McAdie — Neio Units in Aero-physics. 277 



Art. XXX. — iVe?« Units in Aero-physics; by Alexander 



McAdie. 



Pressure. 



Since the time of TorricelH and Pascal, measures of atirios- 

 plieric pressure liave been in linear units ; i. e., tlie height of a 

 column of water or mercury in vacuo. The barometer, in 

 En_£>;]ish-speaking countries, is read in inches and decimal parts 

 tliereof ; and in countries where the metric system prevails, in 

 centimeters or millimeters. For example, a standard atmos- 

 phere is generally defined as the pressure of a vertical column 

 of pure mercury whose height is 29-92 inches or 760 millime- 

 ters, at the temperature of melting ice, in latitude 4.5°, under 

 normal gravity and at sea level. Evidently then, if one wishes 

 to know the ratio of certain changes in atmospheric pressure 

 occurring from day to day and fi-om hour to hour, variations 

 which are of prime importance to the meteorologist, he must 

 refer the change to some normal or assumed standard height. 



•75 

 For example, a ftill or a rise of '75 of an inch would be ^^_^. 



19 . . 



or, if metric values are used, ~— -r mm. Again, in expressing 



lines of equal pressure, as on the daily weather maps, we draw 

 for differences of one-tenth of an inch ; or, if the 2"'™ gradient 

 is used, '078 of an inch. The method of reduction is cumber- 

 some ; and it is doubtful if the average reader of the synoptic 

 charts obtains any clear conception of the actual meaning of 

 pressure variations. The entire process of representing pres- 

 sure conditions would be much simplified if variations could be 

 given in percentages or permillages of a standard atmosphere. 

 Such an arrangement was suggested by the writer in a paper 

 published in the Monthly Weather Review in August, 1908. 

 Dr. W. Koppen'* of the Deutsche Seewarte, Hamburg, in 

 March, 1909, modified the proposal, suggesting instead of the 

 sea-level pressure, the new base, the pressure represented by 

 the value one million dynes. In other words, instead of using 

 the pressure indicated by 760"'™ (29-92 inches), which in force 

 units would be 1,013,303 dynes (obtained by multiplying 

 1033-291 grams per square centimeter by normal acceleration 

 of gravity 980-65), use a megadyne, corresponding to a pres- 

 sure of 750-1""" (29-532 inches). This quantity is called a 

 small atmosphere (with Bjerknes and Sandstrom) or a £ar. 



* See also Koppen's Memoir before the Aerological Congress at Monaco, 

 April, 1909. 



