346 SCIENTIFIC RECORD FOR 1884. 



producing cloud and rain; it is the liownfall of rain which, acting like 

 the water-dropper of Sir William Thomson, discharges the electricity 

 from uprising manses ol moist air and cloud, and produces those difiei 

 ences of electric potential that give rise to lightning and thunder.J 



(3.) There are definite regions that are especially favorable to the form- 

 ation of thunder- storms, such as for instance the swampy lowlands be- 

 tween the larger lakes and the Alps ; so also the west slope of the Bo- 

 hemian forests very often gives rise to thunder-storms. 



(4.) In those cases where the formation of a thunder-storm entirely 

 within the area of observation can be clearly proven, the remarkable 

 phenomena is noticed that over long distances the electric discharges 

 begin simultaneously, so far as this can be determined with ordinary 

 clocks ; we are thus forced to the idea that the disturbance of electric 

 equilibrium due to the first stroke of lightning is immediately commu- 

 nicated by induction from cloud to cloud, and thus provokes a simul- 

 taneous outbreak at various places. 



(5.) The observations of heat lightning or distant silent lightning are 

 interesting as showing the extraordinarily great distances to which this 

 can be observed ; thus on August 26, 1880, lightning was observed in 

 the horizon at stations in Saxe-Meiningen that belonged to a thunder- 

 storm then prevailing 240 kilometers distant near Ulm ; and on Decem- 

 ber 9, 1882, lightning was observed in the south that seems to have 

 belonged to storms 270 kilometers distant. 



296. [Observations similar to these have doubtless been made by 

 many others, and the present writer, after having satisfied himself that 

 with Washington as a center lightning was frequently" observed oc- 

 curring near Norfolk and central Pennsylvania, proposed in 1872 to 

 map out the course of all such storms by means of observations of 

 azimuth and time to be made at all the regular Signal- Service stations j 

 special directions were then given to signal-service observers to rec- 

 ord carefully these phenomena in the journal kept at each station. The 

 result proved that the observed azimuths were not generally recorded 

 with sutBcient accuracy; an apparatus to obviate this difidculty was 

 designed but not introduced ; the lines of equal thunder-storm frequency 

 in the monthly weather review for July, 1874, were largely based upon 

 the study of such records.] 



(6.) The diurnal periodicity of the distribution of thunder-storms 

 shows that besides the maximum between 2 and 5 p. m., there is also a 

 secondary maximum between 2 and 3 A. M. {Z. 0. G. M., xviii, p. 200.) 



297. Von Bezold returns again to this subject in August, 1883, and 

 dwells especially on the circumstance, that with the outbreak of a thun- 

 der-storm there often occurs a decided fall of temperature and sudden 

 rise of pressure. As he looks upon the crowding together of isobars as 

 the cause of a thunder-storm he enforced the following sentences: 



On the front side of the band of simultaneous electric discharges, 

 which band is perpendicular to the direction of progress and therefore 



