190 



SYMPOSIUM ON AERONAUTICS. 



observations in the region navigated. These observations on the 

 spot are of fundamental importance and in practice cannot be safely 

 set aside for forecasts or the indications of means as to the upper 

 air conditions. 



Fig. 2. Mean velocity of winds from each of eight directions for the year 



at selected stations. 



Charts of means are in a sense the aeronauts' charts of the 

 medium he navigates, but it must be kept in mind that these charts, 

 in which results of observations are usually shown with reference 

 to surface pressure distribution, are to be used with the current 

 weather map. 



Observations are made by means of kites, captive balloons and 

 free balloons. Kites and captive balloons carry automatically re- 

 cording instruments wdiich record continuously temperature, pres- 

 sure, humidity and speed of movement of the air. The free balloons 

 used are of two sorts, sounding balloons and pilot balloons. The 

 former carry an instrument which automatically records tempera- 

 ture, pressure and humidity of the air. Observations of air move- 

 ment are obtained by means of continuous theodolite observations 

 upon the balloons. In the case of sounding balloons, heights may be 

 computed from the pressure record, and o1)servations with one theo- 



