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PHYSICAL SCIENCES. 



centre of motion. As the orbit of the moon does not differ much from 

 a circle, let it be represented by mdgh, fig. 4, the earth being in C. 

 The centrifugal force arising from the velocity of the moon in her orbit 

 balances the attraction of the earth. By their joint action, the moon 

 moves through the arc m n during the time that she would fly off in the 

 tangent m T by the action of the centrifugal force alone, or fall through 

 mp by the earth's attraction alone. T n, the deflection from the tangent, 

 is parallel and equal to m p, the versed sine of the arc m n, supposed to 

 be moved over by the moon in a second, and therefore so very small that 

 it may be regarded as a straight line. T n, or mp, is the space the moon 

 would fall through in the first second of her descent to the earth, were 

 she not retained in her orbit by her centrifugal force. 



NOTE 19, p. 5. Action and reaction. When motion is communicated 

 by collision or pressure, the action of the body which strikes is returned 

 with equal force by the body which receives the blow. The pressure of a 

 hand on a table is resisted with an equal and contrary force. This neces- 

 sarily follows from the impenetrability of matter, a property by which no 

 two particles of matter can occupy the same identical portion of space at 

 the same time. When motion is communicated without apparent contact, 

 as in gravitation, attraction, and repulsion, the quantity of motion gained 

 by the one body is exactly equal to that lost by the other, but in a contrary 

 direction ; a circumstance known by experience only. 



NOTE 20, p. 5. Projected. A body is projected when it is thrown : 

 a ball fired from a gun is projected ; it is therefore called a projectile. 

 But the word has also another meaning. A line, surface, or solid body, 

 is said to be projected upon a plane, when parallel straight lines are 

 drawn from every point of it to the plane. The figure so traced upon a 

 plane is a projection. The projection of a terrestrial object is therefore its 

 daylight shadow, since the sun's rays are sensibly parallel. 



NOTE 21, p. 5. Space. The boundless region which contains all 

 creation. 



NOTE 22, pp. 5, 11. Conic sections. Lines formed by any plane cutting 

 a cone. A cone is a solid figure, like a sugar-loaf, fig. 5, of which A is 



Fig. 6. 



the apex, A D the axis, and the plane B E C F the base. The axis may or 

 may not be perpendicular to the base, and the base may be a circle, or 



