408 METEOROLOGY. 



Can it be on account of the layers of smoke above the city enfeebling 

 the action of the sun's rays in passing through, which, as to efficiency 

 in this case, I consider more than doubtful ; or is it not rather due to 

 the fact that, upon the pavements in the streets of a compactly-built 

 town, a considerable amount of shade is cast in clear weather, at 2 p.m., 

 from the row of high buildings, which shade is altogether wanting 

 upon an open prairie? While in town the air heated and rarified upon 

 the roofs does not sink down into the streets, and consequently does 

 not affect the surface below, the entire unbroken surface of the prairie, 

 on the other hand, acts as a heating medium at once upon the lower- 

 most portions of the atmosphere. Hence it maybe at times even some- 

 what cooler, at 2 p. m., in town than in the open prairie. Besides 

 this, the customary sprinkling of the streets to lay the dust, and the 

 consequent evaporation, is no doubt helping towards a reduction of 

 temperature. 



In the third antl fourth classes of the annexed table, where the same 

 condition of clearness as in class No. 1 prevails, with the difference of 

 brisk winds instead of a calm or light breezes, we find a great many 

 cases noted down at 2 p. m. ; and hence we can avail ourselves here of 

 a much greater number of instances to illustrate the same principle. 

 Here we have 175 single observations, of which exactly two fifths, 

 namely, seventy instances, came upon 2 p. m.; and here, also, we find 

 the difference of temperature between town and country amounting 

 either to Dothing or a mere trifle, while seventeen cases show even a 

 cooler temperature in town. Here, in addition to the above mentioned 

 causes, a briskly moving atmosphere is actively engaged to carry away 

 from the tops of houses the superincumbent layers of air as fast as they 

 are heated by contact with the roofs. 



Unusually small figures among the morning and evening temperatures of 



Class No. 1. 



But there are also among the morning and evening temperatures in 

 Class No. 1, cases which, by their small figures seem to form exceptions 

 to the high figures of all the rest. They have had all the. advantages 

 of calmness and clearness of weather, the same as the others, and yet 

 we find no corresponding amount of fall in the thermometer. 



There are, in class No. 1, to be found eleven instances not exceeding 

 one degree, four of no difference, and three even warmer than in town. 

 In most of them a very gentle breeze from southeast and south is pre- 

 vailing ; in eight of them we see a rain or thunder storm near at hand ; 

 but beyond this the register gives us no key of explanation. It is, 

 however, very probable that radiation in these cases is interfered with 

 by some kind of haze, dust, or vapor interposed between our vision and** 

 the zenith, too subtle to be recognized by us from the position we oc- 

 cupy in looking towards it. A cyanometer could no doubt solve the 

 question more satisfactorily. It may not be amiss to mention here, 

 also, that in classes Nos. 3 and 4, of the seventeen instances of unusu- 

 ally great differences of temperature, fourteen take place at seven o'clock 

 in the morning, when the air appears to be more refreshing than at 

 any other time, of the day, and when it is the very opposite from sultry. 

 This is what we ought to expect. 



