April 1, 1898.] 



KlSrOWLEDGE. 



89 



The British Colonies : 188R.1S'J7. By Bev. Win. Parr Creswell, M.A. 

 (Blackic) 28. 6d. 



Audubon and his Journals. Two Vols. By Maria R. Audubon. 

 (John C. Ximmo.) Portraits and Illustrations. 



Ethnological Studies among the North- West-Central Queensland 

 Aborigines. Bv AValtor K. Roth. (Queensland Agent-G-eneral's 

 Office!) Illustrated. 



The Year-Book- of British Columbia (1897J. (Victoria, B.C.) 



The Natural ffisfori/ of the British Isles : Vertebrates. By F. d. 

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The Miner s Arithmetic and Mensuration. By Henry Daries. 

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General Elementarg Science. Bv \ViUiamBrigg3,M.A. (Clive.) 3s. (id. 



The Smithsonian Institution: is4G-lS'J(>. (Washington.) 



Sixteenth Annual Report of the Bureau of American TSthnoloyu. 

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Das Weltgebiiude. Von Dr. M. Wilhelm Meyer. (Leipzig.) 



A Treatise on Magnetism and Elecfricifif. By Andrew Gray, 

 LL.D., F.E.s. Vol. I. (Macmillan.) 14s. 



STARS HAVING LARGE PROPER MOTION. 



A RECENT announcement has been made by Prof. Kapteyn 

 that the star Cordoba Z. C. 5h 243 has an annual proper 

 motion of 7'5 ", which is larger than that so far found 

 for any other star (Astronomische Nachriehten, Vol. CXLV., 

 p. 159). 



The effect of this motion is shown in the accompanying 

 illustration, which is enlarged nine times from two photo- 

 graphs taken with the eight-inch Bache telescope, at the 

 Arequipa Station of the Harvard College Observatory. 



■. ^ 



Proper Motion of Cordoba Z. C. oh 243. 



The scale of the original photographs is 180"=01 cm. 

 The plates were superposed so that the images of the stars 

 on one should be a short distance below those on the other. 

 The motion of Z. C. 5h 243, which is indicated by an 

 arrow, is at once apparent from the displacement of the 

 line connecting the two photographic images of this object. 

 The southern of each pair of images, and the right-hand 

 image of 5h 243, are reproduced from a photograph taken 

 October 8th, 1889, with an exposure of fourteen minutes. 

 The northern images are reproduced from a photograph 

 taken November 10th, 189G, with an exposure of twelve 

 minutes. E. C. Pickering. 



THE LEVEL OF SUNSPOTS. 



By the Rev. Arthur East. 



THAT sunspots are holes in the sun most people 

 admit ; that they are black is manifest to everyone 

 who has observed them, even with a field-glass ; 

 but whether they are raised above or sunk below 

 the general level — if there even be a general level — 

 and why anything in the sun should be black, are questions 

 not so easily answered. That the blackness of the " umbra " 

 is probably brighter than the electric light is immaterial. 

 Compared to the far brighter photosphere the inner por- 

 tions of a spot are black or nearly black. To the superficial 

 observer the answer might appear obvious, viz., this: "The 

 deeper a hole is, the blacker are the shadows." But it must 



Fia. 1. — Symmetrical .Spot, elevated Penumbra. Black Umbra 



surrouBded by Penumbra; margins of "Spot" depressed below 



general level. 



be borne in mind that we are not dealing with shadows ; 

 there can be no such thing on a self-luminous body as a 

 shadow, and the reason why one part of the sun is darker 

 than another, and even relatively black, is due to an 

 entirely different cause, namely, absorption of the light. 

 The edge of the sun is darker than the central parts 

 because the light from the edge reaches us after passing 

 through a vast thickness of solar atmosphere, and this is 

 very manifest in photographs of the sun ; for the same 

 reason the middle part of a spot appears black because 

 the light from below has to traverse the depth of the spot, 

 which is known to be filled with comparatively cool and 



Fig. 2. — Symmetrical Spot. Penumbra with dark margin next 

 to Photosphere. 



light-absorbing vapour. If the writer has been fortunate 

 enough to induce anyone to experiment in the way of 

 making artificial sunspots, •'it will have been observed that 

 the spots may be broadly classified under four types : — 



1. Spots more or less elevated above the general level, 

 with deep central part and gaping orifice, as Fig. 1. 



2. Spots with a cup-shaped orifice, where the ascending 

 fluid scours out the sides of the cone of granules, as Fig. 2. 



3. Spots where the hot fluid rushes up obliquely, making 

 the sides much steeper in one part than another, as Fig. 3. 



4. And, lastly, spots which are not cone- or crater-like 

 in form at all, as the others are, but where the sides recede 

 from the orifice, leaving only a black and gaping hole as 

 Fig. 4. 



It is not meant that each spot is restricted to any one 

 type ; it may belong to two or three, or even all four, in 

 different parts of the same spot : e.g., the sides of the 

 penumbra may be nearly flat in one place and concave in 

 another, and almost vertical in a third ; whilst the older a 



* See article on " Artificial Sunspots " in Knowiedob, December, 

 1897. 



