whole vol. notes. north america 69 



Notes on the Methods of Observation. 



(Taken from the Annual Report of the Chief of the Weather 

 Bureau, 1921-1922.) 



Pressure: Two mercurial l)arometers of the well-known Fortin 

 cistern pattern, or a modified form thereof, are fur- 

 nished each station. One of these, the '' station barome- 

 ter," is used in making all regular observations ; the 

 other, the " extra,'" is held in reserve for use in case 

 of emergency, except that once each month five cc^n- 

 parative readings are made on the two instruments for 

 purpose of check upon the deterioration of either 

 instrument. 



Each barometer, before issue to station, is com])ared with the 

 substandard at Washington, and a certificate or correc- 

 tion card furnishecl showing the several constant cor- 

 rections that must be applied to the readings of the 

 instrument in order to derive therefrom the actual pres- 

 sure of the air in standard units at a specified elevation. 

 Each observation as made, therefore, is corrected by the 

 application of the following: 

 (i) Correction for scale errors, capillarity, etc. 



(2) Correction to standard gravity, comprising both lati- 

 tude and altitude terms. 



(3) Correction for removal — a correction applied if any 

 change has been made in the elevation of the barometer, 

 to reduce the readings to the elevation adopted in 1900. 



Corrections i, 2, and 3 are constant for any one station and 

 are combined in a single sum. 



(4) Correction for the temperature of the scale and mer- 

 curial column. 



The monthly mean pressures given in the summary are deduced 

 from the corrected observations of pressure at 8'' and 

 2o'\ 75th meridian time, by taking the mean thereof 

 and applying thereto a correction to reduce to the mean 

 of 24-hourly observations. .At several Alaska stations 

 the mean is printed uncorrected. 



Temperature: The maximum temperature is obtained by the use 

 of the Negretti and Zam])ra mercurial thermometer, 

 having a constriction in the bore of the tube below the 

 .scale. The minimum temperature is obtained l)y the use 

 of the ordinary Rutherford ak-i)hol minimum thermome- 

 ter. Both instruments are read and the values recorded 



