Field Practice ivith Aneroid Barometer. 361 



Kasikh hit RoufiiiKK Methods. 



Wlieiv less precision is needed the following rule may be used. 

 Taking the average temperature of the air as oO" F. in ordinary 

 winter, and 77^ F. in ordinary summer weather. 



Kor each yw'IT i'^ch allow 



In spring or autumn the mean of the summer and wintei- 

 results may be taken. 



To put it another way a difference of readings of 30 to 29 

 inches represents 1000 feet at a temperature of 88° Fahrenheit, 

 950 feet at 62°, and 900 feet at 40° ; 29 to 28 inches represents 

 1000 feet at 75°, 950 feet at 50°, and 900 feet at 25" ; 28 to 27 

 inches represents 1000 feet at 54°, and 950 feet at 32°. 



A still quicker and easier way is to allow 10 feet for each yi^ 

 inch of mercury in summer, and 9h feet in winter for all heights 

 up to 3000 feet above sea. This will usually give results within 

 al)(jut 3 or 4 per cent, of the truth. 



Examples of Successful Work. 



1. In a building, an aneroid was carried upstairs and read at 

 several known levels. It was then carried down and read a 

 second time at each level. From the mean readings the heights 

 were computed as below : — 



By aneroid - - - - 6.7 - 19.2 - 35.5 - 69 

 By actual measurement - - 8.8 - 23.8 - 37.5 - 66.6 



2. Four readings were taken, at intervals of several hours, at a 

 point on the Geelong Waterworks, the level of which was 

 accurately known, and compared with simultaneous readings at 

 the Melbourne Observatory, 40 miles distant. The true differ- 

 ence of level was 760 feet, and the aneroid results varied from 



