318 



METEOROLOGY AND ALLIED SUBJECTS. 



cloudiness appears to favor the development of certain auroras. As to 

 the connection between the magnetic perturbations and the aurora, the 

 observations show that perturbations of the needle may take place 

 without the auroral display, and vice verm, the aurora without pertur- 

 bations of the needle. Those forms of aurora that present irregular 

 outlines, no rays, and no perceptible motions, are seldom accompanied 

 by perturbations: on the other hand those auroras that appear to have 

 a small altitude, and to be very near to us, having definite outlines, 

 rapid movements, and well-marked radial structure, set the needle in 

 rapid movement. {Z. 0. 6. ill., XIY, 1879, p. 190.) 



P. F. Denza has given the following laws relating to atmospheric elec 

 tricity, based on twelve years' observations — six times daily — at Mon- 

 calieri: Regular variations. — He says the daily variations show them- 

 selves clearly in the winter and summer, having two maxima after 

 sunset and sunrise. The yearly variations attain their maxima at the 

 end of February, the minima in September. The annual means show 

 no connection with sun spots or magnetic changes. Irregular varia 

 tions. — Thunder storms affect the tension very much. Eain and snow 

 increase the electricity. Dense fog and haze, cloudiness, etc., increase 

 the intensity very slightly. The least electricity is shown during clear 

 or very clear sky, and especially when it is also very warm. Southerly 

 winds, especially southeast, increase the electricity; but during strong 

 winds the indications of the electrometer are uncertain. During these 

 twelve years, out of a hundred cases of rain and snow, fifty give nega-^ 

 tive and fifty positive electricity; the same ratio holds good for thun- 

 der storms and hail, whether they pass over the observer or ])ass by 

 at a distance. Negative electricity occasionally is observed before and 

 after a thunder storm, and also more rarely before and after a rain or 

 snow. When the sky is clear or completely covered, electricity is always 

 positive, and negative electricity occurs only under dissimilar condi- 

 tions — such as distant storms, clouds, auroras, etc. An electrometer 

 being also placed on St. Bernard, at an altitude of 2,160 meters, the com- 

 parison with observations at Moncalieri'(altitude 259 meters) shows- 

 that under normal conditions the electric tension increases with the alti- 

 tude. The following table gives the mean results of observations at 

 Moncalieri : 



Hourly means. 



6a. m. 

 9a.m. 

 Noon. 

 3 p. m 

 6p.m 

 9p. m 



Moan. 



14.86 

 16.38 

 13.67 

 13.58 

 16.42 

 15.65 



15.09 



Monthly means. 



Annual means. 



January I 18. 



February 1 20. 2 



March..' 1 16.1 



April 



May 



June 



July 



August - 



September 



October 



November 



December 



Mean I 15.09 



I I _. ^ _J 



Z. 0. G. M., Vol. XIV, 1879, p. 484. 



