1843] WRITINGS OF JOSEPH HENRY. 215 



such a current of sufficient intensity to mark the paper, 

 since in some of his experiments on induction, he has devel- 

 oped one which gave a spark between a point and a surface 

 of nearly a fourth of an inch in length. 



The terminal points of the wires from the two screens 

 may be placed very near each other in the same horizontal 

 plane: if then the cylinder revolving horizontally has at 

 the same time a slow ascending motion, the relative position 

 of the dots on the paper will give the number of whole 

 turns and parts of a turn, made by the cylinder while the 

 ball is passing between the two screens. In the same way 

 the terminal points of wires from a number of different 

 pairs of screens may be made to impress their marks on the 

 surface of the same cylinder, and the velocity of the ball at 

 the different points of its path may in this way be deter- 

 mined by a single experiment. 



ON THE APPLICATION OF THE THERMO-GALVANOMETER TO 



METEOROLOGY, ETC. 



(Proceedings of the American Philosophical Society, vol. iv, pp. 22, 23.)* 



November 3, 1843. 



Professor Henry made an oral communication in regard 

 to the application of Melloni's thermo-electric apparatus to 

 meteorological purposes, and explained a modification of 

 the parts connected with the pile, to which he had been led 

 in the course of his researches. He had found the vapors 

 near the horizon powerful reflectors of heat; but in the case 

 of a distant thunderstorm he had found that the cloud was 

 colder than the adjacent blue space. 



* [The title-page of vol. iv, (comprising the proceedings from June, 1843, 

 to December, 1847,) bears date 1847.1 



