TEE BLUE HILL OBSERVATORY. 301 



In the study of the relation of clouds to rainfall are taken up : clouds 

 preceding rain, clouds between intervals of rain and clouds following 

 rain. The methods and cause of cloud formation were also carefully 

 considered. The most important part of this special investigation is 

 the use to which the cloud observations are put in the study of atmos- 

 pheric dynamics, taking up in succession the questions: the relation 

 of clouds to cyclones and anti-cyclones, having regard to the altitudes 

 of the cloud levels; the annual and diurnal periods in the winds in 

 general, at various levels as shown by direct observation near the ground 

 and extended upwards to high altitudes by means of the observed cloud 

 movements; the wind movements in cyclones and anti-cyclones from 

 the ground up to the altitude of the highest clouds ; the relation of the 

 direction of the cirrus clouds to the existing tem.perature gradient ; the 

 relation between the velocity of storms, and the consequent variability 

 of the w^eather, to the general movement of the atmosphere as shown 

 by surface wind and cloud observations; the use of cloud observations 

 in weather forecasts; and the frequency of winds from various direc- 

 tions at different heights above the ground, for different hours of the 

 day, shown by wind and cloud observations. 



The work of making observations of the atmospheric conditions in 

 the free air by means of kites has been carried out with the success 

 achieved only by the persistent endeavors of the observatory staff, not 

 only in overcoming the difficulties in the mechanical construction of 

 the apparatus employed, but also in the actual work of kite flying. 



Experiments were undertaken as to the best forms of kites to use, 

 the best materials for their construction, and the best lines to use for 

 flying them. Special forms of self-recording meteorological instru- 

 ments had to be so designed or so changed as to be adapted to the 

 demands of kite work. Great care was exercised in so exposing the 

 instruments that their possible errors would be reduced to a minimum. 

 During the year 1897 there were thirty-eight successful kite flights, 

 in 1898 thirty-five, in 1899 twenty-five, and in 1900 twenty- four; the 

 average height above sea level at which records were obtained during 

 the respective years being 7,350 feet, 7,400 feet and 8,450 feet, thus 

 showing constant improvement in the methods employed. 



The discussion of the Blue Hill observations has added very materi- 

 ally to our still meager knowledge of the distribution of the meteoro- 

 logical elements in the free air, and their variation with change in alti- 

 tude. The average increase of wind velocities with increasing altitude 

 was determined chiefly for those altitudes for which we have the fewest 

 data because it is so difficult to make measurements there by means of 

 the clouds. The change in direction of air currents at different levels 

 was also clearly and accurately brought out by the changes in position 

 of the kites as they ascended and descended. Such data as these are 



VOL. LIX. — 20 



