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POPULAR SCIENCE MONTHLY. 



equal to 77000 °f an inch. The thickness of 

 ordinary gold leaf is given as jg-giroTr of an 

 inch, from which it becomes evident that the 

 wave length of sodium light, which is an 

 average wave length for the visible spec- 

 trum, is six and a half times as great as the 

 thickness of gold leaf. Such a dimension as 

 4-2 otto °f an i ncn could readily be measured 

 by a suitable micrometer ; but of course the 

 waves of light, as well as the ether particles 

 by which they are transmitted, are entirely 

 invisible, and even were this otherwise the 

 frequency of the undulations is so inconceiv- 

 ably great that the actual phenomena of the 

 movements could never become perceptible. 

 In measuring the absolute wave length, there- 

 fore, we are forced to take the indirect 

 method of observing the results of undula- 

 tions in cases where, by a suitable arrange- 

 ment of the experiment, equal and opposite 

 phases of vibration are made to arrive simul- 

 taneously at the same spot, so producing 

 phenomena of interference. 



The " Cansses " of Southern France. — 



It is surprising to find existing, in a country 

 so old and supposedly so familiar as France, 

 a region similar to our Colorado plateau, full 

 of canons, caves, and cliff dwellings, until 

 recently almost unknown and wholly un- 

 explored. Yet such is the region of the 

 Causses, described and illustrated with a 

 striking series of lantern views, before the 

 American Association for the Advancement 

 of Science, by the well-known cave explorer, 

 Dr. H. C. Hovey, of Newburyport, Mass. The 

 local name Causse, derived from the Latin 

 calx, lime, is applied to a limestone area, and 

 here to a limestone plateau. Along the west- 

 ern slope of the Cevennes Mountains lies an 

 elevated table-land, chiefly of Jurassic lime- 

 stones, which had been cut and carved by 

 the streams, especially the Tarn and its afflu- 

 ents, into a group of high plateaus separated 

 by deep canons. The cliffs of the Tarn Val- 

 ley are from one to two thousand feet and 

 even more in height, and with their precipi- 

 tous sides and the brilliant and varied color- 

 ing of their strata are not unworthy of com- 

 parison with our own great canon regions of 

 the West. At some points, where the beds 

 are markedly unequal in hardness, the weath- 

 ering process has resulted in structures as 

 remarkable as Monument Park or the Gar- 



den of the Gods. Such is the " rock city " 

 known as Montpellier-le-Vieux, at the junc- 

 tion of the Jonte and the Durbais, on the 

 Causse Noir. This strange area of natural 

 ruins covers some two thousand acres with a 

 fantastic similitude of castles, palaces, streets, 

 and temples. It seems surprising that a 

 country so picturesque for the tourist and so 

 interesting for the geologist should have re- 

 mained almost unknown till the present time. 

 Fine roads pass over and around it, but they 

 avoid the wild and rugged portions that pos- 

 sess such scenic interest, and leave the Causses 

 — as they have been for ages — barren soli- 

 tudes, occupied only by shepherds with their 

 huts and flocks. The people, also, as is so often 

 the case in such regions, have a superstitious 

 dread of the deeper caverns and the seeming 

 ruins, and do not lend themselves readily to 

 exploration. The cliffs are full of caves, 

 some of which — the more accessible and 

 simple — are used as sheepfolds, and even in 

 some cases inhabited, but the wilder ones 

 are held in dread. It seems that cliff dwell- 

 ings are actually still in use to some extent 

 in this region. The French Societe de Spele- 

 ologie has now for some years been inves- 

 tigating the Causses with great interest. 

 Ere long this will become a favorite re- 

 gion for tourists ; but at present one must 

 leave all ordinary facilities of travel and 

 take to canoes and mules. This was done 

 by Dr. Hovey and his party, under the lead- 

 ership of M. Edouard A. Martel, of Paris, 

 who has been one of the most active ex- 

 plorers. They entered and traversed many 

 remarkable caves, some never before visited, 

 and some that have been previously explored 

 by M. Martel and others of the societe. 

 One of these, known as the Baumes Chaudes, 

 is a great triple cavern, one of the main 

 branches of which had yielded a large num- 

 ber of prehistoric skeletons to Dr. Pruni- 

 eres, of Marvejol. In the third division 

 are a number of deep pits, locally called 

 " wells," from forty to a hundred and thirty 

 feet deep; these communicate with lower 

 passages and subterranean streams. They 

 are death-traps to animals, the remains of 

 which, of many kinds and in all stages of 

 decomposition, accumulate at the bottom, and 

 are gradually covered by stalagmitic de- 

 posits. Another remarkable cave was dis- 

 covered and named after its daring and en- 



