98 MASS AND FORCE. [CHAP. V. 



A knowledge of the acceleration g can then be used to deduce 

 the mass of the Earth, and Astronomical observations can then be 

 applied to determine the masses of the Sun and the Planets. It is 

 only in this way that these masses can be assigned. 



95. Measurement of Mass. To any particular body we 

 may assign the mass unity, then the mass of any other body has a 

 ratio to the mass of the chosen body which is a real positive 

 number. This number measures the mass of the body. 



The unit of mass is not expressible in terms of the units of 

 length and time. 



In British measure the unit of mass is the mass of a particular 

 lump of platinum, kept in the Royal Exchequer, and defined 

 by Act of Parliament to be a pound. The mass of any body 

 which when weighed against this standard pound would balance it 

 is a mass of one pound. 



In the C.G.S. system of units the unit of mass is the y^th part 

 of the mass of a certain lump of platinum known as the "Kilo- 

 gramme des Archives," made by Borda, and is defined to be a 

 gramme. 



The mass of a cubic centimetre of pure water at a temperature 

 of 4 Centigrade under a barometric pressure represented by 760 

 millimetres of mercury is very approximately equal to 1 gramme. 



96. Quantity of matter in a body. The mass of a body is 

 the quantity of matter in the body. 



This is a definition of quantity of matter. 



We are accustomed to estimate the quantities of matter in 

 bodies by weighing them, that is we think of one body as contain- 

 ing more or less material than another according as, when weighed, 

 the former weighs more or less than the latter. This method 

 manifestly cannot be applied to determine the quantity of matter 

 in the Earth, the Moon, or any other celestial body. 



The acceleration with which a free body falls towards the 

 Earth is variable with latitude, and increases in passing from the 

 Equator to the Poles. Thus the weight of a body, measured by 

 the product of this acceleration and the mass of the body, is 

 variable with latitude, and increases as we pass from the equator 

 to the poles. 



