316 H. MOHN. METEOROLOGY. [norw. pol. exp. 



THE PRESSURE OF THE AIR. 



THE DIURNAL PERIOD. 



For the determination of the diurnal period of the pressure of the air, 

 I have taken the regular readings of the barometer made each second or 

 fourth hour given in the "Observations", and the registrations of the baro- 

 graph. The latter were controlled by, and reduced to, the corrected observa- 

 tions made with the mercury barometer. 



The hours of the barograph were controlled and verified by simultaneous 

 comparisons between the position of the pen of the barograph and the stand 

 of the chronometer HoHWfJ 639 (Vol. II, No. 6, XIX), from which the exact 

 local hour was computed from Prof. Geelmuyden's astronomical tables in 

 Vol. II, No. 6, H, p. 86. These comparisons were noted at the beginning and 

 the end of the barograph-sheets for each week, and also in the meteorological 

 Journal for almost every day. By these means it has been possible to find 

 the exact time corresponding to each registered value of the pressure of 

 the air. 



For each observed and duly corrected height of the mercury barometer, 

 the corresponding height registered by the barograph was read off the sheets. 

 The difference is the correction to be applied to the registered values. For 

 each hour the registered value was read off from the barograph-sheets, and 

 corrected to the true pressure by means of the corrections thus found. As 

 these generally varied somewhat, their magm'fude tor each hour was taken 

 from a linear interpolation from one hour of observation to the next. This 

 interval is generally 4 hours. 



