CHAPTER III. 



PHYSICS THE MEANING OF PHYSICS FORCES OF NATURE GRAVITY 



COHESION CHEMICAL ATTRACTION CENTRE OF GRAVITY 



EXPERIMENTS AUTOMATON TUMBLERS. 



HAVING now introduced our readers to Science which they can find for 

 themselves in the open air, and the pursuit of which will both instruct and 

 amuse, we will proceed to investigate the Branch of Science called PHYSICS. 



PHYSICS may be briefly described as the Branch of Natural Science 

 which treats of such phenomena as are unaccompanied by any important 

 changes in the objects wherein such phenomena are observed. 



For instance, the sounding of a bell or the falling of a stone are physical 

 phenomena, for the objects which cause the sound, or the fall, undergo no 

 change. Heat is set free when coal burns. This disengagement of heat is 

 a physical phenomenon ; but the change during combustion which coal 

 undergoes is a chemical phenomenon. So the objects may be the same, but 

 the circumstances in which they are placed, and the forces which act upon 

 them, may change their appearance or position. 



This brings us at once to the Forces of Nature, which are three in 

 number ; viz., Gravity, Cohesion, and Affinity, or Chemical Attraction, The 

 phenomena connected with the last-named forms the Science of Chemistry. 

 We give these three Forces these names. But first we must see what is 

 Force, for this is very important. Force is a CAUSE the cause of Motion 

 or of Rest. This may appear paradoxical, but a little reflection will prove 

 it. It requires force to set any object in motion, and this body would never 

 stop unless some other force or forces prevented its movement beyond a 

 certain point. Force is therefore the cause of a change of " state " in matter. 



We have said there are three forces in nature. The first is Gravity, or 

 the attraction of particles at a distance from each other. We my say that 

 Gravity, or Gravitation, is the mutual attraction between different portions of 

 matter acting at all distances, the force of attraction being, of course, in 

 proportion to the mass of the bodies respectively. The greatest body is the 

 Earth, so far as our purposes are concerned. So the attraction of the Earth 

 is Gravity, or what we call Weight. 



We can easily prove this. We know if we jump from a chair we shall 

 come to the floor ; and if there were nothing between us and the actual ground 

 sufficient to sustain the force of the attracting power of the earth, we should 



