64 



except that the matter of light is not so essential. Every precaution, 

 however, must be observed to prevent the instrument from being 

 exposed to great changes of temperature and to direct influence of 

 sunshine, etc. 



The instructions following apply particularly to the Richard 

 barograph. 



153. AdjustTnent to standard 'pressure. — ^When the instrument is 

 first set up at a place, the pen should be made to mark, as nearly as 

 possible, the corrected pressure (see par. 172) given by a standard 

 mercurial barometer at the same place. This adjustment is made by 

 raising or lowering the whole series of aneroid shells by means of a 

 screw reached through a hole in the base of the instrument just under 

 the aneroid shells. This screw is turned by one end of the key sup- 

 plied for winding the clockwork. The adjustment of the barograph 

 to agree with a standard barometer will rarely prove permanent and 

 will require a little alteration from time to time, there being a slight 

 tendency for an aneroid barometer to read too high with age. It 

 is generally necessary to set the barograph to standard at the time a 

 new sheet is put on, but if the error is small it is better to allow for 

 it than to readjust. 



154. Special adjustment for high elevations. — The adjustment 

 afforded by the screw underneath the base of the instrument is not 

 sufficient to bring the pen to the proper pressure at stations four 

 thousand feet or more above sea level. In this case, and also to pre- 

 vent injury to the barograph while in transit over lofty mountain 

 passes, it is sometimes necessary at this office to disconnect the system 

 of levers from the aneroid shells and to provide one or more extra 

 holes in the stem projecting at the top. Barographs found discon- 

 nected in this manner upon arrival at stations need simply that the 

 links be united again, placing the small pin in whichever hole will 

 bring the pen nearest the middle of the record sheet. When shipping 

 barographs, if the instrument has to pass over a greater elevation 

 than 3,000 feet, the system of levers should be disconnected, as above. 



155. When the pressure at any particular station is such as not to 

 be included in the rulings on the record sheets furnished with the 

 instrument, observers will change the numbering of the lines by 

 some convenient whole number and adjust the pen of the instrument 

 accordingly. 



156. The sheets should be changed at about noon on the 1st, 8th, 

 15th, 22d, and 29th of each month. 



When a barograph is first put in operation the trace on the sheet 

 should start at the proper date and hour, even if near the end of the 

 sheet. The Roman numeral, XII, at the top of the sheet indicates 

 noon and the letter M midnight. 



If not already done, the lower edge of the sheet should be trimmed 

 accurately parallel to the longitudinal lines and should rest closely 

 against the flange at the bottom of the cylinder and the pen be 

 adjusted to the proper pressure, in accordance with instructions in 

 paragraph 153. 



157. Barograph clocks. — Every effort should be made to regulate 

 the barograph clock to keep correct time, winding once a week, or 

 oftener if found to give better results. The instrument should be 

 inspected each day by the observer in charge and properly adjusted 

 by him when necessary. Whenever a clock is adjusted, a marginal 



