much more clearly than any drawings can. We know nothing better. For explaining the Rules for Mensuration or Solid Measurement, they afford the only proper means. Price, $1 25 to $1 75.

37. The Numeral Frame was designed for Primary Schools, but has proved of nearly equal service in intermediate and Grammar Schools; wherever young pupils require illustrations to enable them fully to comprehend operations with abstract mathematical quantities, this frame furnishes the readiest mode of giving the desired instruction. Price

38. Teachers' Guide, a Manual to accompany the Apparatus, revised by the Educational Department, U. C. Price 50 cts.

39. Price for the entire set, packed in a neat box, containing the following articles, $20:

The Mechanical Powers are certain agents, applied to machinery constructed in accordance with the laws of nature, to enable man to overcome resistance in raising weights, moving bodies, &c.; they consist of the Lever, the Wheel and Axle, the Pulley, the Inclined Plane, the Wedge and the Screw, although by these contrivances man is enabled to perform any work in proportion to the speed of power he can command, yet, the advantage gained is simply an advantage of pressure, not of work; for what is gained in pressure is lost in speed; whenever power is gained something is lost to counter-balance it, practical men express this law by saying "what

you gain in power you lose in speed." If a man can lift a hundred pounds with his hands alone, he may be able to lift one thousand pounds by the aid of a lever, but it will take him ten times as long to lift it through the same space.

The real advantage to be derived from the use of these models, is not only a practical knowledge of the appliances of the simple machines, but, to impress upon the mind of the pupil that one man, by working proportionably

SET OF MECHANICAL POWERS-(CANADIAN MANUFACTURE.)

Gilz

COMPOUND LEVERS AND WEDGE.

longer, can perform the work of many men acting at once, whom it might be impossible from various causes to bring together at one time. To illustrate this a printer, by means of a screw, can press a sheet of paper against the type, so as to take of a clear impression; to do which without the aid of the press, the pressure of men would scarcely be sufficient, besides fortynine out of the fifty men would be idle, except just at the instant of pressing. Hence the screw may be said to do the work of fifty men.

40Jchnson's Philosophical Diagrams.-Natural Philosophy.. Rotary Map-stand.-This stand contains six maps or a continuous web of cloth same as engraving No. 1, page 9. Natural Philosophy Map-case same as engraving No. 6, page 11.

List of Plates.

I. Properties of Bodies, containing | IV. Hydraulics, containing 36 Dia

II. Mechanical Powers, containing V. Physiology, plate 1, containing

47 Diagrams.

III. Hydrostatics, containing 28
Diagrams.

28 illustrations.

VI. Physiology, plate 2, 42 illustrations completing the subject.

Size of map 4 feet 2 inches by 3 feet 6 inches; size of stand 6 feet 9 inches by 4 feet 8 inches. Price of the six, already published, in the Rotary stand $25. Price singly, with hand-book, $2.38.

41. Chambers' Scientific Charts of Natural Philosophy.-L Laws of Matter. II. Laws of Motion. III. Laws of, etc. Price $1 each, as published, on rollers.

COLLISION BALLS,

42. Mechanical Powers, mahogany frame, with three sets brass pulleys, with silk cord, and balanced, two sets brass weights, simple and compound levers, wheel and axle, screw and lever with nut, screw as an inclined plane, wedged in sections, inclined plane with arc and binding screw carriage, ship's capstan, &c., complete (as above.) Price $16.

43. Collision Balls, mahogany stand, six 14 inch balls. Price $3.50.

44. Bent Lever, convertable into a Toggle Joint Press with weights, and description. Price $1.50.

45. Inertia Apparatus, (a card being projected by the spring and leaving the ball upon the pillar.) Price 60 cts.

46. Centre of Gravity Apparatus.—A set of eight illustrations for centre of gravity, viz., 1, 2, 3, blocks of various figures, with centres of gravity, and suspension 4, two balls on rod, with centre of gravity; 5 Leaning Tower of Pisa, with two centres of gravity; 6, loaded wheel on

INERTIA APPARATUS.