We may now recall a fundamental fact in connection with any celestial body large or small. It is well known that, with the most powerful pieces of artillery that can be forged, a projectile can be launched with a speed of about half a mile per second. If the cannon were pointed vertically upwards the projectile would soar to a great elevation, but its speed would gradually abate, the summit of its journey would be duly reached, after which it would fall back again on the earth. Such would undoubtedly be the case if the experiment were made on a globe resembling our own in size and mass. But on a globe much smaller than the earth, not larger, for instance, than are some of the minor planets, it is certain that a projectile shot aloft from a great Armstrong gun would go up and up and would never return. The lessening gravitation of the planet would fail to recall it. Of course we are here reminded of Jules Verne's famous projectile. According to that philosopher, if a cannon was pointed vertically, and the projectile was discharged with a speed of seven miles a second it would soar aloft, and whether it went to the moon or not, it would at all events not return to the earth except by such a marvellous series of coincidences as those which he has described. But the story will at all events serve to illustrate the fact that for each particular globe there is a certain speed with which if a body leaves the globe it will not return.
It is a singular fact that hydrogen in the free state is absent from our atmosphere. Doubtless many explanations of a chemical nature might be offered, but the argument employed by Dr. G. J. Stoney is most interesting, inasmuch as it shows that the continued existence of hydrogen in our atmosphere would seem to be impossible.
