352 CLOUD WORK OF THE WEATHER BUREAU 



terial that could be used to determine the action of the higher 

 strata with reference to the formation and the progressive motion 

 of storms. Our observations are generally so exclusively made 

 in the lowest level of the ocean of air that comparatively imper- 

 fect information exists regarding the higher currents upon which 

 to found intelligent theories, and it is with the purpose of sup- 

 plying this deficiency that the series of international observa- 

 tions was undertaken. By the liberal policy of the United States 

 government the Weather Bureau was able to do its part of this 

 work. The discussion of the data is now nearly finished for the 

 report which it is expected to issue before the end of the present 

 year. While it is not practicable to give any detailed account 

 of the results, it may be interesting to have presented in The 

 National Geographic Magazine a brief synopsis of the scope of 

 the report now being prepared by the writer. 



The observations are divided into, two classes : (1) The primary, 

 which are made by means of two theodolites placed at the end 

 of a long base line adapted to triangulations in the vertical direc- 

 tion. These give the absolute heights, velocities, and direction 

 of motion of individual clouds ; between 6,000 and 7,000 of such 

 observations were made at Washington, D. C. (2) The secondary, 

 executed with nephoscopes at fourteen stations distributed at 

 nearly equal distances from each other over the districts east of 

 the Kocky mountains, give the relative velocities and direction 

 of motion, and with the help of the results obtained by the pri- 

 mary system can be translated into absolute values; there were 

 25,000 to 30,000 of these observations made in the United States. 



The discussion of these data has been divided into a number 

 of parts, of which the following may be mentioned in this con- 

 nection : (1) The distribution of the cirrus, cirro-stratus, cirro- 

 cumulus, alto-cumulus, alto-stratus, strato-cumulus, cumulo- 

 nimbus, nimbus, cumulus, stratus, was so determined that we 

 now know the average height of each type for every month in 

 the year and the depth of the zone or horizontal belt in which 

 they may severally occur. Thus the upper types are found in 

 layers as much as six miles thick, though they form most fre- 

 quently near the middle of their respective belts ; the lower are 

 thinner, and have some peculiar characteristics besides. When 

 we consider that the height and shape of these belts, changing 

 from month to month, indicates some very delicate physical 

 process going on in the aqueous vapor of the atmosphere, it is 

 easy to see that they become the best means for studying the 



