APPENDIX A 



[Extracts from a Lecture on " The Tides" given to the 

 Glasgow Science Lectures Association, not hitherto pub- 

 lished, and now included as explaining in greater detail 

 certain paragraphs of the preceding Lecture.} 



(i) Gravitation. The great theory of gravitation put 

 before us by Newton asserts that every portion of matter in 

 the universe attracts every other portion ; and that the force 

 depends on the masses of the two portions considered, and 

 on the distance between them. Now, the first great point 

 of Newton's theory is, that bodies which have equal masses 

 are equally attracted by any other body, a body of double 

 mass experiencing double force. This may seem only what 

 is to be expected. It would take more time than we have to 

 spare were I to point out all that is included in this statement ; 

 but let me first explain to you how the motions of different 

 kinds of matter depend on a property called inertia. I 

 might show you a mass of iron as here. Consider that if 

 I apply force to it, it gets into a state of motion; greater 

 force applied to it, during 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 influence of equal force for equal times as coro- 



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