THE INTENSITY OF TERRESTRIAL MAGNETISM. 265 



persons, about seven letters can easily be signaled in one minute, 

 as many experiments have shown. If^ instead of manipulation, 

 appropriate mechanical arrangements were adopted, the velocity 

 and precision would undoubtedly be considerably increased. 



It is precisely in this kind of telegraphing that the new appa- 

 ratus possesses a considerable advantage over the magnetometer ; 

 and for the following reasons. Although the two opposite im- 

 pulses, of which one simple signal consists, are exactly equal 

 m force, — and consequently the second destroys just as much 

 velocity as the first produced, — yet the needle cannot be in 

 perfect quiescence between the signals, because this perfect 

 quiescence is only possible when the needle is in its natural 

 position of equilibrium. Even if it is in this position pre- 

 vious to a signal, it is somewhat disturbed therefrom by the 

 signal itself, and the directive force acting on the needle causes 

 it to tend to return. Though a single signal causes only a very 

 slight movement, yet a considerable disturbance from the na- 

 tural position of equilibrium will arise from the accumulation 

 of a great number of signals ; and the result will be so much 

 motion between the signals that they will lose somewhat of 

 their sharpness of expression. It will easily be seen, on con- 

 sideration, that under circumstances otherwise similar, this dis- 

 advantage is greater when the needle employed has a short in- 

 terval of vibration, than Avhen it has a long one. Its effect is 

 greater, therefore, on the magnetometer in the Magnetic Obser- 

 vatory, than on the 25-lb. needle suspended in the Astronomical 

 Observatory ; and is least of all on the new appai'atus, when its 

 magnet bar, by being placed in the reverse position, is converted 

 almost into an astatic needle. Thus, even when the needle is at 

 a considerable distance from its position of equilibrium, the com- 

 paratively weak directive force, with which it tends to return to 

 that position, does not produce in it any movements which can 

 materially disturb the signals, while the current in the multiplier 

 acts as strongly on the needle, and consequently produces quite 

 as rapid movements, as if it belonged to an ordinary magneto- 

 meter. 



A peculiar apparatus, which I have lately caused to be con- 

 structed, is highly useful in preventing the disadvantages and 

 inconveniences arising from untimely vibratory motions, both in 

 this kind of telegraphing, and in many other applications of mag- 

 netic apparatus. I give it the name of a damper, as its action con- 



