102 'APPENDIX I. 



These historical details, perhaps, will not be 

 deemed superfluous, inasmuch as they are calculated 

 to bring out more fully the beauty and the high 

 scientific value of the following researches. 



Description of M. Pouillefs Clironoscope. More 

 than twenty years ago, M. Pouillet suggested a very 

 ingenious plan for measuring the velocity of pro- 

 jectiles. If an electric current flows constantly 

 through the coil of a galvanometer, the needle is 

 deflected to an amount which depends upon the 

 intensity of the current, and upon the sensitiveness 

 of the galvanometer. But if the current be sent 

 through the coil only for a time so short that it 

 vanishes when compared to the duration of one 

 oscillation of the needle, things happen differently. 

 The needle then receives as it were a single impulse, 

 yielding to which it slowly recedes, till its velocity 

 has been annihilated by the magnetic force of the 

 earth, which draws it back to zero. And the initial 

 velocity imparted to the needle by the current, pro- 

 vided this be of constant intensity, will be propor- 

 tional to its duration, so that from the velocity, 

 which can be calculated from the deflection of the 

 needle, the duration of the current may be inferred. 

 The galvanometer is thus transformed into a chrono- 

 scope, which can be used for measuring the duration 

 of rapidly transient processes, whenever there is a 

 possibility of making the beginning and the end of 

 the process coincide with the beginning and the end 

 of the chronoscopic current, for so we will style the 

 current, which, by its action on the galvanometer, 

 becomes an indicator of time. And similarly we 



