130 



KNOWLEDGE, 



[June 1, 1898. 



How does America show in temperature from our present 

 point of view ? E is a " gain and loss " curve, as we may 

 call it, for Harvard Observatory, from 1841. It is distinctly 

 opposite in character to that for Greenwich. Up to 1875, 

 more cold years than hot ; since 1875, more hot years 

 than cold. A similar kind of variation might be shown for 

 Chicago, and, doubtless, other places. 



In all these curves, we may here note, it is the general 

 trend of the curve that has to be considered, rather than 

 the relation to the average line. For the starting point is 



Fig. 3. — Gain and Loss Curves of Kainfall, Edinburgli and 

 Greenwich (years), and London (moutlis of 1897). 



chosen arbitrarily, and we always commence near the 

 average line. Referring to curve D, if we commenced in 

 1824 we should get a curve wholly above the average line, 

 instead of mostly below it, as in the diagram. 



We may now, in conclusion, briefly glance at rainfall. 

 The annual rainfall at Edinburgh from 1841 to 1896 

 (according to Mr. Mossman's figures), treated by our 

 method, yields the curve F (Fig. 3) ; and that for Green- 

 wich the curve G. 



In both of these we may observe a gradual rise from 

 1858 to 1882; more wet years than dry. Thus, in the case 

 of Edinburgh, of these twenty-five years, sixteen were wet 

 and only nine dry. Since 1882, again, the dry years have 

 preponderated ; in Edinburgh ten dry to four wet. From 

 the general aspect of these curves we may, perhaps, be 

 inclined to think the preponderance of dry years may 

 continue some time further. 



The method here illustrated does not seem to have been 

 much used in this country, but is capable, I believe, of 

 throwing some light on the vagaries (as we often call 

 them) of our weather. 



THE PRISMATIC CAMERA AT THE RECENT 

 ECLIPSE. 



By -J. Etebshed, f.e.a.s. 



SPECTROSCOPIC research formed an important 

 feature in the work of most of the astronomical 

 parties who went to India to observe the total 

 solar eclipse of last January, and photographic 

 methods, which have so largely replaced eye obser- 

 vations at recent eclipses, were, it is needless to add, em- 

 ployed at every station where this analytical method was 

 in vogue. 



Owing to the ideal condition of the weather ail along 

 the line of central eclipse, a large number of very beautiful 

 photographs are the result. The amount of interesting 

 material thus secured, and which is now available for 

 discussion, certainly exceeds anything obtained at any 

 previous eclipse. 



Among the various lines of research undertaken by 

 spectroscopists, perhaps the largest share of attention was 

 given to the study of the spectrum of the layer of gases 

 lying immediately above the dazzling photosphere, and 

 known as the " flash spectrum." 



It may be well to explain, for the benefit of those who 

 are unfamiliar with the subject, the conditions under 

 which this spectrum is revealed during an eclipse. Out- 

 side the visible surface of the sun, and covering the entire 

 sphere pretty uniformly, there exists a stratum of gas of 

 considerable depth and comparatively simple composition, 

 known as the " chromosphere." Its spectrum of bright 

 lines indicates the presence of the three elements calcium, 

 hydrogen, and helium. This layer and the prominences 

 arising from it may be seen at all times by the aid of 

 powerful spectroscopes ; but the base of the chromosphere — 

 that is, the region lying within one or two seconds of arc 

 of the photosphere — is not accessible to ordinary spectro- 

 scopic observation, on account of the perpetual unsteadiness 

 of the telescopic image of the sun, and the very intense 

 atmospheric illumination so near to the sun's edge. 



Now, during the progress of a total eclipse, the moon, 

 advancing from the west, gradually covers up the photo- 

 sphere until only a thin crescent remains on the east side, 

 and this rapidly narrows down and finally vanishes alto- 

 gether ; when this occurs, however, the chromosphere 

 lying outside still remains uneclipsed on the east limb, 

 and even the very lowest strata are uncovered near the 

 point where the last streak of photosphere disappeared. 



It is just here that the most interesting and beautiful 

 spectral phenomena are revealed ; bright lines flash out in 

 hundreds — there seems literally to be a shower of bright 

 lines all along the spectrum the moment the photospheric 

 light is withdrawn — but it is only momentary ; the steadily 

 advancing moon almost immediately occults the lowest 

 gaseous strata and only the ordinary chromospheric spec- 

 trum remains. 



