THE ANEROID BAROMETER 107 



3. Any considerable blow is likely to throw the instru- 

 ment out of order for the time being, if not permanently. 

 Two instruments carried are a considerable insurance. 



4. The aneroid should always be held in the same posi- 

 tion when read, and be given a little time to adjust itself. 

 By gentle tapping on the face the observer should assure 

 himself that its various parts are all free and in working 

 order. 



5. In starting out for work it is well to carry the instru- 

 ment a while, so as to get it into its regular field working 

 order, before reading on the base station. 



6. One should check on points of known elevation as 

 often as possible, and, if there is a choice of readings to 

 refer to, he should depend on that which is nearer, time 

 and elevation both considered. 



7. A general caution may be needed that the proper 

 use of the instrument is to obtain relative elevation of 

 points by means of readings on the two. One must not 

 expect by one reading to obtain his height above sea 

 level. 



REDUCTION OF ANEROID READINGS BY USE OF THE 

 TABLES AND WITH CORRECTION FOR TEMPERATURE 

 AND WEATHER CHANGES 



(See tables on pages 111 and 112) 



PROBLEM I. Given barometric readings on two stations 

 and temperature at each, to find the difference in elevation 

 of the two points. 



Rule. Enter the first column of Table T with the read- 

 ings of the barometer on the two stations, and take out the 

 corresponding numbers from column 2 (column 3 is for 

 help in interpolating). Take the difference between these 

 two figures. Call this result for the present a. 



Add the two temperatures together (or if the tempera- 

 tures of the two stations do not differ materially, multiply 

 that of the region by two). With this enter Table II, that 

 for temperature correction, and find in column 1 the near- 

 est number of degrees given. Take out of column 2 the 

 number corresponding, noting the + or sign, and 



