Feb. 5, 1874] 



NA TURE 



271 



The only case of exception on record to this statement is 

 Pr' f. Piazzi Snuth's observations on the Peak of Teneriffe. 

 Theie, during several months of perfectly fair weather, the sur- 

 face of the mountain was, if the electric test applied was correct, 

 .positively electri6ed ; but Prof. Piazzi Smyth lias, I believe, 

 pointed out that the observations lnu^t not be relied upon. The 

 instrument, as he himself found, was not satisfactory. The science 

 of observing the atmospheric electricity was then so much in its 

 infancy tha', thouijh he went prepared with the best instrument, 

 and the only existing rules for using it, there was a fatal doubt as 

 to whether the electricity was positive or negative after all. But 

 the fact that there has been such a doubt is important. Now I 

 suppose there will be a telegraph to Teneriffe before long, and 

 then I hope and tru^t some of the operators will find time to 

 climb the Peak. I am sure that, even without an electric 

 object, they will go up the Peak. Now they must go up tlie 

 Peak witli an electrometer in fine weather, and ascertain 

 whether the earth is positively or negatively electrified. If ihey 

 find that on one fine day it is negatively electrified, the result 

 will be valuable to science ; and if on several days it is found to 

 be all day and all ni ^lit 4iegitively electrified, then there will be 

 a very great accession to our knowledge regarding atmospheric 

 electricity. 



When I say the surface of the earth is negatively electrified, I 

 make a statement which I believe was due originally to Peltier. 

 The more common form of statement is that the air is positively 

 electrified, but this form of statement is apt to be delusive. 

 More than that, it is most delusive in many published treatises, 

 bath in books and encyclopaedias upon the subject. I have in 

 my mind one encyclop.-edia in which, in the article " Air, Elec- 

 tricity of," it is said that the electricity of the air is positive, and 

 increases in rising from the ground. In the same encyclop.xdia, 

 in the article "Electricity, Atmospheric," it is stated that the 

 surface of the earth is negatively electrified, and that the air in 

 contact with the earth, and for some height above the earth, is, 

 in general, negatively electrified. I do not say too much, then, 

 when I say that the statement that the air is positively electrified 

 has been at all events a subject for ambiguous and contradictory 

 propositions ; in fact, what we know by direct observation is, 

 that the surface of the earth is negatively electrified, and positive 

 electrification of the air is merely inferential. 



Suppose, for a moment, that there were no electricity whatever 

 in the air — that the air «ere absolutely devoid of all electric 

 manifestation, and that a charge of electricity were given to the 

 whole earth. For this no great amount would be necessary. 

 Such amounts as you deal with in your great submarine cables 

 would, if given to the earth as a whole, produce a very con- 

 siderable electrification of its whole surface. Vou all know the 

 comparison between the electricity of one Atlantic cable — the 

 electro-static capacity of one of the Atlantic cables — with the 

 water round its gutta-percha for outer coating, and the earth and 

 air with infinite space for its outer coaling.* I do not remember 

 the figures at this moment ; in fact, I do not remember which is 

 the greater. Well, now, if all space were non-conducting — and 

 experiments on vacuum tubes seem rather to support the possi- 

 bility of that being the correct view — if all space were non-con- 

 ducting, our atmosphere being a no.i-conductor, and the" rarer 

 and rarer air ab.jve us being a non-conductor, and the so-called 

 vacuous space, or the interplanetary space beyond that (which 

 we cannot admit to be really vacuous) being a non-conductor also, 

 then a charge could be given to the earth as a whole, if there 

 were the other body to come and go oway again, just as a charge 

 could be given to a ])ith ball electrified in the air of this room. 

 Then, I say, all the phenomena brought to light by atmospheric 

 electrometers, which we observe on a fine day, would be ob- 

 served just as they are. The ordinary observation of atmo- 

 spheiic electricity would give just the result that we obtain from 

 it. The result that we obt.iin < very day in observations on 

 atmospheric electricity is precisely the same as if the earth 

 were electrified negatively and the air had uo electricity in it 

 whatever. 



Well, now I have asserted strongly that the lower regions of 

 the air are negatively electrified. On what foundation is this 

 assertion made? Simply by observation. It is a matter of 

 fact ; it is not a matter of speculation. I find that when air is 

 ili.iwn into a room from the outside, on a fine day, it is 

 negatively electrified. I believe the same phenomena will be 

 observed in this city as in the old buildings of the Uni- 



* The earth's radius is about 630 million centimetres, and its electrostatic 

 capacity is therefore 630 microlarads, or about that of 1,600 miles of cable. 



versity of Glasgow, in the middle of a very densely- 

 peopled and smoky part of Glasgow ; and therefiire I doubt not 

 that when air is drawn into this room from the outside, and a 

 water-dropping collector is placed in the centre of the room, 

 or a few feet above the floor, .and put in connection with a 

 sufiiciently delicate electrometer, it will indicate negative electri- 

 fication. Take an electric machine; place a spirit lamp on its 

 prime conductor ; turn the machine for a time ; take an um- 

 brella, and agitate the air with it till the whole is well mixed 

 up ; and keep turning the machine, with the spirit-lamp burning 

 on Its prime conductor. Then apply your electric test, and you 

 find the air positively electrified. 



Again— Let two rooms, with a door and passage between 

 them, be used for the experiment. First shut the door and open 

 the window in your observing room. Then, whatever electric 

 observations you may have been performing, after a short time 

 you find indications of negative electrification of the air. Then, 

 during all that time, let us suppose that an electric machine has 

 been turned in the neighbouring room, and a spirit-lamp burning 

 on its pi ime conductor. Keep turning the electric machine in 

 the neighbouring room, with the spirit-lamp as before. Make 

 no other difference but this — shut the window and open the 

 door. I am supposing that there is a fire in your experimenting 

 room. Then, when the window was open and the door closed, 

 the fire drew its air from the window, and you got the air direct 

 from without. Now shut the window and open the door into the 

 next room, and gradually the electric manifestation changes. 

 And here somebody may suggest that it is changed because of 

 the opening of the door and the inductive effect from the pas- 

 sage. But I anticipate that criticism by saying that my ob- 

 servation has told me that the change takes place gradually. 

 For a time after the door is opened and the window closed, 

 the electrification of the air in your experimenting room 

 continues negative, but it gradually becomes zero, and a little 

 later becomes positive. It remains positive as long as you keep 

 turning the electric machine in the other room and the door 

 is open. If you stop turning the electric machine, then, after a 

 considerable time, the manii'estatlon changes once more to the 

 negative ; or if you shut the door and open the window the 

 manifestation changes more rapidly to negative. 



It is, then, proved beyond all doubt that the electricity which 

 comes in at the windows of an ordinary room in town is ordi- 

 narily negative in fair weather. It is not always negative, how- 

 ever. 1 have found it positive on some days. In broken 

 weather, rainy weather, and so on, it is sometimes positive and 

 sometimes negative. Now, hitherto, there is no proof of 

 positive electricity in the air at all in fine weather ; but we have 

 grounds for inferring that probably there is positive electricity in 

 the upper regions of the air. To answer that question the direct 

 manner is to go up in a balloon, but that takes -us beyond 

 telegraphic regions, and therefore I must say nothing on that 

 point. But I do say that superintendents and telegraph operators 

 in various stations might sometimes make observations ; and I 

 do hope that the companies will so arrange their work, and 

 provide such means for their spending their spare time, that each 

 telegraph-station may be a sub-section of the Society of Tele- 

 graph Engineers, and may be able to have meetings, and make 

 experiments, and put their forces together to endeavour to arrive 

 at the truth. If telegraph operatois would repeat such experi- 

 menis in various parts of the world, they would give us most 

 valuable information. 



And we may hope that besides definite information regarding 

 atmospheric electricity, in which we are at present so very 

 deficient, we shall also get towards that great mystery of nature 

 — the explanation of terrestrial magnetism and its associated 

 phenomena, — the grand secular variation of magnetism, the mag- 

 netic storms, and the aurora borealis. 



NOTES 

 We have frequently had occasion to refer to the energy and 

 work of the Perthshire Society of Natural Science, and we re- 

 joice to see that at its last meeting it has shown an example 

 which we hope will be followed sooner or later by all scientific 

 societies ; it has resolved to make its influence felt in parliamentary 

 elections. On the motion of the secretary. Dr. White, the fol- 

 lowing resolution was unanimously adopted : — " That in respect 

 that Britain is apparently rapidly losing that commercial and 



