Meteorites to the Earth? s Orbit. 13 



sonably say that the 94 stone-falls, about which we know compara- 

 tively little, seem decidedly to follow the same laws as the 116 

 falls about which we know so much more. 



This conclusion is greatly strengthened if we take account 

 of the effect of the earth's attraction in carrying the meteor's 

 quit toward the zenith. Any stone must be moving down- 

 ward when it enters the air. But the earth's attraction must 

 change the direction of its motion during the approach to the 

 earth. Hence the region ' of the heavens from which a stone 

 can approach the earth is not bounded by the actual horizon, 

 but by a curve which may be treated as a depressed horizon. 

 This depression of the horizon is far greater toward the quit 

 than toward the goal side of the horizon. The maximum de- 

 pression for a stone moving in a parabolic orbit is about 17°. 

 It hence follows that when the zenith is more than 73° and 

 less than 90° from G, both the points G and Q are above the de- 

 pressed horizon, and therefore that the 14 falls whose zeniths are 

 between these limits, that is, are between the circles AA and 

 PEPS, figure 2, should be left out of the count. The correspond- 

 ing region on the Q-hemisphere is less than one degree in 

 breadth, and contains one zenith point. We have left only 20 

 falls when the earth's goal alone was above the depressed hori- 

 zon to be compared with 59 falls when the earth's quit alone 

 was above the depressed horizon. 



Of the 50 observed falls constituting the third group, of 

 which the hour of fall is not stated, very few particulars other 

 than the fact of fall are known. Although we are left without 

 the power of saying that they indicate the same law as the 

 other 210 falls, we find at the same time no reason to suspect 

 the contrary. It is not unreasonable to assume that the well- 

 observed stone-falls are good representations of the whole 

 group, and to affirm the three propositions with which I set 

 out as true, in general, not only for the 210 stone-falls of the 

 first two groups, but for the whole 260 stone-falls which are 

 represented by stones in our cabinets, and in which the stones 

 were seen or known to fall. 



It also seems a natural and proper corollary to these proposi- 

 tions (unless it shall appear that stones meeting the earth are 

 destroyed in the air), that the larger meteorites moving in our 

 solar system are allied much more closely with the group of 

 comets of short period than with the comets whose orbits are 

 nearly parabolic. All the known comets of shorter periods than 

 33 years move about the sun in direct orbits that have moderate 

 inclinations to the ecliptic. On the contrary, of the nearly 

 parabolic cometic orbits that are known only a small propor- 

 tion of the whole number have small inclinations with direct 

 motion. 



