486 Intelligence and Miscellaneous Articles, 



In any system of conducting wires isolated from the ground, or 

 communicating therewith by one or several points, and submitted to 

 the simultaneous action of several batteries acting with any force 

 and in any direction, the galvanometric effects which must be pre- 

 sented by any one of these wires are precisely the same, whether we 

 regard the currents produced by the differences as independent or 

 not, that is to say, either according to one or the other of the two 

 following theories : — 



1 . That each battery determines its particular system of currents, 

 and that each wire is traversed simultaneously by all the unaltered 

 partial currents, on condition that the currents of each battery taken 

 separately are subjected to the laws of Ohm, and submitted not only 

 to the resistance of the battery which produces it, but also to that 

 of all the other batteries. 



2. That in each wire we have a single current furnished by the 

 composition of the forces with which the different batteries set elec- 

 tricity in motion. 



M. Belli has taken up a certain number of particular cases, and 

 shown that the facts constantly agree with the two theories (at least 

 if we admit the conditions enunciated above), and that consequently 

 we must seek for some other method of determining which of the 

 two hypotheses is the true one. — // Nuovo Cimento, vol. ii. p. 401. 



MEASUREMENT OF THE SPEED OF A RAILWAY TRAIN BY MEANS 

 OF ELECTRO-MAGNETTSM. BY W. C. M'^REA. 



The wheels of a car rotate a certain number of times in going over 

 a given space of road ; for example, suppose a car- wheel to be eight 

 feet in circumference, it will rotate 660 times in going the distance 

 of a mile. Now, if the car were so constructed that the body would 

 always remain at an equal distance from the axle, which is prevented 

 by the motion given it by the springs, it would not be difficult to 

 bring the wheel at each revolution to so bear upon a lever, as to ro- 

 tate a wheel inside the car, w^hich might have a.& many cogs as the 

 car-wheel rotates times in going a mile. Or, by a series of clock- 

 work wheels, the indicating wheel might contain a fractional number 

 of cogs in proportion to the number of times which the car- wheel 

 would rotate in a given distance. 



The difficulty occasioned by the unsteady motion of the car, owing 

 to the springs, may be overcome by the use of a magnet, battery, 

 and galvanic circuit ; the latter to be so arranged as to be broken at 

 each revolution of the car-wheel or axle. So by this means the 

 operation of the magnetic contrivance would be to move the speed- 

 indicator the distance of one cog. 



A contrivance of this kind may be so constructed as not to require 

 winding, in order to have it in readiness for use. 



The indicator may be so constructed as to have an index placed 

 over it, in such a position that the hand on the indicator should move 

 it a sufficient distance to show at each successive rotation the num- 

 ber of miles already travelled. 



The kind of battery most suitable for such a contrivance would be 

 that of the '* sand battery/' which, if properly constructed, may b6 



