THE BAROMETER. 217 



which formerly required a year have been made in 

 eight months by means of the assistance afforded by 

 the barometer, admitting a full spread of canvas by 

 night as well as by day, from the certainty of its pre- 

 dictions. On land its indications are not so certain, and 

 at some places less so than at others. Sometimes, and 

 more commonly during autumn and winter, the sink- 

 ing of the mercury is followed only by wind instead 

 of rain. There is, however, no doubt that its use would 

 be of much advantage in large farming establishments, 

 more especially during the precarious seasons of haying 

 and harvesting. 



The barometer is an instrument of great value in 

 determining with little labor, and with considerable ac- 

 curacy, the heights of mountains, hills, and the leading 

 points of an extensive district of country. In rising 

 above the level of the sea, the weight of the air above 

 us becomes less ; that is, the pressure of the air upon 

 the barometer decreases, and the column of mercury 

 gradually falls as we ascend. To determine, therefore, 

 the height of a mountain, we have only to place one 

 barometer at its foot while another stands at the top, 

 and then, by observing the difference in the height of 

 the mercury, we are enabled to calculate the height of 

 the mountain. The following table shows how much 

 the barometer falls at different altitudes, thirty inches 

 being taken for the sea-level :* 



* The mercury rarely stands as high as 30 inches at the level of the 

 sea, the mean height being about 29.5 inches. But this does not af- 

 fect the measurement of heights, which is determined, not by the actual 

 height, but by the difference in heights. 



K 



