"How can I tell them apart on a photograph plate, sir?" asked Ralph. "That doesn't show any colors." "By their distance from the sun," the meteorologist replied. "Halos are seldom seen except at distances of about twenty-two degrees and forty-six degrees from the sun. There are lots of others, but they are rare. You'll soon learn to catch those distances by eye. Coronas are usually much smaller. "I think one of the most striking forms of cirro-stratus is the polar 'band,' which stretches from one side of the sky to the other, like a wide white road." "Ah knows that one, Mistah Levin," put in Dan'l. "Noah, he done stretch that road for the animals to get out of the Ark." The Forecaster glanced at the aged darky. "You certainly did manage to pick up a lot of queer superstitions in that article of yours, Dan'l. I've heard that cloud called a Noah's Ark cloud, but I never knew why." "Yas, suh; oh, yas, suh," Dan'l repeated earnestly, "Noah, he done make that cloud, jest like the rainbow was made to convince Noah that there weren't goin' to be no more floods." "A high cirro-stratus which looks as if some cream had been poured on the blue sky and hadn't mixed properly yet," the Forecaster continued, "is cirro-nebula. It's very hard to photograph, and even when you do get it on a plate, it doesn't look like much. "Now the third one in the classification is very familiar. That's the well-known mackerel sky. What's the rhyme about that, Dan'l?" Proud at being thus appealed to, the darky quoted triumphantly: "Mackerel scales and mares' tails, Make lofty ships carry low sails." "That's correct," said the weather expert, "because those clouds foretell wind. Sometimes the cloud flakes are less solid and look like the foam in the wake of a steamer. "Beneath them come the alto clouds, which are made up of drops of moisture instead of crystals of ice. The fourth class, called alto-stratus, is a thick sheet of gray or bluish color, sometimes thin enough to let the sun shine through. When lower and in heavy roundish masses it's called alto-cumulus, which is the fifth on the list, and when it is lower still and looks like a lot of great blue-gray footballs wedged closely together it is known as strato-cumulus." He shuffled the prints rapidly, selecting types of clouds as he did so, and pencilling on the back the character of the cloud. "Then comes the cumulus, the big round cloud, that looks like masses of fluffy cotton wool piled on top of each other. These are the 'woolpack clouds,' which, in summer time, throw deep shadows on the grass. It is this cloud which, when it comes between you and the sun, gives rise to the old saying that 'every cloud has its silver lining.'" "Those aren't the thunder clouds, sir, are they?" the photographer asked.