FOG STUDIES ON MOUNT TAMALPAIS. 541 



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 ad- 

 vance of other precursory appearances. 



Strangely enough within the past 3'ear and from an unexpected 

 source, suggestions have been made which should be considered witli 

 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 

 ]00 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. iVt 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 'Sio de Janeiro' was lost, sometimes do not exceed 100 feet in depth. 

 The summer afternoon sea fogs are more dense and more sharply de- 

 lined. Sonic 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 electri- 

 cal potential, dust-content and ionization also exist. There is urgent 

 need of bettering our knowledge of these matters. Practical applica- 

 tions 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. 



