SHORT MEMOIRS ON METEOROLOGICAL SUBJECTS. 



473 



(where jp is the pressure and t the temperature of the air whose density 

 is represented by /?). 



Upon the synoptic chart, the natural unit of length is the degree of 

 the meridian, which is represented at any point by the distance between 

 the lines corresponding to the terrestrial parallels. I will call d the 

 value of the distance dr expressed in this unit. As our formula sup- 

 poses that the meter is taken as the unit of length, the value of Ar that 



we shall use will be 



10 000 000-" , 



Ar = 



90 



(90° of the meridian of the earth are equivalent to 10,000,000 meters). 

 We have, finally, 



^ ' 80400 &0100 



After ?Jl substitutions have been made, the formula which gives the 

 value of y becomes 



y_ 0.005 X 13.596 x 9.8088 x 86400 x 90 

 2 X 0.001293 X 2 X 3.1416 x 10 000 000 ^ 

 whence, by making the calculations as indicated, 



©■".TOO .. 1 + a< 



0.760 1-^at 

 X — :— i; 



P 



d sin A 



F=31™.91x 



X 



P 



sin X 



In this formula, there enter four elements — the barometric pressure 2?) 

 the temperature t, the distance of the isobars 5, and the latitude X ; but 

 the daily variations of the two first elements exert only a feeble influ- 

 ence, which may be neglected in a first approximation. The pressure |> 

 in the synoptic chart has reference to the level of the sea, and varies 

 between the limits 715 and 785 millimeters; consequently, the factor 

 0.760 remains always comprised between the limits 0.907 and 1.06. As 

 to the temperature t, its variations have a greater importance ; but, as 

 they are not given upon the synoptic charts,* we are obliged to neglect 

 its daily variation, and can take account only of the mean temperatures. 

 In order to simplify the matter, we have assumed that this mean tem- 

 perature, supposed to be reduced to the level of the sea, depends only 

 upon the latitude X, and we have formed a table of double entry giving 

 the values of the velocity V corresponding to various values of X and d. 

 This table has been formed especially in reference to the zone covered 

 by the synoptic charts, that is to say, the regions of the North Atlantic 

 and of Western Europe comprised between the parallels of 30° and 60° 

 of north latitude. 



' Those, e. g., of the Paris Observatory. 



