GRAVITATION. 27 



3. GRAVITATION. 



Action of gravitation. All masses attract each other and this 

 attraction is known as gravitation. The action of gravitation 

 between the thousands of heavenly bodies moving in the uni- 

 verse is to be considered by astronomy, but some of the phen- 

 omena caused by the mutual attraction of the substances com- 

 posing the earth are of importance for our present considera- 

 tions. 



Such phenomena caused by gravitation are the falling of sub- 

 stances, the flowing of rivers, the resistance which a substance 

 offers on being moved, lifted, or carried. A body thrown up 

 into the air or deprived of its support will fall back upon the 

 earth. In this case the attraction of the earth for the substance 

 has caused its fall. It might appear that in this case the attrac- 

 tion was not mutual, but exerted by the earth only; it has been 

 proved, however, by most exact experiments, that there is also 

 an attraction of the falling substance for the earth, but the 

 amount or the extent of this attraction is in proportion to the 

 mass of the bodies, and consequently too insignificant in the 

 above case to be noticed. 



Weight of a substance is an expression used to indicate the 

 amount of attraction which the earth exerts for that substance. 

 Here again the attraction of the substance for the earth is not 

 taken into consideration. All our weighing is a comparison 

 with, or measurement by some standard weight, such as pound, 

 ounce, gramme. 



Specific weight or specific gravity denotes the weight of a body, 

 as compared with the weight of an equal bulk or equal volume 

 of another substance, which is taken as a standard or unit. 

 The word density is frequently used for specific weight, as den- 

 sity means comparative mass. By the density of a body is con- 

 sequently meant its mass (or quantity of matter) compared with 

 the mass of an equal volume of some body arbitrarily chosen 

 as a standard. The standard or unit adopted for all solids and 

 liquids is water at the temperature of 60 F. = 15.5 C. 



Specific weight is generally expressed in numbers, which de- 

 note how many times the weight of an equal bulk of water is 



