n8 POPULAR LECTURES AND ADDRESSES. 



by building it up from small to large, let him learn 

 the capacity of it in electrostatic measure. Let 

 him begin with two plates or cylinders, or a sphere 

 enclosed within a concentric sphere, and go on 

 multiplying till he gets a capacious enough con- 

 denser of which he knows, in electrostatic measure, 

 the electrostatic capacity. This, again, is a line. 

 Now let him take the rectangle of those two lines, 

 and construct the equivalent square let him, 

 geometrically or arithmetically, take the square 

 root of the product of the two lines and let him 

 observe the period of electric oscillation that I 

 have spoken of. Let him imagine the hand of 

 a watch, going once round in the observed period. 

 He has good magnetic eyes, and he sees the 

 electro-magnetic oscillation, or he has appliances 

 by which he can test it : the thing has been done. 

 He sets in motion a little piece of wheel-work, with 

 a hand going once round in the period of the 

 oscillation. Now for a moment let him imagine 

 that hand to be equal in length to the square root 

 of the product of those two lines several million 

 centimetres, or several thousand kilometres, if 



