UNDISCOVERED PLANETS. 133 



heat to our earth and its sister planets, which causes summer to emerge 

 from the dreariness of winter, but it is only affording us gradually a 

 portion of its superfluous temperature with which it was originally crea- 

 ted. The earth gives decided evidences that its temperature was once 

 far more elevated than at present; and we doubt not, that originally the 

 planets too, as well as it, had the same temperature as the sun, but be- 

 ing small bodies, they have, by cooling, long since reached the point o'f 

 equilibrium ; and that at some distant day the sun will wander through 

 space as cold and rayless as the frigid earth. And however, high itg 

 present temperature may be, the total amount of heat which it contains 

 may be very small when compared with that vast amount of insensible 

 heat which fills all space. 



ON THE PROBABLE EXISTENCE OF UNDISCOVERED PLANETS. 



BY D. KIRKWOOD, OF LANCASTER, PA. 



Previous to 1S45, when Aslrea, the fifth asteroid, was first detected 

 by the keen eye of Professor Hencke, the hope of discovering any new 

 planetary members of our system seems to have been generally aban- 

 doned. The two recent additions, however, to the number of planets 

 render it now an interesting inquiry whether the exploration has been 

 sufllciently complete to furnish grounds for deciding upon the probabi- 

 lity or improbability of further discoveries. We shall consider : 



1. Whether there areprohaNy aiuj planets within the orhit of Mercury. 

 The distance from the centre of Jupiier to the nearest satellite is about 

 three times the equatorial diameter of the Primary. If, therefore, we sup- 

 pose the distance of the nearest primary planet to have the same ratio 

 to the diameter of the sun, the orbit of such planet will be somewhat 

 less than three millions of miles from the sun's centre. Consequently, 

 in the interval of thirty-seven millions of miles there may be four pla- 

 nets, the orbit of the nearest having the dimensions above stated, and 

 their respective distances increasing in the ratio of Mercury's distance 

 to that of Venus. Such bodies, however, in consequence of their prox- 

 imity to that luminary, could hardly be detected, except in transiting 

 the sun's disc. 



2. Whether there be yet any undiscovered asteroids between Mars and 

 Jupiter, or any similar bodies ni the other interplanetary spaces. We 

 think no sufficient reason can be assigned for concluding that none of 

 this interesting group of planets have hitherto escaped observation ; but 

 if such bodies exist there can be little or no prospect that they will ever 

 become known by their disturbing influence upon any of the other 

 members of the system. Consequently there remains no other method 



