242 MAciirN-ERY i^r connectio?^ with aie. 



placed downward in a, cup of mercury. The Yf eight of 

 the mercury in the tube causes it to descend until tlio 

 pressure of the atmosphere on the mercury in the cup 

 preserves an equilibrium, which takes place when the col- 

 umn in the tube has fallen to about two feet and a 

 ns.~Go. i^j^if j^jgi^^ ^}^Q upper part of the tube being left a 

 perfect vacuum, as no air can enter (fig. 265). Xow, 

 as the height of the column of mercury depends 

 alone upon the weight of the atmosphere, then, 

 whenever the air becomes lighter or heavier, as it 

 constantly does during the changes of the weather, 

 the rising or falling of the column indicates these 

 changes ; and, what is very important, it shows 

 the approaching changes of the weather several 

 hours before they actually take place. Hence 

 it becomes a valuable assistant in foretelling 

 the weather. When the mercury falls, showing that 

 the atmosphere is becoming lighter, it indicates the 

 approach of storms or rain ; when it rises, a settled or fair 

 sky follows. These are often foreshown before there is 

 any change in the appearance of the sky. For this rea- 

 son the barometer is sometimes called a weather-glass. It 

 is of the greatest value to navigators at sea. Long 

 voyages, w^liich formerly required a year, have been made 

 in eight months by means of the assistance afforded by 

 the barometer, admitting a fall spread of canvas 'by night 

 as well as by day, from the certainty of its predictions. 

 On land its indications are not so certain, and at some 

 places less so than at others. Sometimes, and more com- 

 monly during autumn and winter, the sinking of the mer- 

 cury is followed only by wind instead of rain. There is, 

 however, no doubt that its use would be of much advant- 

 age in large farming establishments, more especially dur- 

 ing the precarious seasons of haying and harvesting. 



The barometer is an instrument of great value in de- 

 termining with little labor, and with considerable accuracy, 



