Page:Popular Science Monthly Volume 59.djvu/551

Rh been claimed by some that the polarization of blue sky light can be used in studying the vertical distribution of fog, and that changes in atmospheric conditions are shown by this means several hours in advance of other precursory appearances.

Strangely enough within the past year and from an unexpected source, suggestions have been made which should be considered with some care and then tested. In discussing the mortar batteries used at Windisch Feistritz, Dr. Pernter has given us some data concerning vortex rings. These are the rings which, according to Burgermeister Stiger and his associates, successfully protect their vineyards from hail. Whatever the real cause may be regarding hail, we are thankful for the opportunity to study such large and energetic vortices. These rings are powerful enough to tear a thick paper screen to pieces at a distance of 100 meters. On leaving the mortars in a horizontal direction the whirls have a velocity of about 170 miles per hour or eight times the velocity of the stiff surface indraft of air on summer afternoons through the Golden Gate. At a distance of 100 meters the velocity was reduced nearly 50 per cent. With the Suschnig apparatus, the charge of powder being 250 grammes, Dr. Pernter found an initial velocity of about 55 meters per second. The probable limit of upward movement was 400 meters. Dr. Hann has suggested that the results obtained by shooting these rings into winter fogs should be carefully studied. The suggestion is pertinent. At Mt. Tamalpais, as we have tried to show, unusually good opportunities exist for experimenting upon fog. Many varieties of formation occur. The tule fogs of winter, in one of which the 'Rio de Janeiro' was lost, sometimes do not exceed 100 feet in depth. The summer afternoon sea fogs are more dense and more sharply delined. Some of the fogs are due to direct cooling by contact; in some the cooling is due to radiation, and, in the great majority of cases, the cooling is due to mixture. The differences in temperature, humidity and air motion are so marked that it is likely that differences in electrical potential, dust-content and ionization also exist. There is urgent need of bettering our knowledge of these matters. Practical applications will speedily follow.

The sacrifice of life on that ill-fated steamship on the morning of February 23 will not have been altogether in vain, if it leads to a thorough study of the conditions governing fog.