218 Royal Institution. 



which would make the eclipse of Agathocles nearly central over the 

 northern station, and excludes the possibility of his passing by the 

 southern route. 



The author then adverts to the principal remaining causes of un- 

 certainty in these conclusions, and points out the values of progress- 

 ive change in the secular mean motions as peculiarly deserving 

 investigation. 



Allusion is then made to a record in the Persian poetical history, 

 preserved by Sir John Malcolm, which appears to point to a total 

 eclipse as occurring under similar circumstances in the province of 

 Mazenderan. It appears however on calculation, that no total 

 eclipse passed over Mazenderan, at least for many years, about the 

 time in question. 



The author then calls attention to the statement of Herodotus, 

 that something like a total solar eclipse occurred when Xerxes was 

 setting out from Sardes for his invasion of Greece. On calculation 

 it appears impossible to explain this by a solar eclipse, and moreover 

 the peculiar turn of the answer of the Magi to the inquiries of Xerxes 

 would seem to be irreconcileable with a solar eclipse. The author 

 thinks it most likely that the phenomenon really was the total eclipse 

 of the moon which occurred on the morning of b.c. 479, March 14. 

 If this were adopted, the date of the invasion of Greece must be 

 brought down one year later than that given by the received chro- 

 nology. 



ROYAL INSTITUTION OF GREAT BRITAIN. 



Jan. 21, 1853. — Observations on the Magnetic Force. By Pro- 

 fessor Faraday. 



Inasmuch as the general considerations to be brought forward had 

 respect to those great forces of the globe, exerted by it, both as a 

 mass and through its particles, namely, magnetism and gravitation, 

 the attention was first recalled briefly to certain relations and differ- 

 ences of the two which had been insisted upon on former occasions. 

 Both can act at a distance, and doubtless at any distan^^e , but whilst 

 gravitation may be considered as simple and unpolar in its relations, 

 magnetism is dual and polar. Hence one gravitating particle or 

 system cannot be conceived to act by gravitation, as a particle or 

 system, on itself; whereas a magnetic particle or system, because 

 of the dual nature of its force, can have such a self- relation. Again, 

 either polarity of the magnetic force can act either by attraction or 

 repulsion ; and not merely so, but the joint or dual action of a 

 magnet can act also either by attraction or repulsion, as in the 

 case of paramagnetic and diamagnetic bodies : the action of gravity 

 is always that of attraction. As a further consequence of the differ-* 

 ence in character of the powers, little or no doubt was entertained 

 regarding the existence of physical lines of force* in the cases of 

 dual powers, as electricity and magnetism ; but in respect of gravi- 



* Proceedings of the Royal Institution, June 11, 1862, p. 216; also 

 Phil. Mag. S.4. 1852, vol. iii. p. 401. 



