aueoi:a boeealis. 241 



distance from it.* Auroras are therefore most abundant along a certain zone 

 ■whicli follows nearly a magnetic parallel, being everywhere nearly at right 

 angles to the magnetic meridian of the place. 



17. WJty auroras do not occur nnthin the tropics. — Auroras do not prevail 

 within the tropics, on account of the high intensity of the electricity, combined 

 with the high temperature of the lower atmosphere. By the rapid evaporation 

 within the tropics a vast amount of electricity is daily elevated into the air ; 

 but on account of the general dryness of the air this electricity is to a great extent 

 insulated, and cannot flow back again to the earth. Whenever there is a general 

 condensation of the vapor of the air, the precipitation is copious on account of 

 the large supply of vapor, and dense clouds are formed which are jiretty good 

 conductors of electricity. The electricity thtis accummulates and acquires great 

 intensity, moving with explosive violence in thixnder-showers, instead of the 

 slow and silent discharges of the aurora. 



By an extensive comparison of observations, t I have determined that — 



Between latitude 0° and latitude 30° the average nnmber of thunder-storms annually is 52 

 Between latitude 30° and latitude 50° the averaj^o number of thunder-storms annually is 20 

 Between latitude 50° and latitude (50° the average number of thunder-storms annually is 15 

 Between latitude 60° and latitude 70° the average number of thunder-storms annually is 4 

 Beyond latitude 70° 



Thus we see* that atmospheric electricity is most abundant in the equatorial 

 regions, where the causes which develop it are the most active ; and as we 

 recede from the equator, thunder-storms diminish in frequency, while auroras in- 

 crease in frequency, because circumstances favor a slow and quiet rather than a 

 rapid and violent discharge. The aurora and lightning differ, then, chiefly in 

 the mode of discharge of electricity from the atmosphere to the earth. 



18. Cause of the diurnal inequality in the frequency of auroras. — The diur- 

 nal inequality in the frequency of auroras is probably due to the same causes as 

 the diurnal variation in the intensity of atmospheric electricity. The intensity 

 of atmospheric electricity is found to vary with the hour of the day. From the 

 mean of three years' observations made at London, it appears that at 4 a. m. 

 the electric tension is represented by 20 on Volta's electrometer ; from this 

 hour the electricity increases to 10 a. m., when it is represented by 88 ; from 

 that time it decreases to 4 p. m., when it is represented by 69 ; it then increases 

 to 10 p. m., when it is represented by 104; from which time it decreases till 

 4 a. m. ; that is, there are two daily maxima of intensity and two daily minima. | 



The variations in the intensity of atmospheric electricity are to be ascribed 

 partly to real changes in the amount of electricity present in the air, and partly 

 to variations in the conducting power of the air. Just before sunrise the elec- 

 tricity has a feeble intensity, because the moisture of the preceding night has 

 transmitted to the earth a portion of the electricity which was jirevicusly present 

 in the air. After the sun rises new vapor ascends, and carries with it positive 

 electricity, and the amount of electricity in the air increases. Towards noon the 

 air becomes dry, and transmits less readily the electricity accumulated in the 

 upper regions of the atmosphere ; so that, although the amount of electricity in 

 the air is continually increasing, an electrometer near the earth's surface indi- 

 cates an apparent diminution. Towards evening the air grows cool, again be- 

 comes humid, and transmits more readily to the earth the electricity accumulated 

 in the upper regions of the atmosphere. The effect produced upon an elec- 

 trometer, therefore, increases until some hours after sunset ; but since during the 

 night there is a constant discharge of electricity from the air to the earth, the 



* De la Eive's Elec, v. 2, p. 243. 



t American Journal of Science, n. s., v. 30, p. 97. 



X Eeport British Assoc. 1849, pp. 117-191. 



16 s 



