Field Fvdctice with Aneroid Barometer. Ji63 



in ransfy country. If it is necessary to go further than this from 

 known levels, new datum points should be established, and con- 

 nected by a series of repeated observations with the railway 

 station or other known point. Aneroid work should not be 

 attempted in stormy weather, or when atmospheric condition:* 

 are specially unsettled, nor in extremely hot or extremely cold 

 weather. 



Should the expense not be too great, it would be well to have 

 two barographs placed at two points in the district surveyed, as 

 far apart in distance and at as great a difference of elevation as 

 is conveniently attainable, and take the mean of tlie two as 

 giving the true variation of pressure. There should in many cases 

 be no insuperable difficulty in doing this, as a decent barograph 

 may be purchased for £5, and it will need attention but once a 

 week. If any meteorological station exists within say 50 miles 

 of the work in open, or 25 miles in rangy conntry, the I'eadings 

 obtained there should be used as far as possible as a check on the 

 barograph. If no such station be available, it will conduce to 

 accuracy to have a mercurial barometer or boiling-point hyp.so- 

 metrical apparatus at the barograph station, and check both 

 barograph and aneroid by its means as often as convenient. A 

 comparison every week is suggested as a minimum. The mer- 

 curial barometer or boiling-point apparatus might be occasionally 

 taken out in the field as a check on the aneroids there. This, 

 however, would probably only be done for specially important 

 points, such as new barograph stations, as it would involve 

 considerable extra labour. Narrow and deep valleys are bad as 

 barograph stations, as in them the pressure varies with the 

 temperature in such a way as to make the station appear too 

 high in the day time and too low at night. The barograph will 

 read correctly only at say 9 a.m. and 2 p.m. 



Ranges of mountains act as atmospheric dams, causing the 

 pressure at the same level on the two sides to vary by amounts 

 corresponding to 50 or even 100 feet difference of altitude. 

 Hence, on passing such a range, new datum points of known 

 level should be sought. 



In reducing aneroid observations taken on a long journey or 

 exploration, the simultaneous readings at stations of known level 

 should be taken into account. For example, in reducing obser- 



