NOTES. 



475 



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 225, p. 316. A copper wire coiled in the form represented iu 

 fig. 73 was the first and most simple form of the 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. 



NOTE 226, p. 344. It is to Halley we are indebted for the first declina- 

 tion chart and the theory of 4 poles of maximum magnetic intensity, since 

 confirmed by observation, as well as the earliest authentic values of the 

 magnetic elements in London and St. Helena, where he went on purpose 

 to make observations on terrestrial magnetism. Since that time M. Gauss 

 has formed charts of the magnetic lines, and published a theory which very 

 nearly represents the magnetic state of the globe. The mass of observa- 

 tions daily making by our cruizers and our Government surveys in every 

 part of the earth is enormous. 



NOTE 227, p. 360. In fig. 74 the hyperbola HPY V the parabola 

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



Flcj. 74. 



through a 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 228, p. 363. In fig. 75, E A represents the orbit of Halley's 

 Fig. 75. 



comet, E T the orbit of the earth, and S the sun. 

 very nearly exact. 



The proportions are 



