110 



SCENERY OF THE HEAVENS. 



the summer solstice that the earth attains its highest temperature, although its daily supply 



of solar influence is then actually diminishing. 

 The comet appears in the diagram at its 

 perihelion passage merely for the sake of 

 illustration, as in that part of its course it 

 was completely lost in the solar blaze. In the 

 other positions it was observed at the times 

 stated by Cassini, Newton, Halley, and 

 Flam stead. 



Cometary statisticians have compiled a 

 record of between seven and eight hundred 

 appearances since the commencement of the 

 Christian era. But little dependence can be 

 placed upon this enumeration, as simple me- 

 teors and such phenomena as new stars were 

 confounded in former times with true comets, 

 and instances of the re-appearance of the 

 same body are no doubt included in the 

 return. During the age of cometary astro- 

 nomy down to the year 1831, the orbits of 

 137 have been observed ; and of these, the 

 perihelia of thirty lie within the orbit of 

 Mercury. The following calculation has 

 been founded upon this fact. The radius of 

 the orbit of Mercury is to that of Uranus 

 as 1 to 49 ; in other words, the latter is 49 

 times more remote from the sun than the former. Assuming the perihelia to be 

 distributed as thickly through the system as within the orbit of Mercury, the number 

 circumscribed by the path of Uranus will V to the number bounded by the path of 

 Mercury as the cube of the respective radii, or I 3 : 49 } :: 30=3,529,470, giving 

 upwards of three millions and a half as the number of comets visiting the system. 

 Some raise the number as high as seven millions, supposing that owing to fogs, daylight, 

 and great southern declination, one out of every two comets that have their perihelia 

 within the orbit of Mercury escape our notice. But the premises upon which this 

 calculation is founded seem to be erroneous. We have no more reason abstractedly to 

 infer the equable distribution of the perihelia of comets in the system than of the 

 planetary orbs; and the observed perihelion place of the 137 whose elements have been 

 studied is against such a conclusion. Of that number, there are 



Pcxihclioa . 



between Mercury and the sun 

 Mercury and Venus 

 Venus and Earth 

 Earth and Mars 

 Mars and Jupiter 

 Further than Jupiter 



30 comets. 



44 



34 



23 



6 







137 



Thus, as far as observation has proceeded, there is no increase in the number of perihelia 

 proportionate to the extent of the planetary spheres, which the preceding calculation 

 assumes. Arago has supplied several interesting and instructive analyses respecting the 



