"The temperature and condition of a body,"continued Cortlandt,"seem to depend entirely on its size.In the sun we have an incandescent,gaseous star,though its spots and the colour of its rays show that it is becoming aged,or,to be more accurate,advanced in its evolutionary development.Then comes a great jump,for Jupiter has but about one fourteen-hundredth of the mass of the sun,and we expect to find on it a firm crust,and that the planet itself is at about the fourth or fifth period of development,described by Moses as days.Saturn is doubtless somewhat more advanced.The earth we know has been habitable many hundreds of thousands or millions of years,though three fourths of its surface is still covered by water.In Mars we see a further step,three fourths of its surface being land.In Mercury,could we study it better,or in the larger satellites of Jupiter or Saturn,we might find a stepping-stone from Mars to the moon,perhaps with no water,but still having air,and being habitable in all other respects.In our own satellite we see a world that has died,though its death from an astronomical point of view is comparatively recent,while this little Pallas has been dead longer,being probably chilled through and through.
From this I conclude that all bodies in the solar system had one genesis,and were part of the same nebulous mass.But this does not include the other systems and nebulae;for,compared with them,our sun,as we have seen,is itself advanced and small beside such stars as Sirius having diameters of twelve million miles."As they left Pallas between themselves and the sun,it became a crescent and finally disappeared.
Two days later they sighted another asteroid exactly ahead.They examined it closely,and concluded it must be Hilda,put down in the astronomies as No.153,and having almost the greatest mean distance of any of these small bodies from the sun.
When they were so near that the disk was plainly visible to the unaided eye,Hilda passed between them and Jupiter,eclipsing it.
To their surprise,the light was not instantly shut off,as when the moon occults a star,but there was evident refraction.
"By George!"said Bearwarden,"here is an asteroid that HAS an atmosphere."There was no mistaking it.They soon discovered a small ice-cap at one pole,and then made out oceans and continents,with mountains,forests,rivers,and green fields.The sight lasted but a few moments before they swept by,but they secured several photographs,and carried a vivid impression in their minds.
Hilda appeared to be about two hundred miles in diameter.
"How do you account for that living world,"Bearwarden asked Cortlandt,"on your theory of size and longevity?""There are two explanations,"replied Cortlandt,"if the theory,as I still believe,is correct.Hilda has either been brought to this system from some other less matured,in the train of a comet,and been captured by the immense power of "Jupiter,which might account for the eccentricity of its orbit,or some accident has happened to rejuvenate it here.A collision with another minor planet moving in an orbit that crossed its own,or with the head of a large comet,would have reconverted it into a star,perhaps after it had long been cold.A comet may first have so changed the course of one of two small bodies as to make them collide.This seems to me the most plausible theory.Over a hundred years ago the English astronomer,Chambers,wrote of having found traces of atmosphere in some of these minor planets,but it was generally thought he was mistaken.One reason we know so little about this great swarm of minor planets is,that till recently none of them showed a disk to the telescope.Inasmuch as only their light was visible,they were indistinguishable from stars,except by their slow motion.A hundred years ago only three hundred and fifty had been discovered;our photographic star-charts have since then shown the number recorded to exceed one thousand."CHAPTER IV.
PREPARING TO ALIGHT.
That afternoon Ayrault brought out some statistical tables he had compiled from a great number of books,and also a diagram of the comparative sizes of the planets."I have been not a little puzzled at the discrepancies between even the best authors,"he said,"scarcely any two being exactly alike,while every decade has seen accepted theories radically changed."Saying which,he spread out the result of his labours (shown on the following pages),which the three friends then studied.
(1)MOVEMENT IN ORBIT.Velocity compared with earth as 1.
(2)MOVEMENT IN ORBIT.Period of revolution in years and days.
(3)MOVEMENT IN ORBIT.Orbital velocity in miles per second.
(4)Mean diameter in miles (5)Surface compared with earth as 1.
(6)Volume compared with earth as 1.
(7)Mass compared with earth as 1.
Planets (1)(2)(3)(4)(5)(6)(7)Mercury.....0.8823to 351.63,0000.140.0560.13Venus.....0.2241/221.91.177,7000.940.920.78The Earth...1.0018.51.07,9181.001.001.00Mars........1.8815.00.814,2300.280.1390.124Asteroids...3.29........From a few to 6.56miles to 300Jupiter.....11.868.10.4486,500118.31309.00316.0Saturn......29.466.00.321,0000.4760.095.0Uranus......84.024.20.2331,90016.365.014.7Neptune....164.783.40.1834,80019.390.017.
(1)Length of day.hrs.min.sec.
(2)Length of seasons (3)DENSITY Compared with earth as 1(4)DENSITY Compared with water as 1
(5)FORCE OF GRAVITY AT SURFACE OF PLANET Compared with earth as 1.
(6)FORCE OF GRAVITY AT SURFACE OF PLANET Bodies fall in one second.
(7)Inclination of axis.
Planets (1)(2)(3)(4)(5)(6)(7)Mercury..................1.247.170.8513.7.....
Venus...232122........0.925.210.8313.453+The Earth......Spring,931.005.671.0016.09231/2Summer,93
Terrestrial days Autumn,90
Winter,89
Mars...243723Spring,1910.962.540.386.2271/2Summer,181
Martian days Autumn,149
Winter,147
