92 ATMOSPHERIC ELECTRICITY. 



Essex, on August 12 of the same year. Franklin himself describes 

 having used a kite in Philadelphia in a letter dated October 19, with- 

 out giving the date of his observations. But this must be supplied in 

 some passage which I have not been able to find, for Eosenberger 

 ("Geschichte der Physik," Vol. II, p. 316) mentions that it was done in 

 June. 



Franklin's disbelief in the dangerous character of the experiment 

 must have received a severe shock when he heard of the death of 

 G. W. Eichinann, who, in the year 1753, was killed by an electric dis- 

 charge drawn from the clouds by means of a kite. 



The thunderstorm is the most impressive effect of atmospheric elec- 

 tricity, though it is rivaled in beauty by the aurora, and in interest by 

 the many phenomena of daily occurrence which are only made percepti- 

 ble to us by proper instruments. In a lecture delivered before this 

 institution on May 18, 1860, Lord Kelvin described the delicate elec- 

 trical appliances constructed by him for the more accurate observation 

 of atmospheric electricity. The problems then for the first time clearly 

 stated gave a powerful and still lasting impulse to the investigation of 

 atmospheric electricity, and though no decisive answer can be given 

 to all the questions raised in that lecture, recent researches have 

 brought us somewhat nearer to their solution. 



Observations which may be made every day and at every place have 

 shown that the earth is electrified, whatever the weather may be. In 

 the language of the older theories, which we can not as yet altogether 

 abandon, we say that the earth is covered with negative electricity, or, 

 in modern phraseology, we express the same idea by the statement 

 that we move about in an electrified field, that electric lines of force 

 stretch through the air from the ground, from our bodies, and from 

 everything which is exposed to the sky overhead. The strength of this 

 electric field is not at all insignificant. If we wish to produce it arti- 

 ficially between two parallel plates kept at a distance of 1 foot, we 

 should have to apply an electromotive force sufficient — and sometimes 

 more than sufficient — to light up the incandescent lamps which illu- 

 minate our dwellings. The electric force is comparatively weak in our 

 country, but 50 volts per foot are constantly observed, and 100 volts 

 are not uncommon; but in drier climates the amount of the force may 

 be considerably in excess of these figures. 



If we fix our minds on the lines of force starting from the surface of 

 the earth, we are at once led to ask, Where is their other end? Do 

 they curve round and back again to earth? Do they end in the dust 

 which everywhere surrounds us, or do they reach up to the clouds? 

 Do they pass through the clouds and end where invisible particles 

 separate the sunset red from the midday blue? Or, finally, do they 

 leave the earth altogether, and form intangible bonds between us and 

 the sun, the stars, the infinity of space? These are not idle questions, 

 and we can not be said to have solved our problem unless some definite 



