Intelligence and Miscellaneous Articles. 247 



the cause of an increase of potential of the storm-cloud, and when 

 this potential is sufficient, discharge takes place in the form of 

 lightning. Moreover, the continual renewal of the charge indicates 

 that the cause must reside in the storms themselves. 



Air being a dielectric, the vapour becomes the natural reservoir 

 of the electricity. Imagine a mass of electrified aqueous vapour in 

 suspension in the air : at night, when the sun has disappeared, 

 there will be a cooling of the cloud-mass. For the reason above 

 given its potential will increase. 



Now if the potential readies a certain extent, the vesicles considered 

 separately can no longer retain their charge — that is to say, they will 

 escape into the air in the form of silent discharges. 



It is calculated that, at the ordinary pressure, a small sphere, 

 0*6 millim. in diameter, cannot retain electricity at the potential 

 obtained with our ordinary machines. 



From the small radius of the vesicles it will be seen that a cloud 

 cannot retain a charge at a potential exceeeding a certain amount. 

 In this way would be produced the phenomenon known as heat 

 lightning. 



As these lightning flashes are seen by different observers in the 

 horizon, it is logical to suppose that they are also produced at the 

 zenith, and that they are not the reflections of the lightning due to 

 storms at a distance. A similar phenomenon occurs with a 

 traveller who always sees a layer of fog towards the horizon, while 

 the sky above is clear. This is only due to a different thickness 

 of the lavers traversed by the visual rays. — Comptes Rend us, 

 Jan. 28, i884. 



NOTE ON THE TEMPERATUKE OF SUN-SPOTS. 

 BY PROF. EILHARD WIEDEMANN*. 



Messrs. Liveing and Dewar, in one part of their very interesting 

 paper (Phil. Mag. vol. xvi. p. 402), have endeavoured to show that 

 the temperature of the sun-spots may be higher than that of the 

 surrounding surface of the sun, although their thermal and chemical 

 effects are much less. The authors are of the opinion that the 

 intensity of the total radiation from the spots may be much larger 

 than that from the rest of the surface, containing a greater propor- 

 tion of violet and ultra-violet rays which are absorbed at the border 

 of our atmosphere. In these conclusions I cannot agree with them. 

 In increasing the temperature of a luminous source it is true that 

 the intensity of the more-refrangible part of the spectrum increases 

 more rapidly than that of the less -refrangible part ; but, never- 

 theless, the intensity of the latter does increase ; so that, of two 

 sources of different temperature, the hotter emits in every part of 

 the spectrum more light than the colder, at least as long as the 

 spectrum is continuous. 



* Communicated by the Author. 



