730 Transactions of the American Institute. 



our knowledge of the time of the formation of a sun-spot is derived 

 from vision, depending upon the motion of light. The instant a per- 

 son can see a sun-spot, that instant the eifect of the heat of the sun 

 is felt upon the earth, and, of course, at that instant it should affect 

 the needle. As to the other objection, the position of the magnetic 

 pole, it will be found, upon examination, that if a magnetic pole were 

 to be produced as the effect of the solar heat, it would be produced 

 exactly where it now exists, and that is one of the strongest reasons 

 for accepting this theory. 



Dr. Yan der Weyde — If the solar, heat were the cause, I should 

 expect the magnetic currents to run parallel to the equator, for at all 

 seasons of the year the apparent rotation of the sun is always at right 

 angles with the axis of the earth. Besides, the currents should be 

 the greatest where the heat is the greatest. The magnetic meridians 

 and parallels are very irregular indeed. 



The President — That is another argument in favor of the theory, 

 because the earth's surface is very uneven, and the thickness of the 

 earth's crust is very unequal, and the currents produced by unequal 

 heating should be the strongest where the heat is the least. 



III. Distance or Double Stars. 

 The distance of nearly all the fixed stars being so great that obser- 

 vations of the same star, at opposite points of the earth's orbit, form 

 no parallax, the astronomer has heretofore been baffled in his 

 attempts to estimate, even approximately, the remoteness of these 

 celestial bodies. Mr. Fox Talbot has proposed to use the spectro- 

 scope in the following manner for effecting this object in binary 

 systems : Suppose the plane of the orbit of a binary system to pass 

 through the sun, *. e., that the observer is in the plane of the orbit, 

 and that in the spectra of the individual stars there are lines belong- 

 ing to the same element, the spectra of the two stars, taken through 

 the same slit, should be observed and compared. When the stars 

 appear in the same straight line, it is clear that their velocities, rela- 

 tive to the earth, are the same, since both are moving, perpendicu- 

 larly, to the line of vision; the lines from the two stars will, there- 

 fore, coincide. But when their apparent distance from each other 

 is greatest, the difference of their velocities relative to the observer is 

 equal to the velocity of either star, in its velocity in its orbit about 

 each other. This difference will produce a displacement of the lines, 

 which may be observed and measured. This gives the value of that 

 velocity ; but we know, also, the periodic time. We have, then, at 



