May 1, 1900.] 



KNOWLEDGE. 



113 



■'The Norwegian North Polar Expedition, 18951896. Scientific 

 Results." Volume I. Kdited by Fridtjof Nausen. (Longmans, i 

 Illustrated. 40s. net. 



I'BELIMIN.VHY NOTICE. 



The publication of the memoirs relating to the now famous 

 " Fram ' expedition will be eagerlv welcomed by the scientific 

 world. The first volume, a handsome quarto, very finely illustrated, 

 is before us ; it is to be followed by four or five of the same 

 character. 



The present volume contains memoirs on " The Jurassic Fauna 

 of Cape Flora, ' by l)r. Pompeckj ; the fossil plants from Franz 

 Josef Land, by Dr. A. G. Xathorst ; an account of the birds 

 collected bv Mr. Collet and Dr. Xansen ; and the Crustacea, bv Mr. 

 G. O. Sar^. 



The second volume w ill in all probability be taken up with the 

 astronomical and magnetic results of the expedition. The mono 

 graph on the Celanographv of the Polar Regions bv Dr. Nansen is 

 expected to appear in the third volume, together with " The 

 depths and submarine features of the North Polar Regions. ' by 

 the same author. The whole work will be completed in the course 

 of about two years. 



Our countrymen will be gratified to learn that the work is [lub 

 lished in the English language orJy, which is regarded by the 

 Editor as the most international tongue. 



BOOKS RECEIVED. 



Illustrated Annual of Microscopy — 1900. (Percy Lund.) 



Opfn Access in Public Libraries Eaposed. By Edw. Foskett. 

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Calendar of the Science and Art Department — 1900. 8id. 



Official Proceedings of the International Commercial Congress. 

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The Theorii and Practice of Interpolation. By Herbert L. Rice. 

 (The Nichols Press, Lvnn. Mass.) 



The Humane Retie'ir. April. 1900. (Bell.) Is. 



Practical Phi/sics — Descriptive Catalorjue of Apparatus. (GrilI'm.) 



Francis Marif Buss Schools — Jubilee Magazine. April, 1900. 



A Surgical Operating Table for the Horse. By Jno. A. W. Dollar. 

 (Douglas.) 2s. Bd. 



The Seliquart/ and Illustrated Archeeologist. April. (Bcmrose.) 

 2.-. 6d. 



Inorganic Evolution. By Sir Jfonnan Loekyer. (llacmUlan. ) ^s. 



Sexual Dimorphism in the Animal Kingdom. By J. T. Cunning- 

 ham. (Black.) Illustrated. 125. 6d. net. 



A 2'reatise on Zoology. Edited by E. Kay Lankester, f e.s. Part 

 III. — The Echinoderma. (Black.) Illustrated. 15s. net. 



Micro-Organisms and Fermentation. Third Edition. By Alfred 

 Jorgensen. (Macmillan.) 10s. net. 



Mental Culture. By George A. Ilight. (Dent.) 3s.6d.net. 



Man and His Ancestor. Bv Chas. Morris. (Macmillan.) 5s. 



The Struggle for Empire. By Robt. W. Cole. (Elliot Stock.) 



Lessons in Flementarg Physiology. New Edition. By Thomas 

 H. nuiley. (Macmillan") Illustrated. 43. 6d. 



Wireless Telegraphy and Hertzian Waves. By S. R. Bottone. 

 (Whittaker.) Bs. ~ 



The Concise English Dictionary. New Edition. By Clias. 

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Smithsonian Institution — The Secretary's Report for the t/ear 

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The Studio. April, 190O. Is. 



WIRELESS TELEGRAPHY.-II. 



By G. W. DE TUNZELMANN, B.SC. 



After the digressions made in the last article I will 

 now ask my readers to return to the main subject of the 

 present series, Hertzian Telegi-aphy, viz., a system of 

 telegraphic communication by means of electric disturb- 

 ances set up in, and transmitted from place to place 

 through, the ether. The general outline of the subject 

 contained in the earlier portion of the article referred 

 to suflSciently indicates the simplest and most logical 

 order of development of the subject to be the study, in 

 the first place, of the medium in which the disturbances 

 are excited, secondly, an inquiry into the nature of these 

 disturbances; and here so liltle is definitely known 

 that I can do little more than set forth hypotheses 



which would more or loss fully explain the observed 

 phenomena and which probably have a more or less 

 distant resemblance to the actual facts; and, finally, 

 an account of the app:u-atus and methods which have 

 been employed in the winning of such knowledge as we 

 have so far obtained, or which are now being employed 

 in putting it to practical use. 



THE ETIIEK. 



Sir Isaac Newton formulated a theory of light known 

 as the corpuscular theory, according to which light was 

 supposed to bo due to very minute particles or corpuscles 

 projected with enormous velocity from luminous bodies 

 Newton adopted this merely as a working hypothesis 

 which gave a fairly reasonable explanation of what was 

 known of light, but he was by no means satisfied with it. 

 It accounted for the ordinary phenomena of rcficction 

 and refraction, but in order to account even for the 

 simpler phenomena of polarised light it was necessary 

 to make various more or less com23licated additional 

 assumptions. Still for a long time the corpuscular 

 theory found a number of adherents to maintain it 

 against the theory developed by Huyghens and others 

 that light was a wave motion, the great objection to the 

 acceptance of the undulatoi-y theory being the necessity 

 of assuming the existence of a medium filling the whole 

 space occupied by the visible universe and having pro- 

 perties of a character hitherto quite unfamiliar. It 

 was not until it became possible to make comparative 

 measurements of the velocity of light in media of vary- 

 ing density that the coi-puscular theory was definitely 

 overthrown, since it demanded that the speed of trans- 

 mission of light should increase with the density of the 

 medium, whereas it is found that it decreases as the 

 density increases, as is required by the wave theory. 

 Since then new phenomena have been predicted from the 

 wave theory and experimentally verified, and the whole 

 theory of spectrum analysis rests upon it, so that it, 

 and therefore the existence of the luminiferous ether, 

 is no longer regarded as a working hypothesis but as 

 a fact, the evidence in its favour being quite as strong 

 as that for the truth of Newton's law of Universal 

 Gravitation. 



At the present time we not only know that light and 

 radiant heat are due to etheric vibrations but we know 

 the exact nature of the vibrations, and as regards light 

 we know the lengths of the waves corresponding to the 

 various coloui's of the spectrum. We know, too, that 

 heat waves are exactly similai' to light waves except 

 that they are of greater length, and the only reason 

 that we cannot make measurements of heat waves with 

 the same degree of accuracy as of light waves is that we 

 have no special organ for the heat sense coiTespondiug 

 to the eye, which forms an instrument of extreme sen- 

 sitiveness for light observations. 



Sound waves ai'e transmitted by ordinary matter, 

 either solid, liquid or gaseous, and, as is well known, 

 sound cannot be transmitted through a si^ace which 

 does not contain matter of very sensible density, a com- 

 paratively thin stratum of even such an imperfect 

 vacuum as can be obtained by the aid of a good 

 ordinai-y air-pump being sufficient to entirely stop it. 

 When somid travels through a solid mass of matter the 

 vibration takes place in all possible directions, but when 

 it is transmitted through fluids, whether liquid or 

 ga-scous, the vibrations are entirely longitudinal, that is 

 to say the motion of the moving particles is always 

 parallel to the direction in which the sound is travelling. 

 The reason of this is that while fluids possess volume 



