WIND TABLES. XXV 



records and is widely practiced. Although anemometers are used at meteor- 

 ological observatories, the majority of observers are still dependent upon 

 estimates based largely upon their own judgment, and so reliable can such 

 estimates be made that for many purposes they abundantly answer the 

 needs of meteorology as well as of climatology. 



A great variety of such arbitrary scales have been adopted by different 

 observers, but the one that has come into the most general use and received 

 the greatest definiteness of application is the duodecimal scale introduced 

 into the British navy by Admiral Beaufort about 1800. 



Table 39 is taken from the Observer's Handbook of the Meteorologi- 

 cal Office, London, edition of 191 7. The velocity equivalents in meters 

 per second and miles per hour are based on extensive observational data 

 collected by Dr. G. C. Simpson and first published by the Meteorological 

 Office in 1906. Several other sets of equivalents have been published in 

 different countries. For a history of this subject see Rept. 10th Meeting 

 International Meteorological Committee, Rome, 1913, Appendix VII. (Lon- 

 don, 1914.) 



In the Quarterly Journal of the Royal Meteorological Society, volume xxx, 

 No. 132, October, 1904, Prof. A. Lawrence Rotch has described an instru- 

 ment for obtaining the true direction and velocity of the wind at sea aboard 

 a moving vessel. If a line A B represents the wind due to the motion of a 

 steamer in an opposite direction, and A C the direction of the wind relative 

 to the vessel as shown by the drift of its smoke, then, by measuring the 

 angle DBA that the true wind makes with the vessel — which is easily done 

 by watching the wave crests as they approach it — we obtain the third side, 

 B C, of the triangle. This represents, in direction and also in length, on the 

 scale used in setting off the speed of the ship, the true direction of the wind 

 relative to the vessel and also its true velocity. The method fails when the 

 wind direction coincides with the ship's course and becomes inaccurate 

 when the angle between them is small. 



CALCULATION OF THE MEAN DIRECTION OF THE WIND BY LAMBERT'S 



FORMULA. 



Lambert's formula for the eight principal points of the compass is 



m E-W+ (NE + SE- NW - SW) cos 45 

 an a N-S+ (NE + NW - SE- SW) cos 45 ' 



a is the angle of the resultant wind direction with the meridian. 

 E, NE, N, etc., represent the wind movement from the corresponding 

 directions East, Northeast, North, etc. In practice, instead of taking the 

 total wind movement, it is often considered sufficient to take as propor- 

 tional thereto the number of times the wind has blown from each direction, 



