The barometer has evidently fallen during the time 

 through a space equal to 30 feet, making the B. M. ap- 

 pear to be 30 feet higher than at starting, and, as a 

 matter of course, the other stations too high also in due 

 proportion. We assume that the fall was gradual, and 

 equally distributed over every portion of the two 

 journeys ; therefore half 30 feet (i.e. 15 feet) must be 

 deducted from the mean of the two observations at each 

 place, the result gives the required elevation. 



B.M. 1st 2nd Last 



o +200 +80 + 100 



+ 30 + 225 + 100 + 100 



J/ 30 y 425 y 180 |/ 200 



Mean of two 1 .. - 



observations j 



Deduct error 15 



Elevation 



(6.) A somewhat similar method is to note the time 

 of starting (with the indicator at zero), and of making 

 each observation; then, on again reaching the home 

 B. M., without having gone more than once to each station, 

 to observe the time, and the rise or fall of the barometer. 

 This rise or fall must be distributed, in proportion to the 

 time that elapsed before each was taken, over the station 



