448 MEMORIAL OF JOSEPH HENRY. 



solar spots. In connection with his relative, Professor Alexander, 

 he may be said to have commenced a branch of modern solar 

 physics which has since grown to large proportions, by comparing 

 the temperature of the solar spots with that of other parts of the 

 photosphere. The first experiments were made on January 4, 

 1845. A very large spot was then visible upon the sun, the image 

 of which was formed by a four-inch telescope upon a screen in a 

 dark room. A thermopile was placed in such a position that the 

 image of the spot and of the neighboring parts of the solar disk 

 could be thrown upon it in quick succession. The result of obser- 

 vations extending through several days was that decidedly less heat 

 was received from the spot than from the brilliant part of the 

 photosphere. It is believed that it was these experiments which 

 started Secchi on the brilliant investigations in solar physics which 

 he carried on in subsequent years. 



Among Professor Henry's latest electrical researches was his 

 analysis of the dynamic phenomena of the Leyden jar. The one 

 of his discoveries which he most often referred to in later years 

 was that the discharge of a Leyden jar did not consist of a single 

 restoration of the equilibrium, but of a rapid succession of libra- 

 tions back and forth, gradually diminishing to zero. This was 

 proved by passing the discharge through a coil of wire containing 

 needles of different degrees of magnetic force. After the discharge 

 these needles were found to be magnetized in different directions, 

 according to their size and hardness. 



In one of his numerous communications presented to the Philo- 

 sophical Society he appears as one of the inventors of the electro- 

 chronograph. On May 30, 1843, he presented and read a com- 

 munication on a new method of determining the velocity of 

 projectiles. It was in its essential parts identical with that now 

 generally adopted. It consisted, he says, in applying the instan- 

 taneous transmission of the electrical action to determine the time 

 of the passage of the ball between two screens placed at a short 

 distance from each other on the path of the projectile. For this 

 purpose the observer is provided with a revolving cylinder, moved 

 by clock-work at the rate of at least ten turns in a second, and of 

 which the convex surface is divided into a hundred equal parts, 



