of Energy in the Atmosphere. 463 



That the barometer, as a rule, still stands low when the 

 equatorial current itself has already reached the ground is 

 explained for the most part by the fact that, in consequence 

 of convective power of the moving air, all the quiescent 

 masses of air in the neighbourhood of the current suffer rare- 

 faction. But the barometer shows the pressure of the sur- 

 rounding quiescent air, and not the true pressure of the air 

 entangled in the movement. A barometer in the car of a 

 balloon moving rapidly with a storm must therefore indicate 

 a perceptibly higher pressure than a stationary one*. 



The vis viva active in winds and storms has thus its origin 

 essentially in the acceleration which the air rising in the tropics 

 attains in consequence of superheating at the earth's surface. 

 The vis viva equivalent to this is transferred especially to the 

 upper highly rarefied strata of air. In consequence of their 

 momentum these are driven, with small loss of velocity by 

 internal friction, to the polar regions of the earth. They 

 retain the mean velocity of rotation which they possessed 

 when raised in the equatorial latitudes. They must therefore, 

 when they pass to higher latitudes, move more rapidly than 

 the more slowly rotating earth, and hence, as seen from the 

 earth, must approach the poles in spirals of decreasing incli- 

 nation. If on this course they turn still earlier towards the 

 earth's surface, in consequence of the narrowing of the upper 

 current-bed, and so return to the equator united with the 

 returning masses of air from higher latitudes, they strike 

 these, and with more rapid descent the earth's surface itself, 

 with a velocity combined of their own proper velocity and the 

 difference between their velocity of rotation and that of the 

 earth's surface at the point of contact. The source from 

 . which the storms of high latitude really draw their destructive 

 energy is therefore the momentum of the earth itself. In 

 order that its rotation may remain unchanged, the law must 

 hold good that the acceleration which the earth receives from 

 the difference in velocity in high latitudes is compensated by 

 the retardation which it suffers in low latitudes, in which the 

 mean rotation of the air is smaller than that of the earth's surface. 



* Experiments that I have made, and of which I reserve a fuller 

 account, have shown that a current of air, which passes by the mouth of 

 a narrow tube placed at right angles to its direction, produces in this 

 tube a rarefaction proportional to the velocity of the air, which corre- 

 sponds, within wide limits of velocity, to the pressure of a mercury- 

 column of 0-025 millim. for each metre of air-velocity. I have constructed 

 an anemometer upon this principle, which shows the velocity of the air 

 very simply and without complicated apparatus. It consists essentially 

 of a narrow vertical tube, which is carried as high as possible above the 

 roof of the house. Any simple arrangement for measuring pressure 

 placed in the room then gives directly the velocity of the air in metres. 



