198 CONTINUITY. 



ances, and he invites further co-operative observation on the 

 subject, one of the highest interest, but at present remaining 

 in great obscurity. 



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 vis viva occasioned by friction of the 

 tidal waves, as well as by their forming, as it were, a drag upon 

 the earth's rotatory movement, the velocity of the earth's rota- 

 tion must be gradually diminishing, and that thus, unless some 

 undiscovered compensatory action exist, this rotation must 

 ultimately cease and changes hardly calculable take place in 

 the solar system. 



M. Delaunay considers part of the acceleration of the 

 moon's mean motion, which is not at present accounted for by 

 planetary disturbances, to be due to the gradual retardation of 

 the earth's rotation ; to which view, after an elaborate inves- 

 tigation, 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 insuperable, in this theory. Sup- 

 posing such cosmical bodies to exist in sufficient numbers, they 

 would, 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 motive force would 

 be lost, and heat generated in space by friction against such 

 medium. When these bodies arrive at the sun they would, 

 assuming them, like the planets, to have revolved in the same 

 direction, all impinge in a definite direction, and we might ex- 

 pect to see some symptoms of their action in the sun's photo- 

 sphere ; but though this is in a constant state of motion, and 

 the direction of these movements has been carefully investi- 

 gated by Mr. Carrington and others, no such general direction 

 is detected ; and M. Faye, 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, and 

 believes he has removed certain apparent anomalies and re- 



