FROM THE HIGHER ATMOSPHERE. 491 



exchange of influence must evidently tend to accelerate that 

 progress to an equilibrium which the gradual intermixture of 

 the different strata, if left quite undisturbed, would in time 

 produce. The air of a close apartment, exposed to the action 

 of a steady fire, is hence kept agitated through its whole mass 

 by a series of opposite tremors, which continually disperse, in 

 all directions, the irregularities of temperature. 



If the action of the pulses excited in the air of a small room 

 be made thus apparent, how much more striking should we 

 expect to find the effect produced by the mingled tide of 

 commotion collected from the vast body of the atmosphere 

 itself? Taking even the lowest range of strata, to the height 

 perhaps of two miles, including scarcely one-third part of 

 the whole aerial mass, the difference of temperature between 

 its extreme boundary will amount to 20 centesimal degrees, 

 or 36 on Fahrenheit's scale. The order of the series, however, 

 is exactly the reverse of what takes place in a close room, the 

 air of the superior regions being invariably colder than at the 

 surface of the earth. Accordingly, the simple pyroscope, ex- 

 posed in ealm weather to a clear and open sky, will, at all 

 times, if not disturbed by the influence of a strong light, indi- 

 cate large impressions of cold, amounting to 5 or perhaps even 

 10 degrees. In most cases, it may be sufficient to screen this 

 instrument from the direct action of the sun's ravs. But the ac- 

 tion of light will be almost neutralized, by opposing a diapha- 

 nous ball to one gilt with silver, or contrasting a ball of the 

 different shades of green or blue, to another coated with pure 

 gold leaf. But to procure consistent results, it is still more 

 necessary to guard against the deranging influence of winds. 



The same sectoral form of the sethrioscope discloses also the 

 peculiar influence of clouds in obstructing the frigorific pulses 

 excited in the atmosphere. When the sky was completely ob- 

 scured by a dense canopy of clouds, the instrument being 



3Q2 pointed 



