considerable atmosphere. For besides the permanent spots on its surface, I have often noticed," he says, "occasional changes of partial bright belts and also once a darkish one in a pretty high latitude. And these alterations we can hardly ascribe to any other cause than the variable disposition of clouds and vapours floating in the atmosphere of that planet." From the fact that the dark belts or spots and the red colour of Mars manifestly belong to the surface of the planet, we may accept Herschel's idea "that its inhabitants probably enjoy a situation in many respects similar to ours." It has been shown in our own day that the vapour of water, and with that we may associate clouds, is present in the atmosphere of Mars. But there is reason to believe that the atmosphere of Mars is comparatively rare.
Jupiter was not one of the planets from which Herschel reaped an ungathered harvest. The field had been so thoroughly worked by others in searching for a method of easily discovering the longitude at sea, that it does not seem to have presented the same attractions to him as other planets did. A paper which he wrote on Jupiter in 1797—and he wrote no other—gives many curious quotations from his journal regarding the planet and its satellites. So minute are the discoveries made of change of colour and apparent size of the satellites that if the Red spot, detected on the planet in 1878, had been visible in his day, he could scarcely have failed to see it. The bands or belts on the body of the planet, the white and dark spots they showed, the length of day they indicated, and the rotation of the four satellites round their primary were the principal points attended to by
