300 PETER GUTHRIE TAIT 



observer's umbrella by a slight fall of snow, to such an extent that he could draw 

 sparks from it with his ringer. 



In calm clear weather the atmospheric charge is usually positive. This is very 

 commonly attributed to evaporation of water, and I see no reason to doubt that the 

 phenomena are closely connected. I will show you one of the experiments upon 

 which the idea is based. I can take no other form of experiment than a somewhat 

 violent one, as the effects of the more delicate ones could not easily or certainly be 

 made visible to so large an audience. [A few drops of water were sprinkled on a 

 heated crucible, insulated, and connected with the electrometer.] 



There can be no doubt that, whatever be the hidden mechanism of this experi- 

 ment, the steam has carried with it a strong charge of positive electricity, for it has 

 left the rest of the apparatus with a strong negative charge. We might reverse the 

 subject of measurement by connecting the electrometer with the escaping steam, but 

 I omit it, to save time, and because we will now try that form of the experiment 

 in another way. [High-pressure steam escaping from a little boiler was made to play 

 upon an insulated conductor furnished with spikes, and connected with the electro- 

 meter, which then showed a strong positive charge.] 



There are many substances which produce on evaporation far greater electric 

 developments than water does, some of positive, others of negative, electricity. By 

 far the most remarkable in this respect to which attention has yet been called is an 

 aqueous solution of sulphate of copper. (Proc. R.S.E., 1862.) The smallest drop of 

 this solution thrown on a hot dish gives an intense negative effect so great, in fact, 

 that it may be occasionally employed to charge a small Leyden jar. But this, like 

 the smaller effect due to water under similar circumstances, is not yet completely 

 explained. 



The next striking features are the flashes of lightning which at intervals light 

 up the landscape with an intensity which must in the majority of cases far exceed 

 that produced by the full moon. To the eye, indeed, the flash does not often 

 appear to furnish more than the equivalent of average moonlight, but it must be 

 remembered that it lasts for a period of time almost inconceivably short, and that 

 the full effect of light on the eye is not produced until after the lapse of a con- 

 siderable fraction of a second. Professor Swan has estimated this interval at about 

 one-tenth of a second ; and he has proved that the apparent intensity of illumina- 

 tion for shorter intervals is nearly proportional to the duration. (Trans. R.S.E., 1849.) 

 I can illustrate this in a very simple manner. [Two beams of light were thrown 

 upon the screen by reflection from mirrors, each of which was fixed nearly at right 

 angles to an axis. When matters were so adjusted that the brightness of the two 

 illuminated spots was the same, one mirror was made to rotate. The corresponding 

 light spot described a circle about the other, and its brightness became less the 

 larger the circle in which it was made to revolve.] The lightning flash itself on this 

 account, and for the further reason that its whole apparent surface is exceedingly 

 small, must be in some degree comparable with the sun in intrinsic brilliancy 

 though, of course, it cannot appear so. The fact that its duration is excessively 

 short is easily verified in many ways, but most simply by observing a body in 

 rapid motion. The spokes of the wheels of the most rapidly-moving carriage appear 



