THE FORCE OF GRAVITATION 9 



which I shall have to refer. For instance, here is water 

 it is heavy; but let us examine it with regard to the atnotwit 

 of its heaviness or its gravity. I have before me a little 

 glass vessel and scales [nearly equipoised scales, one of 

 which contained a half-pint glass vessel], and the glass 

 vessel is at present the lighter of the two ; but if I now take 

 some water and pour it in, you see that that side of the 

 scales immediately goes down; that shows you (using com- 

 mon language, which I will not suppose for the present 

 you have hitherto applied very strictly) that it is heavy, 

 and if I put this additional weight into the opposite scale, 

 I should not wonder if this vessel would hold water enough 

 to weigh it down. [The lecturer poured more water into 

 the jar, which again went down.] Why do I hold the 

 bottle above the vessel to pour the water into it? You will 

 say, because experience has taught me that it is necessary. 

 I do it for a better reason because it is a law of nature 

 that the water should fall toward the earth, and therefore 

 the very means which I use to cause the water to enter 

 the vessel are those which will carry the whole body of 

 water down. That power is what we call gravity, and you 

 see there [pointing to the scales] a good deal of water 

 gravitating toward the earth. Now here [exhibiting a small 

 piece of platinum^)] is another thing which gravitates 

 toward the earth as much as the whole of that water. See 

 what a little there is of it; that little thing is heavier than 

 so much water [placing the metal in opposite scales to the 

 water] What a wonderful thing it is to see that it re- 

 quires so much water as that [a half-pint vessel full] to 

 fall toward the earth, compared with the little mass of 

 substance I have here! And again, if I take this metal 

 [a bar of aluminium ( 2 ) about eight times the bulk of the 

 platinum], we find the water will balance that as well as it 

 did the platinum ; so that we get, even in the very outset, 

 an example of what we want to understand by the words 

 forces or powers. 



I have spoken of water, and first of all of its property of 

 falling downward : you know very well how the oceans sur- 



1 Platinum, with one exception the heaviest body known, is 21% time* 

 heavier than water. 

 * Aluminium is 2% times heavier than water. 



