504 



NOTES. 



While the electricity flows through the helix, the magnet S N remains sus- 

 pended within it, but falls down the moment it ceases. The magnet always 

 turns its south pole S towards P, the positive wire of the battery, and its north 

 pole towards the negative wire. 



NOTE 219, p. 361. A copper wire coi led 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 becomes 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. 380. 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 

 Fig. 74. 



small space on each side of the perihelion P ; and, as a comet is only visible 

 when near P, it is difficult to ascertain which of the three curves it moves in. 



NOTE 221, p. 391. In fig. 75, E A represents the orbit of Halley's comet, 

 E T the orbit of the earth, and S the sun. The proportions are very nearly 

 exact. 



Ftf- 75. 



NOTE 222, p. 411. Fig. 74 represents the curves in question. It is evident 

 that, for the same focal distance S P, there can be but one circle and one para- 

 bola p P R, but that there may be an infinity of ellipses between the circle and 

 the parabola, and an infinity of hyperbolas H P Y exterior to the parabola 

 P PR. 



NOTE 223, p. 427. Let A B, fig. 26, be the diameter of the earth's orbit, and 

 suppose a star to be seen in the direction A & from the earth when at A. 



