ADDRESS. Ixiii 



nial periods of terrestrial magnetic intensity, though only empu-ical at pre- 

 sent, might tend to lead us to a knowledge of the connexion wo are seeking ; 

 and the President of the Eoyal Society considers that an additional epoch of 

 coincidence has arrived, making the fourth decennial period ; but some doubt 

 is thrown upon these coincidences by the magnetic observations made at 

 Greenwich Obsei-vatory. In a paper published in the ' Transactions of the 

 E.oyal Society,' 1863, the Astronomer Royal says, speaking of results ex- 

 tending over seventeen years, there is no appearance of decennial cycle in the 

 recurrence of great magnetic disturbances ; and Mr. Glaishcr last year, in the 

 physical section of this Association, stated that after persevering examination 

 he had been unable to trace any connexion between the magnetism of the 

 earth and the spots on the sun. 



Mr. Airy, however, in a more recent paper, suggests that currents of 

 magnetic force having reference to the solar hour arc detected, and seem to 

 produce vortices or circular disturbances, and he invdtes further cooperative 

 observation on the subject, one of the highest interest, but at present re- 

 maining in great obscuritj'. 



One of the most startling suggestions as to the consequence resulting from 

 the dynamical theory of heat is that made by Mayer, that by the loss of vix 

 viva occasioned by friction of the tidal waves, as well as by their forming, 

 as it were, a drag upon the earth's rotatoiy movement, the velocity of the 

 earth's rotation must be gradually diminishing, and that thus, unless some 

 undiscovered compensatory action exist, this I'otation must ultimately cease, 

 and changes hardly calculable take place in the solar system. 



M. Delaunay considers that part of the acceleration of the moon's mean 

 motion which is not at present accounted for by planetaiy distiu-bauces, to be 

 duo to the gradual retardation of the earth's rotation ; to which view, after 

 an elaborate investigation, the Astronomer Royal has given his assent. 



Another most interesting speculation of Mayer is that with which you are 

 familiar, viz., that the heat of the sun is occasioned by friction or percussion 

 of meteorites falling upon it : there are some difficulties, not perhaps in- 

 superable, in this theory. Supposing such cosmical bodies to exist in suffi- 

 cient numbers they woTild, as they revolve round the sun, fall into it, not as 

 an aerolite falls upon the earth directly by an intersection of orbits, but by 

 the gradual reduction in size of the orbits, occasioned by a resisting medium ; 

 some portion of force would be lost, and lieat generated in space by friction 

 against such medium ; when they arrive at the sun they would, assuming 

 them, like the planets, to have revolved in the same direction, all impinge in 

 a definite dii'ection, and we might expect to see some symptoms of such in 

 the sun's photosphere ; but though this is in a constant state of motion, and 

 the direction of these movements has been carefully investigated by Mr. 

 Carrington and others, no such general direction is detected ; and M. "Paye, 

 who some time ago wrote a paper pointing out many objections to the theory 

 of solar heat being produced by the fall of meteoric bodies into the sun, has 

 recently investigated the proper motions of sun-spots, antl believes he has re- 

 moved certain apparent anomalies and reduced their motions to a certain re- 

 gularity in the motion of the photosphere, attributable to some general action 

 arising from the internal mass of the sun. 



It might be expected that comets, bodies so light and so easUy deflected 

 from their course, would show some symptoms of being acted on by gravita- 

 tion, were such a number of bodies to exist in or near their paths, as are 

 presupposed in the mechanical theory of solar heat. 



Assuming the undulatory theory of light to be true, and that the motion 



