388 ANNUAL OF SCIENTIFIC DISCOVERT. 



ops, partially but not entirely surrounding the nucleus, and the emission of 

 streams or jets of luminosity, bright sectors, etc., in a direction inclined or 

 opposite to that of the tail. "With great variety of detail in other respects, 

 these have all a well-marked tendency to appear in the first instance on the 

 side of the nucleus next the sun. The great comet of the present year 

 undoubtedly takes a foremost rank in respect of the multiplied and most 

 curious changes which it has exhibited, and especially in the complete illus- 

 tration which it afforded of the origin, construction, and final dissipation of 

 a succession of envelops. In these phenomena, the process of the forma- 

 tion of the tail, from the substance in immediate contact with the nucleus, 

 is intimately concerned. The astronomer, night by night, sees the work of 

 evolution going on with an amazing rapidity, and seemingly in defiance of 

 the best established properties of matter, the laws of gravitation and of 

 inertia. The results are evident to all, but the secret cause is a profound 

 mystery admirably calculated to stimulate speculation and intelligent in- 

 vestigation. 



As regards the motion of comets in space, it is a well-established fact, so 

 far as our present means of observation extend, that their nuclei alone move 

 in obedience to the attractive force of the sun and planets. This property, 

 which has been recognized with consistency and uniformity, is not the least 

 singular peculiarity of their constitution. Immense volumes of matter, 

 apparently of the identical substance of the nucleus, go to compose the en- 

 veloping nebulosity and the tail; but from the moment of leaving the cen- 

 tral body, their motion is perfectly inexplicable, without assuming them to be 

 under the influence of laws of force which greatly modify that of gravita- 

 tion. 



The shape of the cometary orbits, described about the sun, is nearly that 

 of a parabola, or of an elongated ellipse, with periods of revolution varying 

 from a few years to many centuries. The point in the orbit which is nearest 

 the sun is called the perihelion ; the distance of this point from the sun, the 

 perihelion distance, and the time of the comet's passing it, the perihelion 

 passage. 



LUMINOUS THEORY OF COMETS' TAILS. 



The following is a resume of a paper on " Comets, and the Curvature of 

 their Tails," recently read before the Boston Society of Natural History, by 

 Dr. C. F. Winslow. 



A comet, strictly defined, is that portion of it called by astronomers its 

 nucleus. 



Comets consist of gaseous and very elastic matter, whose physical consti- 

 tution and luminous functions appear to be similar to, or identical with, the 

 gaseous envelops that surround the sun. 



The tail of a comet is not a material element, nor a physical constant, like 

 the nucleus, but only results from a transient evolution of luminous waves, 

 and is a sort of magnetic sequence, like a terrestrial aurora, depending on 

 the progressive gravitation and repulsion (i. <?., condensation and reaction) 

 that take place between the particles of the nucleus, as it approaches and 

 passes its perihelion. 



The luminous effect of solar action on comets, is similar to that exerted by 

 the solid mass of the sun itself on its photosphere. 



