1895.] on Atmospheric Electricity. 461 



Chelmsford, Essex, on August 12th of the same year. Franklin 

 himself describes having used a kite in Philadelphia in a letter dated 

 October 19th, without giving the date of his observations. But this 

 must be sujDplied in some passage which I have not been able to find, 

 for Eosenberger * 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. Richmann, who, in the year 1753, was killed by an electric 

 discharge drawn from the clouds by means of a kite. 



The thunderstorm is the most impressive effect of atmospheric 

 electricity, though it is rivalled in beauty by the aurora, and in 

 interest by the many phenomena of daily occurrence, which are only 

 made perceptible to us by proper instruments. In a lecture delivered 

 before this Institution on May 18, 1860, Lord Kelvin described the 

 delicate electrical 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 cannot 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 artificially between two parallel plates kept at a distance 

 of one foot, we should have to apply an electromotive-force sufficient 

 — and sometimes more than sufficient — to light up the incandescent 

 lamps which illuminate our dwellings. The electric force is compara- 

 tively weak in our country, but 50 volts per foot are constantly 

 observed, and 100 volts are not uncommon ; but in dryer 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 cannot be said to have solved our problem unless 



* ' Geschichte der Physik,' vol. ii. p. 316. 



