NATURAL PHILOSOPHY. Ill 



stituting the immediate instrument of the induction, assures the perfect iso- 

 lation of the several convolutions, and if a spark too violent passes through 

 the instrument, the presence of a non-conductinir fluid stops its passage and 

 insures safety. M. Quet. thus using the Ruhmkorff machine, has produced 

 results which have hitherto defied the Voltaic battery, and, of course, the 

 ordinary electrical machine. 



STUDY OF THE THERMO-MULTIPLIER. 



This instrument, valuable to the experimentalist for its extreme delicacy, 

 and for its extensive scale, has been the object of careful theoretic and prac- 

 tical study, by M. de la Provostaye, whose results as reported to the Acad- 

 emy of Sciences, at Paris, may be summed up as follows : 



First, as to the galvanometer. 



1. "Whatever may be the position of the needles, the forces which act upon 

 each half are reducible practically to one, perpendicular to the plane of the 

 meridian. 



2. That the amount of deviation makes but a slight change in the amount 

 of this resultant : hence, the apparatus may be regarded as a tangent-needle 

 of a very considerable degree of perfection. 



Secondly, as to the thermo-electric pile. 



1. The progress of heating the thermometer takes place by the same 

 degrees, and in the same time, as if it were placed in a space at the constant 

 temperature which the pile attains under the influence of the source of heat. 



2. When the rise of the temperature is sufficiently small, if we withdraw 

 the caloi-ific action, it cools again, in the same time, and by the same degrees, 

 as if heated. 



Thirdly. He terminates the integral expression for the movement of the 

 needle ; shows that its position of rest under the action of the current is pro- 

 portioned to the constant quantity of the current when the anterior face of 

 the pile has assumed a stationary excess of temperature, and to the intensity 

 of the incident heat : and then derives expressions for the times correspond- 

 ing to the maximum and minimum excursions of the needle, and the extent 

 of these excursions ; and terminates with the following observation : " If, 

 after making an observation with the thermo-pile in the common way, and 

 awaiting the fixed deviation of the needle, the screen is replaced, the energy 

 of the current diminishes, and the needle returns to zero. I have found that 

 the retrograde motion of the needle, counted from the fixed deviation, takes 

 place by oscillations of the same extent and times as the primitive motion 

 counted from zero." 



HUGHES'S TELEGRAPH. 



The following is a full description of this somewhat famous instrument, 

 with its latest improvements, as it has been employed during the past year 

 on one of the lines between New York and Philadelphia. By it, messages 

 are transmitted simultaneously to and fro at the rate of two hundred letters 

 a minute each way. With all other telegraphs, the current runs through the 

 magnet of the instrument, and a sign is transmitted by breaking the current 

 for an instant; with this one, the line is connected with the ground, and the 

 current is made to pass through the machine only for an instant, when a sign 

 is to be transmitted. This arrangement constitutes one of its most important 

 advantages, namely : Any surplus of electricity produced by an overcharged 



