VI.] THE ATMOSPHERE. 99 



This gives the readings of the barometer for six consecutive 

 days, taken daily at 7 p.m., and duly corrected as just 

 explained. The table also gives the direction of the 

 wind at the same hour each day; and indicates the humidity 

 of the atmosphere, by a comparison of the w^et and dry 

 bulbs of Mason's hygrometer, represented in Fig. ig. It 

 likewise gives the highest and lowest temperature during the 

 day, and the depth of rain which had fallen in the course 

 of the twenty-four hours. The column left blank, but 

 headed " Maximum solar radiation in vacuo" is intended to 

 receive the readings of a radiation thermometer. This 

 generally consists of a delicate thermometer, having a dull, 

 blackened bulb, and inclosed in a glass tube, from which 

 the air has been removed. The instrument is freely exposed 

 to the heat of the sun, and its maximum reading is registered. 

 The greatest amount of solar radiation which occurs during 

 the day is then indicated by the excess of this temperature 

 over the maximum temperature of the air in the shade. 



Although, in this chapter, the subject of atmospheric 

 pressure has been dwelt upon at some length, it must not 

 be supposed that we have travelled away from our specific 

 purpose — the study of the basin of the Thames. It has 

 been pointed out that difterences of atmospheric pressure 

 give rise to the winds, and the character of the winds 

 determines the supply of atmospheric moisture by which 

 the river is fed. It is, therefore, hardly too much to say 

 that, in the long run, the flow of the Thames is regulated by 

 the changes in the atmosphere which are registered by the 

 barometer. 



Moreover, every phenomenon of oxidation and combustion 

 and the well-being, and even the very existence, of every living 

 thing upon the surface of the Thames basin is absolutely 

 dependent upon the composition of the air which covers it. 



H 2 



