65 



note stating the fact should be connected with the proper hour. If 

 the clockwork goes too fast or slow, it can be regulated in the same 

 manner as the movement of a watch, through an opening in the 

 cylinder marked "A" (to "accelerate") and "R" (to "retard"), 

 corresponding to the letters F and S near the ends of the regulator 

 itself. 



158. Time error of harograph. — The clock movements of these 

 barographs often keep but imperfect time, and it is important that 

 the recoi'd be checked in this respect, so that proper correction may 

 be made in compiling the records of hourly readings. This result is 

 secured by gently touching the lever of the recording pen, so as to 

 deflect it and cause a slight lateral mark to be made on the record 

 sheet across the barometric trace. A mark of this character will be 

 made each day at noon, seventy -fifth meridian time; the record sheet 

 will thus always show how much the barograph clock may be in 

 error. 



In producing these marks great care must be exercised not to strain 

 the lever mechanism in any way; the weight of any ordinary lead 

 pencil is amply sufficient to make the mark, ivMch should not he Tnore 

 than one-eighth inch in length. 



159. Pens. — Pens should be kept clean. Only the standard register 

 ink should be used. Care must be observed in cleaning a pen not to 

 bend or deform the points and render it unserviceable. (See circular 

 A, instrument division.) 



160. Corrections. — Owing to imperfections in barographs, more or 

 less frequent comparisons should be made with standard instruments, 

 and corrections applied according to the scheme fully described in 

 circular A, instrument division. 



VI. Concerning the Elevation of Stations 



161. In the system of the Weather Bureau the elevation of a 

 station is the height above mean sea level of the zero point — that is, 

 the " ivory point " — of the barometer scale, and all measurements and 

 levelings for elevations must be made in reference thereto. 



162. Elevation determined hy spirit level. — ^Whenever a station is 

 established or an office moved and the elevation of the barometers 

 changed, observers will secure the services of a competent surveyor 

 or city engineer, who will run a line of levels to determine accurately 

 the elevation of the station above or below the " plane of reference." 

 In many instances this survey can be secured without expense 

 through the courtesy of the Government or of the city engineers. 

 In the remaining cases the cost will be included with other items of 

 expense incident to the establishment or removal of the office, 

 authority for the expenditure being procured in the usual manner. 



All heights will be given in feet and hundredths or thousandths of 

 a foot. 



163. Fixed point. — The engineer will establish a so-called " fixed 

 point " or bench mark in a permanent manner on the outer stone, 

 work of the building, from which direct measurements of the height 

 of the barometers can readily be made, or a line of levels conveniently 

 run to the " plane of reference " at any time. 



164. Plane of reference. — The " plane of reference " should, in 

 general, be the top of rail at depot. In case, however, a bench mark 



