172 Miscellaneous Intelligence. 



an experiment of this kind, the deviation was about 15° in the 

 morning and evening, but more considerable in the course of the 

 day* This first attempt has made me suppose that the galvano- 

 meter might become, in the hands of an attentive and skilful 

 philosopher, a kind of atmidometer. If by means of a single 

 couple of two different metals, bismuth and copper, a deviation 

 of 15° has been obtained, a much greater one would be produced 

 by employing several pairs, conveniently immersed in the same 

 vessel of water ; and, perhaps, one might succeed by increasing 

 the scale of observation, in ascertaining more exactly the diurnal 

 rate of evaporation. I propose, also, to ascertain the effect of a 

 current of air, excited by any means over the surface of the 

 water used in the experiment ; it would, without doubt, augment 

 the evaporation, and by increasing the difference between the 

 temperature of the air and the water, increase the effect on the 

 instrument. — Bib. Univ. xxix. 119. 



3. On the Length of the Electric Flash producing Lightning. By 

 M. Gay Lussac. — The length of the flash during storms is always 

 very great, and one may readily ascertain, in a mountainous 

 country, that it frequently exceeds a league. This extraordinary 

 length, and the awful sound produced by the flash, induces us 

 naturally to admit, that the quantity of electricity which forms it 

 is incomparably greater than that which may be accumulated in 

 the largest electric batteries. We cannot produce explosion ex- 

 cept at the distance of a few centimetres, (an inch or two,) and the 

 intensity which we must suppose is required in batteries to make 

 an explosion at the distance of a few metres, (or a few yards,) 

 only, would be so great as to make it impossible it could be re- 

 tained on a coated surface by the pressure of the air. On the 

 other side, when lightning falls on a lightning-rod, it frequently 

 happens that only a small portion of the point, perhaps three or 

 four millimetres, (0.12 to 0.16 of inch,) is fused; and this effect 

 is not very different to what may be produced by large electrical 

 batteries. 



But we cannot really judge of the intensity of electricity ac- 

 cumulated on our conductors, and on a thunder-cloud by the 

 length of the spark. The electricity is retained on our conductors 

 by the pressure of the air, the spark only occurs when this pres- 

 sure can be overcome by the electricity. On the contrary, the 

 electricity is retained on a cloud only by the resistance it affords 

 to it as a non-conducting body, and equally pressed as it is by 

 this fluid which surrounds it on all sides, it should obey the 

 slightest attractive or repulsive forces by which it is affected. 

 We may therefore conceive, that as soon as the electricity has 

 formed a stratum, no matter how attenuated, so that it be con- 

 tinuous, the flash may occur and pass through considerable dis- 



