ABOUT THE 20tH OF DECEMBER, 1836. l6l 



5. Leslie ascribes them to the centrifugal force arising from violent winds. 

 But, in the case of a hurricane, this would not produce an oscillation of the 

 barometer amounting to the thousandth part of an inch. 



6. The opposition of winds. This might produce a small movement of the 

 barometer. We shall presently inquire whether this cause operated in the 

 case under consideration. 



7. The barometer has frequently been observed to fall under the influence 

 of a whirlwind. But in the present case there was no whirlwind. 



8. These oscillations have been ascribed to sudden changes in temperature 

 and in the amount of aqueous vapour. An elevation of temperature of the 

 entire atmosphere could not directly affect its pressure, for, in proportion as 

 its density is diminished, its height will be increased. But if, by any means, 

 a portion of hot air can be made to displace an equal bulk of cold air, the 

 weight of the column must be diminished. It is obvious that this cannot be 

 a state of permanent equilibrium; yet it is worthy of inquiry whether it may 

 not temporarily exist under the influence of winds. On the 20th of Decem- 

 ber, 1836, the air over nearly the whole of the United States became unu- 

 sually heated, and its specific gravity was, of course, diminished. If, then, 

 the height of the atmosphere remains invariable, a diminution of pressure 

 ought to be the consequence. But, although a fall of the barometer is usually 

 accompanied by an elevation of temperature, the reverse is sometimes the 

 case. Thus the fall of the barometer in Europe, which I have represented 

 on Plate 2, and which, at most places, amounted to more than an inch, 

 was accompanied by a steady fall of the thermometer. The barometer, in 

 this case, fell in spite of the increased specific gravity of the air. We may 

 naturally presume, then, that a change in the specific gravity of the air pro- 

 duces only a secondary effect on the oscillations of the barometer. 



9. A wind blowing upward or downward would affect the pressure of the 

 air. This is a cause whose existence we have proved in the case in question. 

 Its effect upon the mean pressure of the air in the equatorial regions is une- 

 quivocally maintained in the Instructions for the British Scientific Expedi- 

 tion to the Antarctic Regions, recently prepared by the President and Council 

 of the Royal Society, causing the barometer at the equator to stand perma- 

 nently lower than in latitude 30°, by about a quarter of an inch. The as- 



VIT. — 2 Q 



