March 1, 1899.] 



KNOWLEDGE. 



67 



TRIPLE ATMOSPHERIC CORONA. 



By the Rev. Samuel Barber. 



ABOUT thirty years ago there was seen from the 

 landing stage at Liverpool a solar halo and a 

 corona at the same time. The writer hazarded 

 the suggestion that a drift of sleet was the 

 originating medium of both. This was confirmed, 

 soon after, by a downfall of half-melted snow. 



About the same time appeared a lunar corona of similar 

 type to the one now figured and described, seen on 



Christmas Day, 1898, about 8.30 p.m. The first-mentioned 

 had the blue ring of a very intense colour, and appeared 

 more steady, and was probably formed in a higher drift of 

 particles. In this recent instance, the moon, two days off 

 " full," was pretty bright, and a drift of broken cirro- 

 cumulus, or " alto -cumulus " was rapidly passing from 

 about west-north-west. The corona was very variable, and 

 did not appear in perfection for more than a minute or two 

 at a time, recurring again and again. 



There was the usual small pale disc close to the moon, 

 with diameter equal to about three times that of the moon's 

 disc. Then a larger disc of yellowish-grey edged by reddish- 

 orange, and rounded ofi' so as to give it a semi -spherical 

 effect, f The diameter of this was seven or eight times as 

 great as that of the inner disc. Outside the orange, again, 

 was the beautifully coloured blue ring which makes this 

 kind of corona so attractive in form. Then a ring of grey 

 light as narrow or narrower than a rainbow. The blue 

 ring was slightly wider than this outer grey one, and not 

 EC well defined on its inner edge. 



The barometer was falling, and the temperature had 

 risen before this appearance. 



The weather for several weeks after was exceedingly 

 stormy and changeable. A high wind was almost con- 

 tinuous for a long period. 



For the explanation of these corona? the reader is referred 

 to Kaemptz, and any good treatise on optics, or Ganot's 

 Physics. Not long ago the writer saw the corona 

 illustrated beautifully in a Cornish field, where an immense 



* This small disc was seen seTeral times of a pale green colour 

 during the drought, 1898. 



t The size of this disc is contracted in the sketch. 



deposit of gossamer had taken place during the night. 

 Upon the fine lines myriads of drops from the autumnal 

 hoar-frost produced exquisitely coloured concentric rings, 

 the result of diffraction and interference of the declining 

 solar rays. 



In concluding this notice I may chronicle a crurious 

 observation made upon the effect of st^eam, when intercepting 

 the view of a lunar corona. I was in a train near the 

 engine from which the steam burst in large volumes right 

 across the double corona which I had been watching in the 

 bright moonlight. Every time the steam intervened it 

 cut off completely in a moment the outer ring which again 

 burst into view the moment the steam was dissipated. 

 The large inner disc remained entirely unaffected, bemg 

 seen as plainly through the steam as before, so that the 

 corona was alternately single and double. It is undei - 

 stood that the size of these rings is related to that of tie 

 drops or vesicles, and that where their magnitude greatly 

 varies, the overlapping and interference originate, not 

 concentric rings, but a disc only, or " luminous aureola." 



[It seems a pity that the term " corona " should be used 

 in the above connection. " Corona " has so definite and 

 special a significance, that to apply it, not only to the 

 outer appendages of the sun, but also to phenomena quite 

 unlike it, and due entirely to influences within our own 

 atmosphere, is distinctly to be deprecated. — E. Walteb 

 Maundeb.] . ^ 



MAMMOTH IVORY. 



By E. Lydekker. 



IN spite of all their ingenuity and skill, there are two 

 animal products of high commercial value which our 

 manufacturers have not hitherto imitated with such 

 success as to make the substitutes equivalent in 

 utility and beauty to the originals. These products 

 are elephant ivory and whalebone, and although the 

 imitations in the former case make a much nearer approach 

 to the true article than has been found practicable in the 

 latter, they still leave much to be desired. Consequently, the 

 demand for natural ivory is not only likely to be maintained 

 on the same level as heretofore, but would undoubtedly 

 increase if an adequate supply were forthcoming. 



True ivory, to which the name should properly be re- 

 stricted, is the constituent of the tusks of elephants of 

 different species, and is found in no other animals. In 

 making this statement it must not be assumed that its 

 presence in mastodons is denied, since those extinct 

 animals are nothing more than elephants in a wider sense 

 of the term. From other so-called ivory, such as that 

 of hippopotamus tnsks, sperm-whale teeth, and narwhal 

 "horns," elephant ivory is readily distinguished at a glance 

 by the "engine-turned" pattern — similar to that on the 

 back of a watch case — which it displays in cross-section, as 

 may be seen by looking at the butt-end of the handle of a 

 table knife. And it is probably due to this peculiarity of 

 internal structure that elephant ivory displays the elasticity 

 which forms one of its most valuable properties. 



As all the readers of this journal are doubtless aware, 

 there are only two living species of elephants at the present 

 day, namely the Indian, or, as it might with more propriety 

 be called, Asiatic, and the African. As regards the pro- 

 duction of ivory, the latter is, or, perhaps, was, much the 

 more valuable animal of the two. In the first place, till 

 within the last few years, it existed in almost incredible 

 numbers in many parts of its habitat ; and in the second 

 place, it produced more ivory, animal for animal, this being 

 due to the circumstance that whereas in the African species 

 both sexes are furnished with tusks of large size, in its 

 Asiatic cousin they are generally restricted to the male 



