SHORT MEMOIRS ON METEOROLOGICAL SUBJECTS. 477 



more the isobars are crowded together, the more numeroua are the 

 pilings of the wind-arrows. In an earlier initial memoir, entitled 'On 

 the Thunder-storms of Norway in 18G8,' I have attempted a determina- 

 tion of the numerical relation between the magnitude of the gradient 

 and the force of the wind, with the following results : 



"Gradient 20 30 40 50 60 70 80 90 100 kilometers. 



" Force of wind 4.9 3.9 3.1 2.3 1.8 2.0 1.5 1.8 1.4 on a scale of 6. 



" By a graphic interpolation, we deduce from these numbers the de- 

 grees* of force of wind on the scale employed and the corresponding 

 barometric gradients, as is shown in the following table. The approxi- 

 mate velocity of the wind, expressed in kilometers, per hour, which cor- 

 responds to the figures of the scale of force, are also added. 



Force of wind. 



6. Violent . . . 



5. Tr^s-fort .. 



4. Fort 



3. Assez fort 

 2. Mod6r6.-.. 

 1. Piiible 



"The force of the wind depends so strongly upon local causes that a 

 table like this can have only a general value. We can deduce au 

 inverse ratio between the gradient and the wind-velocity, such that 

 their product is equal to 14G0 (Barometric gradient X wind-velocity 

 = 1460). On the sea, the force of the wind will be for the same baro- 

 metric gradient much greater than in the interior country, where the 

 wind finds far more impediments to its movement over the surface of 

 the earth." 



We see that Mohn, in a work independent of ours and probably an- 

 terior thereto, has recognized the law of inverse ratio between the 

 velocity of the wind and the distance of the isobars. 



Let us compare the numbers resulting from his observations with 

 those that our theoretical formula gives. Mohn measures the velocity 

 of the wind in kilometers per hour ; we have taken for the unit the me- 

 ter and the second. In order to pass from these latter units to the first, 



the coefacient - ^^ = 3.G0 is to be used. 



The barometric gradient is defined by Mohn, page 15, as '' the number 

 of kilometers that it is necessary to go in the direction of the gradient 

 in order that the pressure of the air may diminish by one millimeter." 

 Our units for d are a degree of the meridian, or 111 kilometers, and a 

 barometric variation equal to five kilometers. Consequently, the value 



of the barometric gradient is -p-^, and its product by the velocity of 



o 



the wind, 3.G0 F, in our system of units, will have the value 



iii 5X3.60 "r=80F5. 

 5 



