-1859] 



WRITINGS OF JOSEPH HENRY. 



369 



slow approach of the cloud, or by its increase in intensit3\ 

 The fluid therefore will escape into the ground without being 

 perceptible in the form of sparks, but when the repulsion is 

 suddenly relieved, at least in part, by a discharge of the cloud, 

 the natural electricity rushes back and exhibits itself in 

 flashes and sparks, and may even give shocks to persons in 

 the vicinity. Although this sudden return of the electricity 

 from the earth into which it has been driven, (in ordinary 

 cases of conductors in a house supported by bad conducting 

 materials,) is usually attended with but slight effects, yet it 

 may under certain circumstances produce serious accidents, 

 particularly when a person is in good conducting connec- 

 tion with the earth. A remarkable instance of this kind was 

 described by Mr. Brydone, in a letter to the president of the 

 Royal Society, in 1787. 



Two laborers, each driving a cart loaded with coal, and 

 sitting upon the front part, ascending a slight eminence, the 

 one following the other at a distance of about twenty-four 

 yards, as represented at M and L, Fig. 20, were conversing 



Fig. 20. 



about the thunder which was heard at a distance, when in 

 an instant the man in the hinder cart was astounded by a 



24-2 



