406 METEOROLOGY. 



of some calm summer's night, and, sitting upon the doorsteps of his 

 house, he happens to look up towards the zenith of a sky entirely free 

 from clouds. He will perhaps say, now here are all the conditions 

 necessary to a powerful radiation, and yet no relief from this intolera- 

 ble heat by cooling, which ought to he a natural consequence upon 

 radiation. He may even he apt to compare the atmosphere between 

 his eye and the blue sky to an ether of extreme purity. 



But should he take a different position, and view this same atmos- 

 phere from a distance outside of the city, he would perhaps pronounce 

 it to be a reservoir of smoke and of all the subtile impurities capable of 

 rising out of the different sources of decomposition and combustion 

 from beneath that great hive of human industry and action. He 

 would, in fact, see a continuous mass of smoke extending from one 

 end of the city to the other, and slowly drifting along or hovering 

 over it, as the case may be. And then, in considering the great influ- 

 ence of large bodies of smoke upon the results of radiation, he may 

 perhaps feel that the truths of science still hold good, and that it was 

 only his own neglect to take another view of the same subject from a 

 different direction in order to discover the real condition of things. 



Now, we know it to be a well established fact that strata of smoke, 

 suspended in the air, interfere most effectually with radiation of heat 

 from the surface below. 



By analyzing more minutely the evidence contained in our meteoro- 

 logical registers and referring to individual cases more frequently, 

 we may be able to make the great influence of smoke still more ap- 

 parent. 



In order to do this I have divided all those of my observations made 

 at the regular hours of 7 a. m., 2 p. m., and 9 p. m. into five classes. 



In the first class, in the annexed table, I have put down all cases 

 where a clear sky (cloudiness not exceeding 2) exists in connection 

 with a calm atmosphere or very light breezes of velocity 1. 



In the second class are to be found all the cases of a more or less 

 clouded sky (cloudiness exceeding 2) existing in connection with calms 

 or very light breezes. 



In the third and fourth classes all the cases of a clear sky (cloudi- 

 ness not exceeding 2) existing in connection with brisk winds are enu- 

 merated; in the third, those with winds from west to northwest in- 

 clusive, and in the fourth, those with winds from all other directions. 



The fifth class comprises all the remaining cases, namely, those of 

 a more or less clouded sky (cloudiness exceeding 2) connected with 

 brisk winds. The cases of this class being very numerous and of a 

 more uniform character, it is not deemed necessary to give them here 

 in detail. 



Clearness of sky and calmness of the atmosphere are the two indis- 

 pensable conditions for energetic radiation of the heat of the earth's 

 surface, and consequently for the cooling of the lower strata of the 

 atmosphere. But this favorable combination of the two principal con- 

 ditions we can expect to occur but in a limited number of cases in this 

 part of the Mississippi valley ; and, indeed, we find in our register a 

 great many cases where either the clearness of the sky is made ineffec- 

 tive, as to radiation, by the prevalence of brisk winds, or else, where 

 a calm state of the atmosphere is of no avail toward promoting radia- 



