192 POPULAR LECTURES AND ADDRESSES. 



the same time, gives it increased velocity, and so 

 on. Now, instead of a mass of iron, I might hang 

 up a mass of lead, or a mass of wood, to test the 

 equality of the mass by the equality of the motion 

 which is produced in the same time by the action 

 of the same force, or in equal times by the action 

 of equal forces. Thus, quite irrespectively of the 

 kind of matter concerned, we have a test of the 

 quantity of matter. You might weigh a pound of 

 tea against a pound of brass without ever putting 

 them into the balance at all. You might hang up 

 one body by a proper suspension, and you might, 

 by a spring, measure the force applied, first to the 

 one body, and then to the other. If the one body 

 is found to acquire equal velocity under the in- 

 fluence of equal force for equal times as compared 

 with the other body, then the mass of the one is 

 said to be equal to the mass of the other. 



I have spoken of mutual forces between any 

 two masses. Let us consider the weight or heavi- 

 ness of a body on the earth's surface. Newton 

 explained that the attraction of the whole earth 

 upon a body for example, this 56 pounds mass 

 of iron causes its heaviness or weight. Well, 

 now, take 56 pounds of iron here, and take a 

 mass of lead, which, when put in the balance, 

 is found to be of equal weight. You see we 

 have quite a new idea here. You weigh this 

 mass of iron against a mass of lead, or to 

 weigh out a commodity for sale ; as, for instance, 



