vr.l 



THE ATMOSPHERE. 



97 



for the same day is given in Fig. 24. This shows, by a 

 graphic method, the movements of the barometer for four 

 days, ending at midnight of March 30-31. The thick 

 curved line, running across the diagram, represents the 

 variations of the mercurial column, and it is seen, as before, 

 that the barometer has been slowly rising ; tlie line taking, 

 in fact, a steady upward course from 29*15 to 30"o6 inches. 

 It should be explained that, in all these charts, the actual 

 reading of the barometer has been reduced to certain 

 standards, in order to secure the requisite uniformity for 



Fig. 24. — Daily Telegraph barometer chart. 



comparison. These corrections refer to the height at 

 which the instrument is placed, and to the temperature at 

 which the reading is taken. It is obvious that the baro- 

 meter will be affected by its height above the sea-level ; for, 

 as we ascend, we leave a portion of the atmosphere below 

 us, and, consequently, the pressure is lessened and the 

 mercury falls. Hence a barometer at the top of a house 

 always reads lower than one on the basement; the instru- 

 ment is, indeed, often used for the approximate measurement 

 of heights. Barometric readings from different stations 



