[ 78 ] 

 IX. Intelligence and Miscellaneous Articles. 



THOUGHTS ON THE rORMATION OF THE TAIL OF A COMET. 

 BY J. J. WATERSTON, ESQ. 



ANY attempt to account for the formation of the tail of a comet 

 upon exact physical principles would be assisted, if, in the draw- 

 ings of the telescopic appearances of the nucleus and adjacent coma — 

 which are sometimes provided when circumstances are favourable — 

 we were presented with the projection on the plane of vision of the 

 radius vector, and of the tangent to the orbit. In some cases it 

 might be possible to take such observations as would fix the actual 

 curve of the exterior line of the tail, referred to the radius vector, 

 passing through the centre of the nucleus, — i. e. supposing the central 

 axis of the tail to be in the plane of the orbit. This line is very- 

 marked in the front side of Donati's comet, and might probably be 

 easily fixed by the equatorial. 



If we view the tail as composed of molecules as free from the force 

 of cohesion as the molecules of an uncondensable gas, and raised from 

 the nucleus by the heat of the sun, and suppose that that heat as it 

 strikes upon each molecule is converted into a force centrifugal, that 

 not only effectually counteracts the force centripetal of the sun's 

 gravity, but that greatly exceeds it, such molecules will be quickly 

 removed from the feeble attraction of the nucleus, and assume the 

 motion of bodies entirely free from its influence. This hypothesis 

 is suggested by the mechanical theory as a possibility. 



If we inquire as to the centrifugal power of the sun's rays, we 

 may, with the data afforded by modern research, easily compute the 

 accelerative force it is capable of engendering on a single chemical 

 molecule, if their whole heating power were converted into such a 

 force. This is surpassingly great — no less than 800 miles per 

 second ! The data are, value of sun's radiation in a solar day equal 

 to rS ft. thickness of ice melted; the mechanical equivalent of 

 which is about 13 lbs. raised 1 foot high per second by the heat im- 

 pinging on a square foot. If this heat impinges on a superficial foot 

 of gold-leaf one molecule thick (about ] -200,000,000th of an inch, 

 as deduced from the relation of capillarity to latetit heat, both being 

 the measure of liquid cohesion, the first that of the superficial stra- 

 tum of molecules, the second that of a cubic mass of molecules), and 

 is converted into an impulsive force in one direction, it would in one 

 second communicate a velocity of 800 miles a second ; and in about 

 four minutes a velocity equal to light itself. It is, of course, an ex- 

 treme case to suppose the whole heat absorbed and converted ; but 

 it seems right to have in view the quantitative elements of the ques- 

 tion. Wlnle the earth's atmosphere absorbs heat from the sun's 

 rays, we have reason to suppose that heat-vibrations are converted 

 into rectilineal velocity of gaseous molecules ; but there is no reason 

 to suppose that the impulse is in the specific direction //-o/w the sun, 

 but equally /rom and to, as in vibratory impulses generally. It may 

 be shown that the mechanical equivalent of the solar heat that im- 



