

of Terrestrial Magnetism. 451 



trillions of such bars to represent in space the magnetic force of 

 the earth ! The other is, that magnetism may act upon bodies 

 in a manner quite surprising, and of which we are very far from 

 forming an idea before seeing its effects*. The marvellous 

 experiment performed with RuhmkorfFs apparatus, in which a 

 cube of brass, 2 centims. in the side, rotating with the greatest 

 rapidity is struck motionless, if I may use the expression, by an 

 invisible force at the moment of the completion of the circuit of 

 the great electrical magnet between the poles of which it is 

 situated, and without being drawn to one side or the other, 

 remains there fixed, in spite of the powerful torsion of the wire 

 which tends to cause it to rotate, — and resumes its rapid motion 

 when the current ceases, — proves that non-magnetic bodies in 

 motion may, under the influence of a magnet, give rise to phe- 

 nomena of the most mysterious nature. An action of this kind 

 must take place between the earth and the sun, and thus per- 

 haps may be explained some of those anomalies which still 

 present no small difficulties to every theory which is proposed. 



The object of this memoir having been to coordinate the laws 

 of the diurnal motion of the needle, we leave to another oppor- 

 tunity the discussion of its secular variations, as well as of the 

 lunar action, which appears to Colonel Sabine to have been placed 

 beyond doubt ; and perhaps the time is not distant when the 

 diurnal magnetic variations will have been subjected to laws 

 depending only on the most simple relation of distance and angle 

 between two magnets. 



Conclusion. 

 The matters discussed at length in this memoir may be 

 summed up in the following propositions : — 



1. The action of the sun upon the needle is opposite, according 

 as the sun is north or south of the equator. 



2. The action of the sun on the declination-needle has a 

 period, in part but not entirely, analogous to that of the tempe- 

 rature and of the annual and diurnal meteorological changes. 



* I owe it to the kindness of R. P. Palladini, Professor in the College 

 at Naples, that I was a witness of this magnificent experiment, performed 

 with the apparatus with which he has provided the physical cabinet of that 

 college. In truth, one cannot but be surprised at seeing a body rotating 

 with such velocity, stop at the instant at which the circuit is completed, as 

 if it had encountered an invisible obstacle. The torsion given to the 

 thread was sufficient to make the cube rotate with such velocity as to ap- 

 pear a cylinder, and yet on completing the circuit it stopped instantly, and 

 when the current was sufficiently strong, it stopped without the angles 

 being directed to the poles of the magnet (as is usual in the case of weak 

 currents). The explanation of this fact is involved in the phenomena of 

 the magnetism of rotation, but I wished to adduce it as an instance of how 

 a force already known may act in certain circumstances in a new and 

 astonishing manner. 



2G2 



