OBJECTS.] INTRODUCTORY. 3' 



water is put into the one measure the scale will 

 descend, if the balance is a good one ; showing that 

 the drop has weight. If the measures are graduated 

 accurately, then whatever volume of water is put into 

 one, an exactly similar volume of the same water must 

 be put into the other to make the beam level. This 

 obviously means that the same volume of water 

 under the same circumstances always has 

 the same weight. 



In 18 it was said that bodies tend to move to- 

 wards one another with a relative velocity l which is 

 inversely proportional to the quantity of matter which 

 they contain. But how are we to measure quantity of 

 matter? Is it to be estimated by the space which it 

 occupies ; that is, by its volume ? or are we to esti- 

 mate the quantity of matter in a body by its weight ? 

 You will soon learn that the volume of all bodies 

 is constantly changing in correspondence with the 

 changes in the pressure exerted by other bodies, but 

 more especially in correspondence with the changes 

 of temperature to which they are subjected ; while 

 the weight of the same body, at the same point on 

 the earth's surface, never alters. Hence we may 

 take the weight of a body as a measure of the quantity 

 of matter which it contains ; and it follows that, for the 

 same weight, the larger the volume of a body the less 

 matter it contains proportionally to its volume, and 

 the less the volume, the more matter it contains. The 



1 Velocity, or swiftness, is measured by the distance over which 

 a body travels in a given time. Of two bodies, one of which 

 travels through one foot in a second, while the other travels 

 through two feet, the latter has the greater relative velocity 

 3* 



