METEOROLOGY. 407 



tion on account of a clouded sky. Besides this, there are numerous 

 instances where clearness and calmness are both wanting at the same 

 time. 



Hence if the general result, as exhibited by the mean of the year, is 

 expected to make anything like a show, it is necessary that, amongst 

 such an abundance of counterbalanced cases, the few favorable in- 

 stances should carry along with them pretty large figures. And this 

 they really do, as may be readily seen from the first class of cases in 

 the annexed table. 



Further remarks on this class. — In clear and calm weather without 

 smoke, and other things being equal, the cooling of the lower portion 

 of the atmosphere by radiation ought to go on even somewhat faster 

 in town than in the country ; for the thousands of roofs, sending the 

 caloric off against a serene sky, would cool the immediate superincum- 

 bent strata of air, which, hereby becoming heavier, sink down upon 

 the pavements and lift the warmer strata upwards, until the latter in 

 their turn are cooled in a similar manner. But not only is the advan- 

 tage of the city to reduce the temperature in this way counterbalanced 

 in a mechanical way by the strata of smoke gathering above and pre- 

 venting a rapid ascent of the warmer columns of air ; but by this smoke 

 the radiation itself is greatly interfered with, if not altogether stopped. 

 Besides this procedure of keeping the caloric of the city from radiating 

 more freely into space, comes the accumulation of artificial heat from 

 within the houses and shops, from fires, lights, and the presence of 

 hundreds of thousands of living beings. With a calm atmosphere this 

 heat is not easily carried off. 



The principal cause of the interference with radiation in a large 

 city, viz., the smoke, is wanting in the open country, and hence the 

 reduction of temperature which here may'take place, and, indeed, is 

 found to be considerably lower than in town. 



Comparison of temperature at 2 p. m. behveen city and country. 



It is somewhat remarkable that among the whole number of observ- 

 ations in class No. 1 — that is, among 154 instances of combined clear 

 and calm weather — we can find only seven to occur at 2 p. m. ; and in 

 these seven instances the temperature near the city limits is either 

 warmer than that of the town, or only by one half of a degree or at 

 most one degree cooler. 



Now, it may be asked, "Why are, in these instances, few as they 

 are, the same effects not produced under similar circumstances at the 

 hour of 2 p. m.?" or, in other words, "Why does, in clear and calm 

 .weather, the temperature of the open country, at 2 p. m., not bear the 

 same relation to that of the city as it does at 7 a. m. and 9 a. m.?" 

 Here, however, we must not forget that another new additional factor 

 comes into play at 2 p. m., which is not operating at 9 a. m., and only 

 partially so at 7 a. m., namely, the action of direct rays from the sun 

 upon the surface they may happen to strike. 



But how can this cause, which apparently ought to affect equally 

 the open country as well as the city, change the mutual relation of 

 temperature of these two places ? 



