550 Proceedings of the 



it; this last was also covered with a glass receiver, and was sus- 

 tained on a plate of glass a t about four inches distant from the 

 revolving magnet. When the iron began to rotate, a large mass of 

 copper, of about a foot square, and three inches in thickness, was 

 interposed ; the copper being placed on a convenient carriage, 

 moveable on a rail-way, so as to admit of being interposed very easily 

 without deranging the subject of experiment. The result was, that 

 the motion of the iron became soon sensibly diminished, and at last 

 ceased altogether, on withdrawing the intervening copper, the 

 motion of the iron commenced, and this could be repeated at 

 pleasure. Similar effects were evident with four heavy masses of zinc 

 in blocks, each about an inch thick : Mr. Harris had also, he observed, 

 obtained the same result with a dense mass of silver, of about three 

 inches thick. He, therefore, concludes that this screening power is 

 common to every substance in any degree susceptible of magnetic 

 excitation, and is probably in the direct ratio of its energy, as esti- 

 mated by observing its influence in fettering the vibrations of an 

 oscillating magnetic bar. To exemplify a similar screening influ- 

 ence to that just mentioned, by means of distilled water at 32 

 Fahrenheit's scale, or a little below, Mr. Harris believes it would be 

 requisite to obtain a slight action on the disc of iron at about thirty 

 feet distance, so as to interpose nearly that thickness of ice. Mr. 

 Harris accompanied his observations by occasional experimental 

 illustrations ; he seemed very carefully to distinguish between the 

 magnetic state, which amounts to a case of permanent polarity, and 

 that induced or transient state which vanishes when the exciting 

 cause is removed, to which he considered the immediate effects now 

 in question might be referred ; for, whilst the intervening mass un- 

 dergoes magnetic change by induction, it at the same time neutra- 

 lises, in a greater or lesser degree, the power of the exciting magnet 

 on a third substance. 



In the library, Mr. Harris illustrated, by a few experiments, the 

 operation of two instruments, which he had recently invented for 

 investigating the laws of magnetic forces. First, a magnetimeter 

 for general purposes. A small cylindrical mass of iron, or otherwise 

 a magnet, is balanced by means of a hydrometric counterpoise 

 from the horizontal diameter of a delicate wheel, moveable about an 

 axis on friction wheels. This wheel is furnished with an index 

 formed of a light straw, duly adjusted, so as to indicate divisions on 

 a graduated arc, when an attractive or repulsive force is made to 

 operate on the suspended iron or magnet. By means of a light 

 frame of brass, and an adjusting screw, a magnet or a mass of iron 

 can be brought to act under many varying conditions, as to distance, 

 position, &c. on the suspended body, and the force due to the latter 

 simultaneously observed. Second, an instrument for measuring 

 magnetic intensity by means of an oscillating bar. The bar is sus- 

 pended through a long tube of glass, by means of a filament of silk, 

 and vibrates under a graduated ring of paper with its poles quite 



