24 



NATURAL PHILOSOPHY. 



[Lesson X- 



respecting heat, we shall be able to understand how the heat-measurer or thermometer is 

 constructed, and in what manner it acts, and many other phenomena connected with it. 

 ll:ill:un observed, that when all the forces acting upon a body counteracted each other, 

 it remained in a state of equilibrium, or was balanced; and having gained a knowledge 

 of the properties of matter, he applied it to the study of the laws of motion and forces. 



QUESTIONS. 



121. T. What is MOTION ? 



P. It is the very reverse of rest, and 

 means the change of place, which may be 

 uniform or variable, according to its rate 

 or relative velocity. 



122. T. What do you mean by rest; 

 because it appears a direct contradiction 

 to the laws ot nature ? 



P. There is no such thing as absolute 

 rest, because the earth upon which we stand 

 is always in a state of motion, therefore the 

 term is only relative, and not positive. 



123. T. What do you mean, by saying 

 that rest is only relative ? 



P. If I let a stone fall from my hand, 

 it will remain in the same place and in 

 the same position where it fell, until re- 

 moved by some applied force ; and, there- 

 fore, we say it is now at rest, but we mean 

 to say that it is at rest only as regards the 

 earth, because we know that there is no 

 such thing as absolute rest in creation. 

 The earth, the planets, and the sun move ; 

 in fact, all creation moves. 



124. T. Which is more natural, for a 

 body to be in a state of rest or motion ? 



P. Rest, of course ; because bodies 

 must be arrested in their progress by 

 atmospheric pressure, friction, the attrac- 

 tion of the earth, or their own gravity. 



1 25. T. What causes motion ? 



P. Its causes are various, but it is cer- 

 tain that all bodies must continue at rest 

 unless acted upon by some force or forces 

 -ed upon them. In the animal 

 economy it is produced by a principle of 

 life, which we do not quite understand; 

 inanimate bodies are put in motion or 

 arrested by certain forces or powers. 



126. T. Is motion variable or constant ? 

 P. It is variable, and must ever be so, 



although its degree of velocity is in accord- 

 ance with the force applied. Thus, if I 

 push a cart with one hand, it will move 

 slowly ; with two hands faster, but if six or 

 seven men all push at once, and with con- 



siderable force, its velocity will be aug- 

 mented. 



127. T. What do you mean by velocity? 

 P. It is the degree of speed that bodies 



possess, which is increased by the space or 

 distance passed over. 



128. T. But does not this speed vary ? 

 P. Unless any impediment is presented 



to moving bodies their velocity is uniform, 

 and their speed is calculated by the time 

 they take to pass over a given distance or 

 space. Thus, any body that takes thirty 

 seconds to pass over sixty feet, is said to 

 possess a velocity of two feet per second. 



129. T. Is motion always uniform ? 

 P. No ; it may be retarded or accele- 

 rated. 



130. T. Explain these terms. 



P. If it decreases gradually, it is said 

 to be retarded ; if it increases, it is accele- 

 rated, and the force that governs or regu- 

 lates this, is called the retarding or accele- 

 rating force. 



131. T. How are forces distinguished ? 

 P. They are called instantaneous or coa- 



tinued forces. 



132. T. What is the difference ? 



P. An instantaneous force is an im- 

 pulse ; it is sudden, like a blow, but a 

 continued force has always the same degree 

 of power, and is called a constant force 

 when it has no intermission. 



133. T. Can forces vary 



T. Yes ; of course they vary when the 

 power is not constant, or when there is 

 intermission. 



134. T. What is meant by absolute 

 motion ? 



P. When any body moves towards an- 

 other body at rest, or passes it, or moves 

 from one point of space to another, it is 

 influenced by absolute motion. 



135. T. What do you mean by relative 

 motion ? 



