66 



of a permanent character and of high precision, such as erected by 

 the United States Coast Survey, the Lake Survey, the Mississippi 

 Eiver Commission, or the Engineer Corps, has been established, this 

 will be used as " plane of reference." The same plane should be used, 

 as a rule, in all measurements within the same town, and will not be 

 changed except for good reason. 



165. All levelings must be run by a competent person and the line 

 will be run over a closed circuit which, in the case of a new station, 

 will be from the plane of reference through the " fixed point " to the 

 barometer, and back through the " fixed point " to the plane of refer- 

 ence. In the case of the removal of an office the line will be run 

 from the barometer in the old office to the barometer in the new office, 

 passing through the " fixed points " at both the old and the new 

 office, thence through the " fixed point " at the new office to the 

 " plane of reference," and thence to the point of starting. 



A copy of the field notes, certified to by the surveyor running the 

 levels, will be filed with the report of elevation which will be ren- 

 dered on Form No. 1058-Metl. 



The observer will fully inform the surveyor concerning the fore- 

 going provisions for running the levels. 



166. When, for any reason, it is impossible to run the levels pro- 

 vided for in paragraphs 162 to 165, the observer will, in case of the 

 change of location of the barometers, or removal of the office, make 

 a special set of comparative barometer readings ; that is to say, the 

 comparative barometer readings always required on changing the 

 location of instruments will in case levels can not be run be made in 

 three sets, as follows : 



First. Set before removal. Second. Set during removal; that is, 

 station barometer in new office, extra barometer in old office. This 

 set of readings will be recorded as usual on Form No. 1027-Metl., and 

 will be accompanied by two readings of the exposed thermometer, to 

 be noted on the margin, the first taken immediately before and the 

 second immediately after the set of barometer readings. Third. Set 

 after removal ; that is, both barometers in new location. 



When there is an assistant on station the five readings of the sec- 

 ond set must be synchronous, but in case there is no assistant at sta- 

 tion, eight readings in all will be made alternately in the two offices, 

 in the following order : No. 1 in new office ; Nos. 2 an'd 3 in old office ; 

 Nos. 4 and 5 in the new office; Nos. 6 and 7 in the old office, after 

 which the extra barometer will be carried to the new office and the 

 eighth reading made on the station barometer. Finally, the third 

 set of comparative readings will be made. 



The interval between readings should be as nearly uniform as pos- 

 sible, and is left to the convenience of the observer, depending upon 

 the distance between stations, etc. 



167. When changes in the location of the barometers that do not 

 alter the elevation are authorized, a line of levels need not be run, 

 and only the usual comparative barometer readings before and after 

 removal will be made. 



168. Reduction of observations to a " station elevation^ — On Jan- 

 uary 1, 1900, a specific elevation above sea level was adopted for each 

 Weather Bureau station, and for purposes of record and publication 

 all barometric observations will be correlated to this " adopted or 



