1 74 Mr. T. Hopkins on the Causes of the 



If the two were equal while acting in opposite directions, they 

 would balance each other. But the separate action of these 

 two causes cannot produce such a double undulation of the 

 mercury of the barometer as that which occurs daily in the 

 tropical regions and at Plymouth. 



The double undulation which takes place may be thus ac- 

 counted for. When the sun acts with force on the surface of 

 the earth in the morning, it heats that surface, and the air 

 near it; increases evaporation of moisture from wet surfaces, 

 and sends forth vapour, which presses on the mercury of the 

 barometer and causes it to rise. The lower part of the at- 

 mosphere being heated also rises at the same time, probably 

 in separate vertical streams, until it reaches a height where its 

 expansion and consequent cooling is sufficient to condense a 

 part of the vapour which it contains. A cloud is then formed, 

 and the heat which has been evolved in the condensation of 

 the vapour makes the cloud lighter than the adjoining air. 

 The vapour in the upper pari of the air being thus removed 

 by conversion into water, no longer presses as vapour, or 

 with the same force on that below; and the lower vapour 

 consequently rises more freely to the height of the cloud. 

 Both the air and vapour are also (speaking in popular lan- 

 guage) drawn up by the ascending cloud, and fresh air flows 

 in from adjoining low levels, forming what in some parts 

 is called the sea breeze. Cloud more or less thick is now 

 formed, more heat is liberated, and a larger mass of air heated, 

 which being forced upwards expands and makes the whole 

 atmospheric column lighter, and reduces the pressure on the 

 surface below. Under ordinary circumstances this process 

 proceeds while the sun acts with considerable power on the 

 surface of the earth, which is generally from 10 a.m. to 4 p.m., 

 when day-cloud ceases to form. In this way, from 10 in the 

 morning till 4 in the afternoon, the barometer is caused to fall, 

 through the condensation of vapour in the upper part of the 

 atmosphere making the column of air warmer and lighter. 

 But now as vapour no longer ascends, cloud ceases to form, 

 but that cloud which had been formed remains suspended in 

 the air, where it begins to cool from the influence of evapora- 

 tion of the particles of water that form the cloud. When it 

 cools sufficiently, it becomes heavier and sinks, and additional 

 air flows towards and over it, increasing the weight of the 

 whole column in the locality and causing the barometer to 

 rise. By 10 the heavy air produced by cloud evaporation 

 has partly descended and diffused itself on the surface of the 

 earth, forming what is called the land breeze; and during the 

 same time the cold of the surface condenses some of the va- 



