I have no data at hand regarding this comet, but what I have said will serve to illustrate my ideas. Again, referring to this comet, I remember to have read the statement of an astronomer that, after passing round the sun, a new tail was formed opposite the original one. Now, it seems to me that that is just what would happen, for in moving round the sun the comet would travel say 3,000,000 miles; the greater portion of the tail then, would extend millions of miles upon one side of the sun, while from the nucleus upon the opposite side of the sun a new tail would appear to be formed. Upon this hypothesis, the extraordinary length of their tails and the fact that stars are visible through the densest portion of them is explained; as also the fact that they so rapidly disappear from view when moving from the sun, the light received by them from the sun being in proportion to their distance from it, and but little of that reflected. JOHN M. HUGHES. Brooklyn, N. Y. * * * * * [FOR THE SCIENTIFIC AMERICAN.] ON COMETS. When we see a comet approaching the sun with its tail following in the orbit of the nucleus, we have no great difficulty in believing the common theory that a comet consists of nucleus attracted toward the sun, while the tail is repelled; and that we see the whole of it. But as it approaches the sun, difficulties arise that make us doubt whether the theory be true. Let us suppose a comet with a tail 50,000,000 miles in length, and that it will require two days to pass round the sun. Now the tail, being always in a line drawn through the center of the sun and center of the nucleus, will, when it reaches the long axis of the elliptical orbit, stand perpendicularly to the orbit of the nucleus. That is, the extremity of the tail farthest from the sun, in addition to its onward motion, has acquired a lateral motion that has lifted it 50,000,000 miles in the first day of its perihelion. The velocity of the extremity has been vastly accelerated over that of the nucleus, and it has moreover a sheer lift above the orbit of the nucleus. Now this lift is in opposition to gravity; neither is it in consequence of any previous momentum, for its velocity is accelerated and its previous momentum would be a hindrance; nor is the lift in consequence of any repelling force from the sun, for such force would be diminished in proportion to the square of the distance, and the far end would be acted on less than the nucleus end of the tail, whereas the velocity of the former is increased a hundred fold over that of the latter. A polar force in the comet would merely draw the comet into the sun. We therefore find no force adequate for such a lift, but on the contrary all the forces are opposed to it.