METEOEOLOGY 337 



of other masses, and these latter may so descend bodily and rapidly to 

 the earth as by their inertia to experience a slight compression on reach- 

 ing the earth's surface. This is perhaps best seen during the clear or 

 partly cloudy weather that accompanies our west winds. These winds 

 blow in gusts, the velocity at any one station alternating between and 

 5 to J 5 miles per hour. An observer standing on an open free space 

 may generally easily recognize the fact th^t when such a gust is blow- 

 ing dust and leaves violently towards him, there is a little way beyond a 

 simultaneous wind blowing dust in the opposite direction, and by care- 

 ful circumspection he will assure himself that a downrush of air over a 

 well-defined circular region is being followed by its being pushed out in 

 all directions, thereby constituting the gust as observed by him. This 

 downrush of air may plausibly be supposed to be due to the fact that 

 at a short distance above a quantity of colder or drier and denser air 

 was readj' to come down whenever the hot air below came to a state of 

 unstable equilibrium. The stoppage of any such downrush gives rise 

 to a temporary compression and increase of pressure which usually lasts 

 for short spaces of time, a half minute or more, but when great cumulus 

 clouds pass by this may continue for five minutes.] 



271. An interesting example of such barometric changes is given by 

 the Vienna Meteorological Institute as occurring on the 18th of Octo- 

 ber, 1884. The Kreil barograph (continuous register) shows that from 

 noon of the 17tb to 7.25 A. M. of the 18th pressure fell steadily, then re- 

 mained constant for five minutes, and at 7.30 A. m., entirely unan- 

 nounced, jumped up 2.3'""', and continued ascending steadily until noon. 

 The Theorell barograph (recording every fifteen minutes) shows an in- 

 crease of 2.7""" between 7.30 and 7.45 A. m. The Hipp barograph (regis- 

 tering every ten minutes) shows a similar jumj) between 7.20 and 7.40 

 A. M. The following were the attending meteorological phenomena: 

 A heavy northwest storm jirevailed during the night of the 17th and 

 18th, diminishing somewhat in the morning; at 7.30 A. M. the wind sud- 

 denly sprang i\\) from the north, it grew dark, and a heavy rain shower 

 from the north occurred, the total rainfall being 16'""^, of which 0.3""" 

 fell in the first ten minutes; after this shift in direction the force of the 

 wind diminished, the temperature sank, and the storm died away. 



As this sudden barometric change corresponds to a sudden increase 

 of pressure to the amount of 36 kilograms on the human body it is a 

 subject that may interest physicians as well as meteorologists. {Z. 0. 

 G. M, XIX, p. 236.) 



272. E. Eenou, of Paris, has published an exhaustive and most careful 

 study of the records of barometric pressure at Paris. These records 

 begin in 1689, but only in 1809 does the instrumental temperature begin 

 to be given, and only sincel834 is the series commensurable in accuracy 

 with those of the present day. Assuming that the average pressure 

 has remained the same, he corrects the older series of observations, 



S. Mis. 33 22 



