1864.J ggg [Chase. 



There is a species of mechanical polarity, of which I have never seen 

 any notice, that is apparently produced by motions resembling those 

 to which the air is continually subjected. It may be exhibited in 

 the following ways : 



1. In the middle of a basin of water, lay a long strip of any sub- 

 stance (floating it by corks or otherwise, if it is heavier than water). 

 After the water has become still, lift the basin carefully by one hand, 

 and hold it at arm's length. The intermitting muscular action pro- 

 duces longitudinal vibrations, which tend to bring the floating strip 

 into a line with the outstretched arm, and the tendency may be 

 increased by moving the basin gently up and down. 



2. Hold the gimbals of a binnacle compass so that it can swing 

 only in one direction, and cause it to move like a pendulum in that 

 direction. The needle will tend towards the line of oscillation. 

 Vessels may have been lost from ignorance of this fact, for it is not 

 unusual for compass pivots to become so worn that the needle moves 

 sluggishly, and in order to start it, the compass-box is shaken. If 

 one of the gimbal hinges should be rusty, the shaking would bring 

 the needle into a line perpendicular to the axis of the free gimbal, 

 and the captain might easily suppose that he was sailing north, when 

 his course was due east or west. 



3. Take an ordinary pocket compass, grasp it firmly between the 

 thumb and finger of one hand, and move it quickly up and down 

 through a small arc. The needle, as in the last instance, will tend 

 towards the plane of motion. This experiment may be variously 

 modified, according to the length and directive energy of the needle, 

 the steadiness of the operator's nerves, &c. Sometimes a simple 

 grasp, with a powerful muscular contraction, will bring the needle 

 into line, without any other vibration than that which arises from 

 the irresistible nervous tremor. Sometimes the momentum acquired 

 by each pole in its approach to the operator, carries it forward so as 

 to bring the other pole under the wave-influence, and the needle is 

 thus made to rotate so rapidly as to become nearly invisible. 



The polarity in each of the three cases here enumerated, is easily 

 explained upon purely mechanical principles, but there are some 

 indications that seem to show a close connection between the mecha- 

 nical vibrations and those of nervous electricity. There appears to 

 be a great diS"erence in the control of different individuals over the 

 needle. Some can bring it into line at once, with scarcely any per- 

 ceptible motion, while others are obliged to use considerable effort; 

 the needle does not seem at all times equally susceptible ; it often 



