34 GRAVITY. WEIGHT. UNITS OF WEIGHT. 



has again come to rest upon the ground, the table, or 

 some other support. The cause of this motion of all 

 bodies towards the earth, the attractive force exerted 

 by the earth upon all bodies, has been termed Gravity, 

 and the fact that each body is subject to the action of 

 gravity is expressed by saying that each body possesses 

 Weight. The direction in which this force acts, that is 

 the direction towards the centre of the earth, is called 

 vertical] every line at right angles to the vertical direc- 

 tion is called a horizontal line. 



The greater the mass of a body, that is, the greater 

 the number of material particles which it contains, the 

 more is it attracted by the earth, the greater is the 

 force required to lift it, the greater is the violence with 

 which, when left to itself, it falls back to the earth, and 

 the greater is the pressure which it exerts upon its 

 support when at rest. The total amount of the earth's 

 attraction for a body, the magnitude of the pressure 

 which it exerts upon any support, is termed the weight, 

 or more precisely, the absolute weight, of a body. The 

 gramme -weight is a very convenient unit of weight in 

 many calculations, and therefore much used in physical 

 determinations. 



One Gramme (l gr ) is the weight of one cubic centi- 

 metre of water. 1 A weight of 1000 gr is hence the 

 weight of 1000 or a litre of water, and is called 

 Kilogramme ( l kgr ). The subdivisions of the gramme hav 

 special designations : one tenth of a gramme is calle< 

 a Decigramme ; y-J-oth of a gramme, a Centigramme; 

 j-^Viyth of a gramme, a Milligramme. A milligramme 

 is the weight of a cubic millimetre of water. 



1 The influence of temperature, in consequence of the expansion of water, 

 te discussed in the chapter on Heat. 



