Page:Popular Science Monthly Volume 16.djvu/325

Rh Now, with such clear barometric and thermometric conditions in and around Great Britain, a "cold, wet summer" in 1879 was almost inevitable, and a prediction to that effect, based on the simultaneous international data, would have been justifiable. Of course, new conditions might arise late in June, but the conditions prevalent, according to all physical probability, authorized such a forecast at the commencement of the summer, and would have been of incalculable value. Ask the British farmer what he would have freely paid in June to have gained some idea of the July weather! Or ask the English merchant what he would have freely given in June for a tolerably correct crop-forecast for the summer of 1879! Yet this is no hypothetical case, but one familiar to all, involving a whole nation in agricultural and financial distress, against which, with international reports from the Faroe Islands and Iceland, it could have been forewarned.

The collection of the "international" cloud-observations—especially those taken at sea—opens one of the most fertile and fascinating fields, not only for the elucidation of the profoundest atmospheric problems by the theoretical scientist, but for the popularization of meteoric knowledge. The clouds are Nature's weather-guides, at all times serving to preannounce the approach of storms, or the return of clear weather. Until the middle of the seventeenth century (when Torricelli's experiments led to the invention of the barometer), and long after, the clouds furnished the only weather-indications which the world had. And, the more modern meteorology is developed, the more do the ablest of its leaders seek to understand these unerring monitors of every weather-change. Varied as they are, their forms may be reduced to two or three types, so defined and imposing that the most unscientific can learn to recognize them and construe their meaning. The international observers enter on their blanks the amount and direction of clouds, "The most valuable of weather-signs," says Mr. Ley, "are obtained not so much from the shape of the clouds as from the directions from which clouds of different levels are observed to travel, and it is these weather-signs which, in the present state of our knowledge, can be most readily reduced to definite rules."

Take a single illustration of the utility which such rules would have for the farmer, the sailor, or any close observer of the sky. Our storm-centers are generally preceded by a great bank of those clouds to which the name cirro-stratus is given. They are composed largely of freezing or frozen vapor, floating at great elevations, and often very far in front of the depression and over the belt of country which is to receive its rainfall. They move in parallel lines, and are distinguished by their thread-like and attenuated delicacy, as well as by their altitude—from 20,000 to 40,000 feet—from all local clouds. Outlying streaks of the cirro-stratus, frequently visible from twenty to one hundred miles in advance of the main pack, "like pioneers of the coming army," can easily be detected. But the main body, since it forms the