but in reality they supply proof of the existence of the attribu c. Impenetrability refers rather to tbe ultimate particles of matter than to the corporate mass. A good example of chemical combination is afforded in the mixture of water with sulphuric acid; heat is evolved by the uction of chemical combination. To illustrate absorption, take a glass of water and add some sugar ; the addition of the sugar does not increase the bulk of the liquid, and an apparent anomaly results ; but exactly the same principle is involved if one glass be fill d with marbles, some small shot put in another, and the shot poured in with the marbles ; it will find room in the interstices and the bulK of the mixture would not be increased ; the marbles stand in the same position as th« molecules of the liquid to one another ; the ab* sorption of the shot by the mass of marbles therefore illustrates the dissolution of a solid in a liquid. An application to the air-then mometer indicates that the result is attended with coldness when the sugar is added to water, and more appreciably when sulphate of soda takes the place of sugar. The absorption of gases by solids and liquids is an im portant phenomenon. Fishes breathe the oxygen which is held in water by osmosis : ammonia is capable of being absorbed by water to a remarkable extent ; at freezing temperature, water will absorb 1000 limes its bulk of ammonia and at the ordinary temperature of a room, 700 times. The condensation of ammonia in water causes heat. Hydrogen is capable of being largely absorbed by palladium and platinum : A coil of cold spongy platinum put in a jet of hydrogen absorbs hydrogen, while such a great heat is given out that the jet of hydrogen inflames. The combination of a liquid and a gas evolves heat ; the solution of a salt in a liquid gives cold ; the absorption of a gas in metal gives off heat. By inertia it is understood that every body (particle or aggregate of particles) if at rest, will remain for ever at rest unless acted upon by some outside body, and if in motion will continue in motion at the same rate until affected by some outside body. This principle may be explained under several headings— (l) Moving force ; (2) resisting force ; (3) centrifugal force ; (4) invariability of rotation and oscillation ; (5) Foiicalt'B experiment. It was not until the time of Galileo that a true idea of inertia was conceived. The ancient conception was that matter had a sort of inherent — so to speak — laziness, the tendency of which always was to resist motion ; but really it takes as much force to stop a moving body as it does to move it. Experiment shows that a weak force acting for a long time will have the same effect as a powerful force acting for a short time. In gunnery, it is necessary that a powerful force shall act on the cannon-ball while it is in the bore ot the cannon, and at the instant when propulsion ceases, the cannon-ball has a velocity of from 1,500 to 2,000 feet per second. If the mass of tbe body be divided by 2, the moving force may also be divided by 2 with equal effect. If the moving force be represented by F ; the mass by M ; the accelerating force by f ; then F varies directly as f ; F varies directly as M j F varies directly as M x f j and Fis equivalent toMif, A pendulum being once set swinging would continue to oscillate in the same plane for ever were it not for opposing forces, which are always prtsent, and which we cannot get entirely rid of ; but the more we abate these forces the nearer we come to an exemplification of continuous motion. Heavenly bodies do not experience these resistances, and by observing them we are enabled to form a true idea of the inertia of motion. Centrifugal force is the result of inertia ; it is much greater than tbe force of gravity. If a weight be suspended on an elastic string, and twirled at the end of it, the elastic string will yield to centrifugal force and so stretch, and this, though gravity would fail to stretch it in the least. Centrifugal force may overcome gravity, as when a glass of water is placed in s stirrup and twirled rapidly ; the water does not yield to the force of gravity, but being under the influence of the stronger centrifugal force, is not spilt during rotation. Watt took advantage of this force when regulating the pressure of steam i n his steam : engjne. The air jnsicle a humming top, after having a

circular motion imparted to it, obeys centri fugal force and flies out through reeds so placed as to cause a humming noise. In cyclones, similarly, there is a lessening of pressure in (he centre, and a consequent rush of air to fill up tbe vacuum. A spnning holy, such as a lop keeps the plane of rotation unchanged. Several interesting experi" ments wer performed by Professor Shand to illustrate this principle, a bicycle-wheel being broughtin'o requisition to show that it will keep up and resist any attempt to alter the plane of rotation. Foucalt's experiment wns performed, and mathematical data given, showing that a pendulum once set in motion will continue to oscillate in the same pane, and will indicate the progress of the earth's daily revolution. The lecturer stated that the following problems will be worked out by him next Saturday ; and he would like those who found it convenient, to try them during the week :— (1) A body weighing lOlbs is whirled at the extremity of a string 6ft. long, which is just oapable of sustaining a weight of 1501bs ; find the velocity at the instant the string breaks. (2) A locomotive passing at the rate of 25 miles an hour along a curve of 360 yds radius has a centrifugal force of 1 ton, what is the weight of the locomotive ? (3) What would the period of the earth's revolution round its axis require to be in order that the centrifugal force at the equator may be equal to the force of gravity, viz., 32ft. per second