CHAP vin. PHYSICAL PRINCIPLES. 65 



changed considerably, and in a direction opposite to 

 that of the earth's rotation. At the North Pole a circle 

 drawn on the surface turns completely round in twenty- 

 four hours, so that a pendulum swung there, with a circle 

 beneath it divided like a twenty-four-hour clock dial, 

 would appear to revolve, and would tell the time. At 

 the Equator, however, a circle on the surface of the earth 

 does not itself rotate on its centre as at the Pole, but is 

 merely carried round the earth with the north and south 

 points of the circumference preserving the same direc- 

 tion in space. Therefore, at the Equator a pendulum 

 should show no motion of rotation. At all intervening 

 points it will appear to rotate, but slower and slower as 

 we recede from the Pole; and mathematical calculation 

 shows that, while at the Pole it apparently moves 

 through an angle of 15 in an hour, at London it would 

 move a little less than 12, at Paris 11-J, at New York 

 9f , and at Ceylon somewhat less than 2 an hour. Ex- 

 periments have been tried at each of these places, and 

 the rate of apparent rotation of the pendulum has been 

 found to agree very closely with the calculated amount, 

 thus giving a complete proof that the apparent rotation 

 is really due to the rotation of the earth on its axis. 

 This mode of rendering the earth's rotation visible, in 

 such a simple and convincing manner, is a discovery of 

 considerable interest, even among the many wonderful 

 discoveries of the century. 



One more of these minor applications of scientific 

 principles, leading to very startling results, must be 

 briefly described. All sounds, including the infinitely 

 varied modulations of the human voice, have long been 



