HORIZONTAL SECTIONS. 67 



heights, as additions to, or deductions from, the registered 

 elevation. 



It will be seen that the foregoing methods a and b 

 are founded upon the assumption, that the atmospheric 

 rise or fall of the barometer has been consistent through- 

 out the time occupied in making the observations ; but 

 this probably has not been the case, for the pressure 

 may be increasing at one part of the day, and steady or 

 falling at another. The following method (c) is free 

 from this defect, and the accuracy of its results depends- 

 solely on the perfection of the instruments employed. 



(c.) An assistant and two aneroids are required by 

 this method, the instruments of sufficient accuracy for 

 no perceptible difference between their indications to 

 arise under varying pressure. The readings are taken 

 at each station at some definite time, such as at the 

 hour, half-hour, or quarter; and the assistant, at home or 

 wherever the starting-point may have been, observes 

 and records the reading of the second instrument at the 

 same intervals. The two sets of observations are after- 

 wards compared, and those taken at the stations corrected 

 for variations, if any, of the atmospheric pressure. 



By Level. The principle on which all levels are 

 constructed is that of a perfectly horizontal visual line 

 intersecting a graduated vertical staff, the difference of 

 readings in two observations corresponding to the 

 difference in level of the two places. If the height of 

 one of these be known, that of the other is readily 

 ascertained in relation thereto, and is called a " reduced 

 level." The difference between the heights of any 

 number of places may be thus determined, and the level 



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