50 t«E NATURAL HISTORY Of CLOtfCS. 



Tlie quantity in which it exists inaj'^ he judged of, at e'omfc 

 periods, by the appearance of distant objects seen horizon- 

 tally: at others, by the degree of intensity of the blue 

 colour of the &ky, which becomes paler by it, if irtdeed the 

 blueness is not whcflly due to this part of the naediunti. 



6/ the Nature of the Stratus. 



Nature of ihe This clond is an example of the decomposition of vaponr 

 sTratfls. thrown into uir of a lower temperature. The earth or wa- 



ter on which it reposes is always warmer than the cloud, im 

 is also the clear air above. Thus, in a stratus, formed oi^er 

 a field with ponds, the temperature of the earth just below 

 the turf was 57"; of the water, 5.s)° ; of the air, at an devia- 

 tion of thirty feet, 55°; while that of the cloud, at four feet 

 from the ground, was 49'5''. Hence this cloud preserves a' 

 level surface ; and hence it uniformly vanishes, or begins to 

 be driven upward, as soon as its temperature becomes equal 

 to that of the earth. It is consequently diie to the decohi- 

 position (in a small portion of the atmosphere) of the va- 

 pour which the earth and water continue to emit, after sun- 

 set, by the force o^a temperature previously acquired. But 

 the change in the lower air, which gives occasion to this lo- 

 cal decomposition, is not so easily to be explained : for it ap- 

 pears that very often, in the evening 6f a clear day, the de- 

 crease of temperature in the atmosphere takes place in the 

 same order in which the increase did in the mbrning : fiz. 

 beginning from the surface of the earth and proceeding up- 

 ward. If the air never became colder, on these occasions,' 

 than the contiguous soil, the effect might very well be as- 

 cribed to the absorption of a quantity of caloric by the lat- 

 ter. But we see that, in the present instance,' it became 

 colder by seven degrees, though vapour was still decompbs- 

 ing: and this in a perfect calm, which, in a great degree, 

 forbids another supposition, of the exchange of a quantity 

 of heated air below, for as much cold air from the higher at- 

 mosphere , otherwise this wotjM seem a sufficient account of 

 the matter. 

 Cion<!s not so The electric charge of the stratus, which is always posi- 

 for^ as^sui^*^' ti**, and sometimes highly so^ notwithstanding the contact 

 P'j-ed. of its lower surface with the earth, seems to prove, that a 



cloud 



