Variations of' the Barometer. 109 



scent of the barometer during storms, I conceive to be frequent- 

 ly influenced in no small degree by the reaction of the wind on 

 the acclivities of the earth"*s surface. When a horizontally 

 moving wind encounters an inclined plane, its direction is there- 

 by more or less elevated, and an increase of pressure necessarily 

 takes place on the reflecting surface. The whole pressure on 

 the inclined plane, when estimated in the vertical direction, is 

 obviously reduced in the ratio of the cosine of the inclination to 

 radius ; but still the vertical force exerted within a horizontal 

 square inch will, from the principles of hydrodynamics, be the 

 same as the pressure on an inch of the inclined surface. If 

 the one be equal to a column of 35 or 40 inches of mercury, so 

 will the other. This vertical pressure, therefore, exceeds that 

 of the barometer, in a sheltered place on the same level, in the 

 same ratio as the direct force on the inclined plane does. Even 

 a wall or precipice opposed to the wind, will occasion a greater 

 pressure on the ground at the windward side of its base. Hence 

 the mean of the whole vertical pressures over an extensive dis- 

 trict, exceeds what is indicated by the barometer in a sheltered 

 spot. 



Since, then, these surfaces, on which the wind forcibly acts, 

 sustain a weight greater in proportion to the part of the horizon 

 which they occupy, than the rest of the district does, a part of 

 the weight of the atmosphere is, as it were, supported on pillars 

 during a storm ; but the pressure indicated by our barometers 

 in sheltered spots, being only the diminished pressure between 

 the pillars, is therefore too small. This affords one very satis- 

 factory reason why the barometer should so often be depressed 

 duiing storms, especially where the surface of the country is 

 uneven, and sometimes likewise in a ship riding among moun- 

 tain-like waves. 



Between the tropics, the wind usually blows from the east, 

 diminishing the centrifugal force, which so far accounts for its 

 not depressing the barometer. And if, as is believed, the aerial 

 currents from the south-east and north-east, by meeting near the 

 equator, unite in a current directly from the east, which has no 

 other mode of escape but by accumulating upwards, and flow- 

 ing back to the tropics, this will aflbrd a farther explanation. 

 For the greater the wind, the more will it tend to accumulate 



