May 29, 1913] 



NATURE 



From the Imperial Indian Government's Agricultural 

 Research Station at Pusa has been issued Bulletin 

 No. 28 on '"The Cultivation of Lac in the Plains of 

 India," bv C. S. Misra, a well-illustrated account of 

 the insect (Tachardia lacca), the trees on which it 

 thrives, their culture, the collection of the product, the 

 manufacture of shellac, and its economic uses. The 

 most dangerous enemies of the lac insect appear to be 

 the predaceous caterpillars of four species of moth. 



FORESTS AND CLIMATE. 

 ^pHE very general belief in the influence of forests 

 •*■ upon climate, and especially upon rainfall, is 

 discussed by Prof. R. de Courcy Ward in an interest- 

 ing article in the April number of The Popular Science 

 Monthly. The subject is very complicated, and the 

 author points out that we must be careful not to put 

 the cart before the horse ; in other words, the forests 

 are the result of the rainfall, and not vice versa. 



The various questions involved are discussed in detail, 

 the following being among the points dealt with : — 

 (1) The historical" method; (2) why forests should 

 influence climate; (3) influence upon (a) temperature, 

 (6) humidity and evaporation; (4) the cases frequently 

 cited as showing an influence upon rainfall ; (5) recent 

 European studies. Among the authorities quoted, 

 Hellmann has shown that the increase in the rainfall 

 over a forest is accompanied by a lessened fall to 

 leeward — simply a slight difference in distribution. 

 Both Yoeikof (Russia) and Hann (the leading 

 authority on climate) believe that the vast tropical 

 forests may increase the amount of rainfall. But as 

 regards our own latitudes the author considers that 

 there is at present no conclusive evidence that forests 

 have a significant effect upon the amount of rain- 

 fa.ll, as distinguished from the amount of the rain- 

 iatch in the gauge. 



There is comparatively little popular interest in the 

 possible influence of forests upon temperature ; the 

 forest is a little cooler than the open in summer, and 

 possibly very slightly warmer in winter. Supan sums 

 up the case as follows : — " No one will care to main- 

 tain that the system of isotherms would be radically 

 altered if Europe and Asia were one great forest from 

 ocean to ocean." With regard to moisture, the author 

 thinks that the local supply from forests cannot play 

 any considerable part in the great rain-producing 

 processes. 



SYSTEMS OF LONG-DISTANCE WIRELESS 

 TELEGRAPHY. 



THE Advisory Committee appointed by the Post- 

 master-General to consider and report on the 

 merits of existing systems of long-distance ^wireless 

 telegraphy has made its report. The Committee heard 

 evidence in private from representatives of the Mar- 

 coni, the Telefunken, the Poulsen, the Goldschmidt, 

 and the Galetti interests, and of the Admiralty, and 

 the members visited a number of stations. 



The report is strictly limited to practical considera- 

 tions, and deals with matters of engineering rather 

 than of scientific interest. From the point of view of 

 the building of stations for immediate operation in the 

 Imperial wireless chain, the report is overwhelmingly 

 in favour of the Marconi Company, not only on 

 account of its plant, but also on account 

 of its experience ; though the Committee points 

 out that it would be possible for the Government to 

 get together a highly trained staff and erect the 

 stations, using any desirable patents under the pro- 

 visions of section 29 of the Patents and Designs Act, 

 1007. The Marconi spark plant was tested by the 



NO. 2274, VOL. 91] 



Committee as to duplex working, and as to automatic 

 transmission at the rate of sixty words per minute, 

 across the Atlantic, a distance .of 2300 miles. The 

 Committee found Transatlantic communication prac- 

 tically continuous, though there are periods when the 

 signals become very weak; and there are occasional 

 periods when no signals at all can get through. 

 These weak periods are due to natural causes, and can 

 probably only be overcome by the use of high powers. 

 The Committee received no evidence supporting the 

 reported transmission from San Francisco to Honolulu 

 (2100 miles) by the Poulsen arc, but witnessed trans- 

 mission over a relatively short distance at seventy 

 words per minute. The members also saw the Gold- 

 schmidt alternator transmit at the rate of sixty words 

 per minute. It is interesting to note that the Mar- 

 coni Company and the Telefunken Company are both 

 experimenting with generators of continuous waves. 

 The Marconi machine consists essentially of a rapidly 

 rotating contact-maker in a direct-current circuit with 

 special dispositions of other circuits to give continuous 

 oscillations in the antenna. The Telefunken machine 

 is an alternator constructed to give as high a funda- 

 mental frequency as may be convenient in the first 

 instance, the frequency being doubled or quadrupled 

 by a polarised transformer method. The Marconi 

 machine was witnessed working across the Atlantic. 



SOME FURTHER APPLICATIONS OF THE 

 METHOD OF POSITIVE RAYS. 1 



THE method to which I shall refer this evening is 

 the one I described in a lecture I gave here two 

 years ago. The nature of the method may be under- 

 stood from the diagram given in Fig. 1. A is a 

 vessel containing the gases at a very low pressure ; 

 an electric discharge is sent through these gases, 

 passing from the anode to the kathode C. The posi- 

 tively electrified particles move with great velocity 

 towards the kathode ; some of them pass through a 

 small hole in the centre, and emerge on the other 

 side as a fine pencil of positively electrified particles. 



This pencil is acted on by electric forces when it 

 passes between the plates L and M, which are con- 

 nected with the terminals of a battery of storage 

 cells, and by a magnetic force when it passes between 

 P and Q, which are the poles of an electromagnet. 

 In the pencil before it passed under the influence of 

 these forces there might be many kinds of atoms_ or 

 molecules, some heavy, others light, some moving 

 quickly, others comparatively slowly, but these would 

 all be mixed up together. When they are acted on 

 by the electric and magnetic forces, however, they 

 get sorted out, and instead of travelling along the 



1 Discourse delivered at the Royal Institution on Friday, January 17, by 

 Sir J. J. Thomson, O.M., F.R.S. 



