DanielFs Meteorological Essays. 219 



pretending to the appellation of scientific. It may not, how- 

 ever, be irrelevant to remark, that no standard thermometer 

 is known to exist ; and that most barometers are carelessly 

 made, and filled with mercury from which the air has not been 

 sufficiently extracted. 



Were the earth a sphere of uniform temperature, and at 

 rest in space*; its atmosphere a perfectly dry and permanently 

 elastic fluid ; the height of the latter would be constant over 

 every point of the earth's surface, and its density and elasticity, 

 at equal elevations, every where the same. The column of 

 mercury that it would support in the barometer, would there- 

 fore be the same at every point on the surface of the sphere; 

 and equal at equal heights above the surface. The atmosphere 

 would be absolutely at rest ; and as its elasticity is proportioned 

 to the pressure, the density would decrease in geometrical pro- 

 gression, while the distance from the surface of the sphere in- 

 creases in arithmetical. When air is rarefied, its capacity for 

 heat is increased, and vice versa ,• the sensible heat of the at- 

 mosphere must, therefore, decrease as the altitude increases ; 

 and as this changes the volume of elastic fluids, even under 

 equal pressures, the barometer alone will no longer be the 

 exact measure of the progressive density, but must be asso- 

 ciated with the thermometer. Any change of temperature that 

 affects every part of the sphere, would cause an increase in the 

 elasticity of the atmosphere, and in its consequent height, 

 without producing any motion in the lateral direction, or any 

 change in the pressure upon the surface ; but the pressure will 

 be changed at all other altitudes. 



If the temperature of the sphere, instead of being equal at 

 every point, were greatest at the equator, and decreased to- 

 wards the poles, the pressure on every point of the surface 

 would still continue the same ; but the altitude of the atmo- 

 spheric column would become greatest at the equator, and its 

 specific gravity at the surface less there than at the poles. 

 The heavier fluid at the poles must, by its mechanical action, 

 press upon and displace the lighter, and a current will be es- 

 tablished in the lower part of the atmosphere from the poles 

 towards the equator. The difference in the specific graviiv of 

 the polar and equatorial columns becomes less as we ascend 

 into the atmosphere; while the elasticity, which is constant at 

 the surface, varies with the height, and the barometer stands 

 higher at equal elevations in the equatorial than in the polar 

 column. It will hence happen, that, at some definite height, 

 tin- unequal density of the lower strata will be compensated ; 

 and a counter-current will take place in the higher regions 

 I ! ;iy isi, part 1st. 



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