Physics and Astronomy. 241 



the images of the stars are small and round and the diffraction 

 rings, seldom seen with large instruments, are clearly visible. 

 Even with high powers the fluctuations of the images are very 

 slight. In fact, at this station the limit to observation will prob- 

 ably be the size of the instrument, instead of, as at other observa- 

 tories, the condition of the air. Although the aperture of this 

 instrument is only thirteen inches, it appears to be the largest 

 refracting telescope in use in the southern hemisphere, while 

 about thirty larger telescopes are mounted in the northern hemis- 

 phere. Since all of these instruments are north of +35°, nearly 

 one-quarter of the entire sky, and that containing many objects 

 of the greatest interest, has never been studied by a relractor of 

 the highest grade. For both these reasons an excellent oppor- 

 tunity is afforded to add to astronomical discovery by the erection 

 of a telescope of lai'ge size at this station. It is hoped that 

 patrons of astronomy will consider the advantages of erecting a 

 large telescope where it will be kept constantly at work, where 

 the sky is clear a large part of the year, where the condition of 

 the air is probably more favorable than at any other existing 

 observatory, and where a large part of the sky could be examined 

 for the first time under such satisfactory conditions. Photographs 

 have not yet been obtained with the thirteen-inch telescope, but 

 it is hoped that its advantages for this kind of work will be as 

 great as for visual observations." 



8. On the Causes of the Phenomena of Terrestrial Magnet- 

 ism and on some Electro-mechanism for exhibiting the secular 

 changes in its horizontal and vertical components, by Henry 

 Wilde. — This paper from the Philosophical Transactions pre- 

 sents the author's views on the nature of terrestrial magnetism. 

 These will be evident from his statement of the three successive 

 stages in the earth's history, which he assumes, and which he 

 illustrates by a large globe with its surface wound with copper 

 wire. These stages are (1) The electro-dynamic condition of the 

 terrestrial globe at a period of its history when the crust from its 

 high temperature was non-magnetic, its surface of uniform curva- 

 ture and the electro-dynamic foci of the internal sphere of vapors 

 inclined at a definite angle from the poles of the earth's axis. 



(2) The electro-dynamic and electro-magnetic condition of the 

 earth when its outer crust was uniform in curvature and had 

 cooled down sufficiently to become permanently magnetic with 

 dual foci of intensity coincident with the poles of the earth's axis 

 and separate from the dual polar foci of the internal sphere of 

 vapors. 



(3) The unsymmetrical distribution of the magnetic elements 

 arising from the unequal curvature and foldings of the earth's 

 crust during its secular cooling as indicated by the present distri- 

 bution of land and water on the terrestrial surface. 



As shown in the above, the author finds the seat of the earth's 

 magnetic attraction in the earth's crust and explains the unsym- 

 metrical distribution of the terrestrial magnetism by the irregu- 

 lar configuration of the earth as manifested by the distribution 



