500 PHYSIOLOGICAL PHYSICS. tchap. XLII. 



of 10 pounds were raised by a pulley arrangement 

 by means of a weight of 1 pound, the weight would 

 only rise 1 foot for every 10 feet of rope pulled in. 



CHAPTER XLII. 



GRAVITY. 



Gravity. The tendency which all bodies have 

 to fall to the earth is due to the action of gravity, 

 i.e. the mutual attraction exerted between the earth 

 and the body. This tendency is a particular exem- 

 plification of the universal law that all material particles 

 attract one another. The direction in which gravity 

 acts is always the vertical at the place ; hence, to 

 determine the vertical, a weight is permitted to hang 

 freely from the end of a string, the plumb line. The 

 line of the string gives the vertical. 



Centre of gravity. It is the force of gravity 

 attracting bodies towards the centre of the earth that 

 gives them weight. If a body could be entirely re- 

 moved from the influence of gravity it would have no 

 weight. The force of gravity acts on each particle 

 forming the mass of the body, attracting it with a 

 certain degree of force. A solid body may, therefore^ 

 be considered as under the influence of a vast number 

 of forces, each particle being separately solicited by 

 gravity; that is to say, the body may be considered as 

 operated on by a number of parallel forces, all acting 

 in the same direction. Now we have seen that 

 parallel forces are capable of being compounded into 

 one force equal in amount to the sum of the different 

 parallel forces, and acting through one point in the 

 bodv. The action of gravity on all the separate 



