In the _Challenger_ Report are shown the various movements of the atmosphere, with their corresponding causes. It is thus observed that the prevailing winds are produced by the inequality of the mass of air at different places. The air flows from a region of higher to a region of lower pressure, _i.e._ from where there is an excessive mass of air to fill up some deficiency. And this is the great principle on which the science of meteorology rests, not only as to winds, but as to weather changes. Of the sun's rays which reach the earth, those that fall on the land are absorbed by the surface layer of about 4 feet in thickness. But those that fall on the surface of the ocean penetrate, as shown by the observations of the _Challenger_ Expedition, to a depth of about 500 feet. Hence, in deep waters the temperature of the surface is only partially heated by the direct rays of the sun. In mid-ocean the temperature of the surface scarcely differs 1 deg. Fahr. during the whole day, while the daily variation of the surface layer of land is sometimes 50 deg. The temperature of the air over the ocean is about three times greater than that of the surface of the open sea over which it lies; but, near land, this increases to five times. The elastic force of vapour is seen in its simplest form on the open sea, as disclosed by these Reports. It is lowest at 4 A.M. and highest at 2 P.M. The relative humidity is just the reverse. When the temperature is highest, the saturation of the air is lowest, and _vice versa_. So on land when the air, by radiation of heat from the earth, is cooled below the dew-point, dew is produced, and, at the freezing-point, hoar-frost. The _Challenger_ Reports, too, show that the force of the winds on the open sea is subject to no distinct and uniform daily variation, but that on nearing land the force of the wind gives a curve as distinctly marked as the ordinary curve of temperature. That force is lowest from 2 to 4 A.M., and highest from 2 to 4 P.M. Each of the five great oceans gives the same result. At Ben Nevis, on the other hand, these forces are just reversed in strength. It is also shown by the _Challenger_ observations that on the open sea the greatest number of thunder-storms occur from 10 P.M. to 8 A.M. And, from this, Dr. Buchan concludes that over the ocean terrestrial radiation is more powerful than solar radiation in causing those vertical disturbances in the equilibrium of the atmosphere which give rise to the thunder-storm. CHAPTER XXXV WEATHER-FORECASTING To foretell with any degree of certainty the state of the weather for twenty-four hours is of immense advantage to business men, tourists, fishermen, and many others. The weather is everybody's business. And the probabilities of accurate forecasts are so improving that all are more or less giving attention to the morning meteorological reports.