— 
B. V. Marsh on the Distinguishing Features of Comets. 99 
The waves of light originating at the nucleus and extending 
to the end of the tail are doubtless nothing more than unusually 
large clouds of particles the whole flight of which we can trace. 
Similar phenomena sometimes take place in the auroral streamer, 
the eye Gane able to detect with certainty the upward direction 
of the motion, showing that in these instances the velocity al- 
though so far exceeding any movement of matter with which 
Wwe are acquainted upon the earth that it may fairly be termed 
electrical, still falls much below the velocity of electricity itself. 
The slowest motion of this kind appears to be in the “ auroral 
hes,” which are probably identical with streamers except in 
that they are spread out laterally (in an East and West direc- 
tion) thus occupying what Prof. Pliicker calls a “‘ magnetic sur- 
face.” It may be said that if we admit the height of 500 or 600 
miles claimed for auroral streamers, it may indidate that the at- 
mosphere extends to that height rather than that particles are 
carried up by the current. But if the comet’s tail is admitte 
to be identical with the streamer, it affords an answer to this ar- 
acipg beeause it cannot be claimed that the comet can possibly 
ave an atmosphere co-extensive with its tail. So that the ex- 
planations of the two phenomena support each other. 
ooey dissipate the mass, But this depends upon the rarity of 
the matter emitted, which may be such that the whole amount 
ost during a perihelion passage may be very inconsiderable. 
Some effect of this kind has been suspected by Professors Peirce 
and Mitchel, who conelude that the decrease in the periodic time 
Encke’s comet may be due to this cause. _ 
€ foregoing considerations seem to establish a Dechy ye 
e sola 
sudden 
““eenly burst forth as a comet of great splendor. 
phia, Noy. 26th, 1861. 
