Measurement of Weight 47 



27. It is interesting to note that, although the 

 mass of a body is constant, and its weight at any place 

 is proportional to its mass, the weight of a body is 

 not constant, but undergoes slight variations when its 

 situation on the Earth's surface is changed. At the 

 Poles a body would weigh about one-half per cent, 

 more than at the Equator, e.g. a body weighing 16 Ib. 

 as shown by a spring-balance at either Pole, would 

 only indicate about 15 Ib. 15 oz. when similarly tested 

 at the Equator. In explanation of this we may briefly 

 say that it is due to the combined effect of two causes : 

 (a) The Polar radius of the Earth is less than the 

 Equatorial. Thus the distances between the centres 

 of gravity of the body and the Earth vary, and the 

 force exerted between two bodies is increased when 

 the distance between their centres of gravity is lessened 

 (see Ch. vi). (6) At the Equator the Earth's surface 

 is rotating at over 1000 miles an hour and bodies 

 resting on, or situated near, its surface tend to leave 

 that surface on account of what is usually termed 

 " centrifugal force ", thus appearing slightly lighter 

 than in higher latitudes where the rate of rotation, 

 and hence the centrifugal force, is less. 



28. Let us now suppose that, instead of placing 

 a cubic inch of lead on the pan of our spring-balance, 

 we gradually pour tea into the pan. The spring will 

 stretch as before, and, when a certain quantity of tea 

 has been poured in, the elongation produced will be 

 exactly the same as that produced by the leaden cube. 

 From what has already been said, it is clear that the 

 weight of the tea now in the pan is equal to that of 

 one cubic inch of lead. Applying the principle that, 

 at any given place, the weight of a body is proportional 

 to its mass, we arrive at the conclusion that the masses 



