FOG STUDIES ON MOUNT TAMALPAIS. 



539 



vin meteorograph from the summit to the valley and back, the descent 

 and ascent requiring about 100 minutes, the distance being about 16 

 miles. The instrument was hung at the top of an open canopied car in 

 such a way as to secure a good air circulation. The average dew-point 

 was 50° F. (10° C.) and the maximum weight of a cubic foot of water 

 vapor at this temperature and saturation 100 per cent, is a little over 4 

 grains. Estimating an average fog bank as covering an area of fifty 

 square miles and extending from the 500-foot level to the 1,500-foot 

 level, the maximum weight of the water vapor would be about 400,000 

 tons; and if this condensed vapor could be suddenly precipitated it 

 would be the equivalent of a rainfall of about one-tenth of an inch. 

 But condensation and precipitation are not identical. The processes 



Mond§y2Z. 1893. Tuesday 23. 



Temperature Records. 



wbich cause the collapse, if it may be so called, of a cloud of fog are 

 obscure. Elaborate experimentation is needed at this point before the 

 problems of fog dissipation or rain-making can be solved. With the 

 fogs of the San Francisco Bay district and indeed with every dense 

 cumulo-nimbus or cumulus cloud, the condensation is considerable and it 

 w^ould seem at times as if but a very gentle initiative would lead to pre- 

 cipitation. Various methods of removing dust particles from the at- 

 mosphere have been suggested within the past few years, and possibly in 

 thus removing the dust the essential nuclei of condensation may be re- 

 moved. Conversely, in order to bring about precipitation, it may be 

 fcund necessary to supply at the proper time the proper nuclei. At any 

 rate, it is known that by various methods, of which may be mentioned 

 filtering, clarifying, recondensing, calcining and electrifying, smoke, 

 fog, dust and condensed vapor may be removed from limited spaces. The 



