474 SHORT MEMOIRS ON METEOROLOGICAL SUBJECTS. 



The application of this table to the synoptic charts is easy. Thus, let 

 us suppose that we are studying the charts of the last ten days of June, 

 1864, and that we desire to find the average force of the trade-wind 

 which, during those days, prevailed upon the Atlantic along the coast 

 of Portugal. By comparing the barometric pressures noted by the dif- 

 ferent ships that passed out a short distance from this coast with the 

 pressures observed at Lisbon, which remained very nearly constant dur- 

 ing the ten days, we find for S very accordant values, varying only 

 between 3.0 and 4.0. The latitude is 40° north, since the parallel of 40° 

 cuts the coast of Portugal between Lisbon and Oporto. From this we 

 conclude, by means of the above table, that the force of the wind must 

 have varied between 13 and 17 meters per second, and that its mean 

 value was very nearly 15 meters per second. 



Let us take a second, more complicated example, that of a whirling 

 movement of the air. The synoptic chart of the 14th of January, 1805, 

 shows that on this day a storm raged upon the coasts of France and 

 England. Let us seek the force of the wind which blew over the cen- 

 ter of France, between Paris and P>ordeaux. The barometric pressure 

 at 8 o'clock in the morning was 738.0 millimeters at Paris and 753.G 

 millimeters at Bordeaux and Toulouse. The difference, 15.0 millimeters, 

 corresponds to a distance which, reduced normally to the direction of 

 the atmospheric current, is 4P.5 of the meridian. 



We have, then, to take from the table that value of V which corre- 

 sponds to 



,=^« = lo.5 



at latitude 45^. This value is T = 31.5. Such would be the force of the 

 wind if the atmospheric current were [linear?], but as the air in the storm 

 has a turning movement, we can accept this figure only after having 

 applied to it a correction. I do not think it would be useful to enter into 

 the details of the calculation of this correction. It is made by applying 

 the formula that we have given for the turning movement, and the value 

 of the correction depends essentially on the ratio of the curvature of the 

 trajectory [with reference to the distance of the point considered as the 

 center of depression about which the wind rotates] and the velocity of 

 progression of the center. In the present case, the value of the correc- 

 tion will be found equal to 5 or meters, and we are led to adopt the 

 figure 26 meters per second as representing the average force of the 

 wind that prevailed between Paris and Bordeaux. 



The synoptic chart gives, indeed, the force of the wind at every point, 

 but the sign [or figure] representing this force corresponds to an indefi- 

 nite epithet which cannot be translated into precise figures. We can, 

 therefore, not verify the exactness of the result which our formula gives 

 for the force of the wind ; we can only state whether there exists a gen- 

 eral accordance between the results of the calculation and those of the 



