426 



NA TURE 



[August 22, 1907 



reduced to Stio. As an instance of the protective measures 

 adopted by the department, we may cite the case of rabies. 

 The Transvaal is free from this disease, but it is found 

 in Rhodesia, and in the hope of preventing its introduction 

 a strip of country fifty miles wide, along the northern 

 border of the Transvaal, has been entirely cleared of dogs. 



.\ large part of the time of the chief of the division of 

 botany is taken up by consultative work. Information 

 upon new crops, weeds, poisonous plants, forest trees, &c., 

 is in constant demand, and, apart from interviews and 

 attendances at shows, this work alone involves the writing 

 of some 3000 letters per annum. A herbarium is being 

 formed. Some progress has been made in crossing and 

 selecting maize, but it is remarked that, owing to the 

 pressure of other work, plant-breeding has not hitherto 

 received the attention it deserves. An important section 

 of the work of the division is that which deals with plant 

 pathology. A pathologist was recently appointed by the 

 department, and the number of diseases which he has 

 already observed is referred to in the director's report as 

 "amazing." Special attention has been directed to the 

 rusts, and five have been so far identified, viz. Puccinia 

 graminis on wheat and barley, a second form of 

 P. graminis on oats, P. triticini on wheat, P. coroni- 

 fera on oats, and P. maydis on maize. Some atten- 

 tion has been directed to disease-resistant varieties, and 

 stress is laid on the fact that a cereal which may be immune 

 to the attacks of one rust may be very susceptible to 

 infection by another ; the practical conclusion is drawn that 

 every effort should be made to obtain disease-resisting 

 varieties, and that the continued growing year after year 

 of the same variety of any cereal should be avoided as 

 much as possible.^ 



The chemical division has been engaged in an examina- 

 tion of soils, and attention is directed to the fact that the 

 soils of the Transvaal arc generally well supplied with 

 potash, but are deficient in phosphoric acid, lime, and 

 organic nitrogen. In conjunction with the veterinary 

 division, the chemist has carried out an investigation into 

 the composition of the bone of animals suffering from 

 osteo-porosis, and he finds that affected bones are deficient 

 in total ash, lime, and phosphoric acid. The normal pro- 

 portion of nitrogen to total ash is about, i : 14 ; in diseased 

 animals the proportion is approximately i : 11. 



The " division of publications " issues a quarterly 

 journal, each number of which extends to some 300 pages; 

 there are two editions, an English of some 8000 copies, and 

 a Dutch of about 2000 copies. The journal contains 

 original articles, notes from the various divisions, extracts 

 from foreign journals and Government circulars, market 

 prices, customs returns and other figures of interest to 

 farmers. In addition to the journal, this division publishes 

 leaflets and bulletins ; among the latter, those written by 

 members of the veterinary division upon the common 

 diseases of the live stock of the colony have been of most 

 importance. 



It is satisfactory to learn that the work of the depart- 

 ment commended itself to the Public Service Commission 

 which inquired into the working of all branches of the 

 Civil .Service. The commission report emphasises the im- 

 portance to the Transvaal of agricultural research, and goes 

 on to state that it " has been impressed by the zeal, 

 devotion, and business-like methods which characterise the 

 Department at present, and that it finds itself unable to 

 suggest any improvements in the organisation, or in the 

 distribution of the business." 



T//£ ARC AND THE SPARK IN RADIO- 

 TELEGRAPHY} 

 'T' HE discovery by Heinrich Hertz between 1887 and 

 1889 of experimental means for the production of 

 electric waves, and Branley's discovery that the conduc- 

 tivity of metallic particles is affected by electric waves, 

 form the foundation on which, in 1S96, Signor Marconi 

 built up his system of wireless telegraphy. 



Many of the early investigators certainly had glimpses 



of a future system of being able to transmit messages 



without connecting wires, for as early as 1S92 Sir William 



I Discourse delivered at the Leicester meeting of the British Association 



on Friday evening, August 2, by Mr. VV. Duddell, F.R.S. 



NO, 1973, VOL. 76] 



Crookes predicted in the Fortnightly Review the possibility 

 of telegraphy without wires, posts, cables, or any of our 

 costly appliances, and said, granting a few reasonable 

 postulates, the whole thing comes well within the realms 

 of possible fulfilment. 



Two years later .Sir Oliver Lodge gave his memorable 

 lecture on the work of Hertz, and carried the matter a 

 step nearer the practical stage. 



There will not be lime to dwell to-night on the early 

 history of the art and its development. It will be neces- 

 sary, however, to explaiti some of the fundamental pro- 

 perties of signalling by means of Hertzian waves in order 

 to be able to bring out clearly the relative advantages 

 and disadvantages of the two rival methods now in prac- 

 tical use for producing Hertzian waves for wireless 

 telegraphy. 



The fundamental part of the transmitting apparatus may 

 be said to consist of a long conductor generally placed 

 vertically, in which an alternating or oscillating current 

 is set up by some suitable means. Such a conductor 

 radiates energy in the form of Hertzian waves at right 

 angles to itself into space, in very much the same way 

 that an ordinary candle sends out light in all directions. 

 This radiation, though it is strictly in the nature of light, 

 is invisible to our eyes, as the frequency is loo low. 



If we set up any other conductor approximately parallel 

 to the first, there will be produced in this .second con- 

 ductor alternating or oscillating currents having the same 

 frequency as those in the first conductor, and which can 

 be detected by suitable instruments. 



The simplest and one of the earliest methods for pro- 

 ducing Hertzian waves for use in wireless telegraphy 

 consisted in charging up, by means of an induction coil, a 

 vertical insulated conductor, which was allowed to dis- 

 charge itself to earth by means of a spark taking place 

 between its lower end and another conductor which was 

 connected to earth. To detect the Hertzian waves Marconi 

 etnployed an iinproved form of the Branley filings tube, 

 which is known as the coherer. 



In order to transmit messages the radiation is started 

 and stopped so as to form short and long signals, or dots 

 and dashes of the Morse code, out of which the whole 

 alphabet is built up in the well-known way. 



As 1 have already stated, the radiation takes place round 

 the vertical conductor approximately equally in all direc- 

 tions. Suppose that I set up my transmil'ling apparatus 

 here in Leicester, a receiving station set up either in Not- 

 tingham, Derby, Rugby, or Peterborough would be able to 

 receive the message equally well. Should I wish to send 

 a message from here to Nottingham at the satne time that 

 Derby wishes to speak to Rugby, then the receiving station 

 at Nottingham would receive both the message from 

 Leicester which it should receive, and the message from 

 Derby which it was not required to receive. 



To get over this difficulty, known as " interference," a 

 large number of devices have been patented. The most 

 successful in practice is syntony, or tuning; in this method 

 each station has allotted to it one definite frequency or 

 tune, and the apparatus is so arranged at each station 

 that it will only be affected by messages which are radiated 

 by other stations on its own frequency or tune, and not 

 by any other radiations. To take a musical analogy, sup- 

 posing I had somebody who was either deaf to all notes 

 of the piano except, say, the middle "C," or had such a 

 musical ability that he could tell at once when I struck 

 the middle " C," then 1 could transmit to that person a 

 message in the ordinary Morse code by playing on the 

 middle " C," and that person, whom I shall call Mr. C, 

 would not take any notice of the fact that I might also 

 be playing on the' notes D, E, F, G, S.C., but Mr. C. 

 would confine his attention entirely to what is being done 

 with the middle " C." It is conceivable that I might find 

 a series of persons or train them so that they could each 

 pick out and hear one note only of Ihe piano, irrespective 

 of what was being played on the other notes or of any 

 other noises that were taking place. Taking an ordinary 

 seven-octave piano, and neglecting for a moment the black 

 notes, this would give me fifty-six distinct notes on which 

 I could transmit messages ; so that, transmitting from 

 Leicester, I might send messages simultaneously to fifty- 

 six different towns. 



