12 
METEOROLOGY OF 1 897. 
second sheet was placed on at 2:45 p. m. During the time 
of the observation the wind velocity per hour was as fol- 
lows : 
10 to II a. m. ... 48 miles 1 2 to i p. m. . . . 52 miles 
11 to 12 a. m.. . .49 miles i to 2 p. m.. . .49 miles 
2 to 3 p. m.. . .48 miles 
During the time shown by the second sheet the velocity 
was nearly constant, and was 44 miles per hour. 
§18. The abrupt changes in pressure are noteworthy, 
and indicate the frequency of the more violent gusts, which, 
during the heaviest wind, is from eight to ten variations in 
30 seconds. As the wind becomes slower, its gusty charac- 
ter is lost, the variation of the pressure dissapears, and the 
record shown by the instrument is a straight line. In the 
second diagram are periods of from 30 seconds to 2 min- 
utes where the line is smooth. During these times there 
was a lull in the wind. The greater irregularities indicate 
the violent gusts proceeding and following the calm periods. 
The diagram well shows the character of the wind at 
such times, and that the velocity of the wind per hour does 
not necessarily indicate the power of the wind to do dam- 
age which would be measured by the most violent gusts. 
The greatest velocity is attained only for a few seconds at a 
time, and the strength of structures must be sufficient to 
withstand these gusts of short duration. 
RADIATION THERMOMETERS, 
Fig. 7. 
§19. The Maximum Solar, and the Terrestrial 
