JOSEPH HENRY 129 



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

 of the sun's disk. The first experiments were made on January 4, 

 1845. A verv * ar g e s P ot was tnen visible upon the sun, the image 

 of which was thrown 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. 



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 dis- 

 tance from each other on its path. For this purpose the observer 

 is provided with a revolving cylinder, moved by clockwork at the 

 rate of at least ten turns in a second, and of which the convex sur- 

 face is divided into a hundred equal parts, each part therefore 

 indicating in the revolution the thousandth part of a second or less. 

 Close to the surface of this cylinder, which revolves horizontally, 

 are placed two galvanometers, one at each extremity of a diame- 

 ter; the needles of these being furnished at one end with a pen for 

 making a dot with printers' ink on the revolving surface. In the 

 appendix to the paper he proposes to dispense with the galvan- 

 ometer and produce the marks by direct electromagnetic action, 

 as is now done in the familiar astronomical chronograph. 



It is impossible in the course of this short sketch to present any 

 full account of Professor Henry's scientific researches. Hejyas a 

 born expprimpntajist^ one who knew how to cross-examine Nature 

 as an astute lawyer would cross-examine a witness and thus bring 



