January 30, 1896] 



NATURE 



299 



were of the order of magnitude of the negative charges 

 previously obtained. 



Thus, at the same time as negative electricity is 

 radiated from the kathode, positive electricity travels 

 towards that kathode. 



I endeavoured to determine whether this positive flux 

 formed a second system of rays absolutely symmetrical 

 to the first. 



(4) For that purpose I constructed a tube (Fig. 2) similar 

 to the preceding, except that between the Faraday's 

 cylinder and the opening ^ was placed a metal diaphragm 

 pierced with an opening /3', so that the positive electricity 

 which entered by iS could only affect the Faraday's 

 cylinder if it also traversed the diaphragm ^3'. Then I 

 repeated the preceding experiments. 



When N was the kathode, the rays emitted from the 

 kathode passed through the two openings ^3 and /3' without 

 difficulty, and caused a strong divergence of the leaves of 

 the electroscope. But when the protecting cylinder was 

 the kathode, the positive flux, which, according to the 

 preceding experiment, entered at /3, did not succeed in 

 separating the gold leaves except at very low pressures. 

 When an electrometer was substituted for the electro- 

 scope, it was found that the action of the positive flux 

 was real but very feeble, and increased as the pressure 

 decreased. In a series of experiments at a pressure of 

 20 microns, it raised a capacity of 2000 C.G.S. units to 10 



volts ; and at a pressure of 3 microns, during the same 

 time, it raised the potential to 60 volts.^ 



By means of a magnet this action could be entirely 

 suppressed. 



(5) These results as a whole do not appear capable of 

 being easily reconciled with the theory which regards 

 the kathode rays as an ultra-violet light. On the other 

 hand, they agree well with the theory which regards 

 them as a material radiation, and which, as it appears 

 to me, might be thus enunciated. 



In the neighbourhood of the kathode, the electric field 

 is sufficiently intense to break into pieces {into ions) 

 certain of the molecules of the residual gas. The 

 negative ions move towards the region where the 

 potential is increasing, acquire a considerable speed, 

 and form the kathode rays ; their electric charge, and 

 consequently their mass (at the rate of one valence- 

 gramme for 100,000 Coulombs) is easily measurable. The 

 positive ions move in the opposite direction ; they form 

 a diffused brush, sensitive to the magnet, and not a 

 radiation in the correct sense of the word.^ 



THE FRENCH MAGNETIC SURVEY OF THE 

 WORLD. 



IN Europe, as well as in the United States of America, 

 the study of terrestrial magnetism has for some time 

 played an important part. 



M. le Commandant de Bemardieres has written 



1 .The breaking of the tube has temporarily prevented me from studying 

 the phenomenon at lower pressures. 



* This work has been carried out in the laboratory of the Normal School, 

 and in that of M. Pellat at the Sorbonne. 



a most interesting account of the construction of new 

 magnetic maps of the globe, undertaken by the Bureau des 

 Longitudes. The following are a few facts given by him, 

 which show to what extent it is contemplated to carry out 

 the work. 



From the magnetic determinations already obtained, 

 some maps have been made ; observatories too, permanent 

 and otherwise, have been built. But the work is not 

 entirely satisfactory. The maps are chiefly the result of 

 observations made by navigators, and are only of limited 

 parts scattered over the face of the earth. 



In order to have a general magnetic map, numerous 

 observations would have to be made, distributed over all 

 regions, taken as nearly as possible at the same time, and 

 in the same way with similar instruments. To this end 

 the Bureau des Longitudes have appealed to Vice-Admiral 

 Besnard, Minister of Marine, who has promised help, 

 and put at their disposal officers and sailors, and also a 

 great number of instruments. The Colonial Minister has 

 also shown interest in the matter, and promised his 

 assistance in the colonies. 



Seven sets of observers have been organised, consisting 

 each of a lieutenant, ensign or hydrographer, and one 

 assistant. These expeditions have been arranged as 

 follows : — 



f West Coast of Africa, East \ ^r a^\.„.A.„ i,„.. 

 Atlantic Ocean \ Coast of America, An- } ,„,„, jfut Ujn 

 I tilles &c. . . . ; tenant of the ship. 



Pacific Ocean . West Coast of America ". { ^;hfp°'' ^""'^ °'" "*'' 



M. Monaque, ensign of 

 the ship. 



Pacific Ocean 



Oceana 



Indian Ocean 



Red Sea, South Coast of 

 Asia, Oriental Coasts of 

 Africa, Madagascar, and 

 other islands 

 Chinese and 

 Japanese Seas \ China and Japan 



Madeira, Canary Islands 

 Azores, Cape Verd 

 Islands, Senegamb 



M. Paqu6, ensign of 

 the ship. 



- { 



Coasts of Indo-China, of \ M. Terrier, ensign of 

 . ' 1 



North .Sea, Scand 

 Denmark, Scotland 



inavia, j - 



the ship. 



de Vanssay, hydro- 

 graphic engineer. 



Houette, captain of 

 the frigate, command- 

 ing the Iceland sta- 

 tion ; M. Morache, 

 lieutenant of the ship. 



With expeditions in these various parts of the earth, 

 it will be possible to make observations almost simul- 

 taneously. 



In order to determine the correct value of the mag- 

 netic elements, as well as to ascertain the exact variation 

 of these elements, the missions have been supplied with 

 the finest instruments, which have been adjusted at the 

 observatories of Montsouris and Pare Saint-Maur ; com- 

 parisons will also be made at every magnetic observatory 

 at which they arrive during the expedition. Special 

 instructions have been given with regard to calculations 

 and method of observation, in order to insure a perfect 

 comparison of results. 



Six of these expeditions have started, and have com- 

 municated already the result of some of their first 

 observations ; the work, however, will have to be continued 

 about two more years. 



The ship Manc/te, which left France last spring, for 

 Iceland, has returned with a great number of observa- 

 tions, obtained in Cherbourg, Scotland, the Shetland 

 Isles, Iceland, Norway, and Denmark. In the ob- 

 servatory constructed by the Manche at Keykiawik, two 

 complete observations of variations were obtained, having 

 each a duration of eight days. 



It will be very interesting to compare the results of the 

 present day with those of the Recherche^ obtained sixty 

 years ago, and since then of several other expeditions. 

 The successful return of the Manche certainly indi- 

 cates that great things may be expected of the other 

 expeditions, and makes it certain that a most important 

 step has been taken by the French Government for the 

 advancement of science. 



NO. 1370, VOL. 53] 



