McAdie — I^ew Units in Aero-physics. 281 



Wind. 



It seems desirable to abandon the use of compass points in 

 designating wind direction and to use hereafter degrees, 

 starting from true north as the zero point. The new system 

 eliminates the possibility of confusion of the terms east and 

 west and also saves time in trigonometrical work and com- 

 puting. The air in motion, or wind, will be regarded as 

 flowing in to the center. For example, the wind previously 

 recorded as northeast, because of blowing from that direction, 

 will now be recorded as 45°. Yelocities are to be recorded in 

 meters per second. 



Humidity. 



As at present recorded, the moisture of the air is expressed 

 in terms of relative humidity. This is at best a ratio and 

 means different amounts of water vapor present with different 

 temperatures. The absolute humidity, or weight of vapor in 

 grams, is sometimes given. Likewise the vapor pressure. It 

 would seem best to abandon the relative humidity and here- 

 after give the weight of water vapor in grams per thousand 

 cubic meters of space. There should also be some means of 

 representing the pressure of saturated aqueous vapor at the 

 given temperature. 



Miscellaneous. 



There should also be suitable units for recording amounts of 

 solar energy absorbed and radiated ; also popular units for 

 hours of sunshine and percentages of the possible. Precipita- 

 tion should be recorded in millimeters and snow in its water 

 equivalent. Cloud heights and cloud velocities should be 

 given in the metric system. 



A few of the new units and symbols employed follow : 



Fb equals pressure in units of force, base 1,000,000 dynes 



b " one raegadyne, or bar 



db " decibar 



cb " centibar 



mb " millibar 



B, " pressure at 106 meters, equals 750-1""" (29-632 inches) 



B„ " " " sea level, " 760-0 " (29-92 inches) 



Wj " weight " Bj " 1019-8 grams per sq. cm. 



W,, " " " sea level, " 1030 grams per sq. cm. 



H " height of homogeneous atmosphere 



above B, .•_.. equals 788,504'='" 



T "273 C. plus t. 



R " gas constant for air, " 2,832,250 



Am. Jour. Sci.— Fourth Series, Vol. XXX, No. 178.— October, 1910. 

 19 



