4 GRAVITATION. 



It is my purpose to-day to make you ac- 

 quainted with some of these powers ; not the 

 vital ones, but some of the more elementary, 

 and, what we call, physical powers ; and, in the 

 outset, what can I do to bring to your minds a 

 notion of neither more nor less than that which 

 I mean by the word power or force ? Suppose 

 I take this sheet of paper, and place it upright 

 on one edge, resting against a support before 

 me (as the roughest possible illustration of 

 something to be disturbed), and suppose I then 

 pull this piece of string which is attached to it. 

 I pull the paper over. I have therefore brought 

 into use a power of doing so the power of 

 my hand carried on through this string in a 

 way which is very remarkable when we come 

 to analyse it; and it is by means of these powers 

 conjointly (for there are several powers here 

 employed) that I pull the paper over. Again, 

 if I give it a push upon the other side, I bring 

 into play a power, but a very different exertion 

 of power from the former; or, if I take now 

 this bit of shell-lac [a stick of shell-lac about 

 12 inches long and 1^ in diameter] and rub it 

 with flannel, and hold it an inch or so in front 

 of the upper part of this upright sheet, the 



