432 



NOTES. 



fig- 73. w NOTE 218, p. 316. Fig. 



73 represents a helix or 

 coil of copper wire, termi- 

 nated by two cups con- 

 taining a little quicksilver. 

 When the positive wire 

 of a Voltaic battery is im- 

 mersed in the cup p, and 

 the negative wire in the 

 cup n, the circuit is com- 

 pleted. The quicksilver 

 insures the connection between the battery and the helix, by conveying 

 the electricity from the one to the other. While the electricity flows 

 through the helix, the magnet S N remains suspended within it, but falls 

 down the moment it ceases. The magnet always turns its south pole S 

 toward P the positive wire of the battery, and its north pole toward the 

 negative wire. 



NOTE 219, p. 319. A copper wire coiled in the form represented in fig. 

 73, is an electro-dynamic cylinder. When its extremities P and n are 

 connected with the positive and negative poles of a Voltaic battery, it be- 

 comes a perfect magnet during the time that a current of electricity is 

 flowing through it, P and n being its north and south poles. There are 

 a variety of forms of this apparatus. 



NOTE 220, p. 339. In fig. 74 the hyperbola H P Y, the parabola p P R, 

 and the ellipse A E P L, have the same focal distance S P, and coincide 

 through a small space on each side of the perihelion P ; and aa a comet 

 is only visible when near P, it ks difficult to ascertain which of the three 

 curves it move* in. 4 



H 



NOTE 221, p. 343. In fig. 75, E A represents the orbit of Halley's 

 comet, ET the orbit of the earth, and S the sun. The proportions are 

 very nearly exact. 



NOTE 222, p. 360. Fig. 74 represents the curves in question. It is 

 evident that for the snme focal distance S P, there can be but one circle 

 and one parabola p PR, but that there may be an infinity of ellipses be 



