ON SHOOTING STARS. 17 



2d. That the estimates of timo are in general too small. This is quite probable. Tho 

 mind may not make proper allowance for the time that elapses after tho shooting star is seen 

 before the eye is fairly directed to the place of the path. 



3d. That many of the meteoroids move in hyperbolic orbits about the sun. Whatever 

 may be said of the sporadic meteors, this cannot be true of the members of the August and 

 November groups. 



Tlie sporadic shooting stars cannot all belong to one narroio ring which has a diameter nearly 

 equal to that of the earth's orbit. 



Such a ring would have to be but little inclined to the ecliptic in order to furnish meteors 

 throughout the year. The bodies could not have a retrograde motion, else shooting stars 

 would be seen only in the morning hours, and would moreover have a very distinctly marked 

 radiant. 



They cannot have a direct motion; for their velocity must then be nearly equal to that 

 of the earth, and yet a little less than it, in order that more be seen in the morning than 

 in the evening. Their relative velocity on entering the atmosphere would be not much 

 greater than that of a body falling to the earth from an infinite distance, — that is, not much 

 greater than 11 kilometres per second. So small a msan velocity is entirely inconsistent both 

 with direct observation and with the conclusions given above, on page 1G. 



We might, it is true, suppose the ring to have a considerable breadth, in which case the 

 meteors would have a larger mean relative velocity. But if the breadth be such as to furnish 

 a velocity at all consistent with observation, we have no longer a ring lying between the 

 orbits of Mars and Venus, but a disk extending much beyond these planets. 



A large portion of the meteoroids must, when they meet the earth, have absolute velocities 

 g rater than the earth' s velocity in its orbit; or else — ■ 



The sporadic meteors have a series of radiants at some distance from the ecliptic, and hence 

 come from a series of rings considerably inclined to the earth's orbit; 



For shooting stars cannot have relative motions upwards from the earth when they enter 

 the atmosphere. If, then, the absolute velocities of the meteoroids were all less than those of 

 the earth, their relative motions (disregarding the earth's attractions) would all be from 

 points of the heavens less than 90° from that point to which the earth is moving. To an 

 observer early in the evening but a small part of this hemisphere is above the horizon, while 

 in the morning almost all of the hemisphere is visible. Hence, either the number of those 

 seen in the morning should be very much greater than that of those seen in the evening, or else 

 there should be a radiant in that part of the heavens which is above the horizon in the earlier 

 hours of the night. If the earth's attraction be considered, the numbers seen in the evening 

 would be somewhat increased, as also the numbers seen in the morning. The disproportion, 

 therefore, between the morning and evening hours would be slightly diminished. But even 

 with this allowance the increase through the night would be greater than the observed 

 increase, unless one of the two suppositions above given is true. 



To determine the amount of this increase there is great deficiency of reliable data. Mr. 



(307) 



