20 SCIENCE OF COMMON THINGS. 



TV hen a body weighs the most Systems of weights. 



Because the attractive force of the sun, on account 

 of its greater magnitude, far exceeds the attractive 

 force of the earth. 



11Q Why witt a mass of iron weigh less on the top of a high mountain 

 than at the level of the sea f 



Because the force of the earth's attraction is less at the 

 top of the mountain than at the sea-level ; the attrac- 

 tion (and, therefore, the weight of a body) being greatest 

 at the surface of the earth, and decreasing upward, as 

 the square of its distance from the centre of the earth 

 increases. 



A ball of iron, weighing a thousand pounds at the level of the sea, 

 ~y would be perceived to have lost two pounds of its weight if taken to the 

 top of a mountain four miles high, a spring balance being used. 



113 Where witt a body weigh the most on the surface of the earth ? 



At the poles of the earth, for at these points the 

 attractive power is greatest. 



It must be remembered that the earth is not a perfect sphere, but flat- 

 tened at the poles : consequently, the poles of the earth are nearer the 

 centre of attraction (i. e. the centre of the earth) than any other point on 

 its surface. 



114 Where witt a body weigh the least on the earths surface f 



At the equator, for there the attractive power is less ; 

 the surface at this point being the most distant from 

 the centre of the earth. 



115 What would be the weight of a body carried to the centre of the 

 earth f 



It would have no weight ; for the attraction of gra- 

 vitation acting equally in every direction, no eftect 

 would be produced ; and the body w r ould be fixed as 

 if sustained by a number of magnetic points. 



116 What two systems of weights are employed in tlie United States 

 and Great Britain f 



Troy weight and avoirdupois weight. 



117' What is Troy weight used for } and from whence does it derive its 

 name? 



Troy weight is used for weighing gold and silver. 

 It derives its name from the ancient designation of 

 London, Troy Novant, or from Troyes, in France, 

 where it was first adopted in Europe. It has existed 

 in England from the time of Edward the Confessor. 



