54 Description of the Mountain Barometer^ 



purpose, therefore, of computing on the spot, and very near 

 to the truth, any observations made on a journey, and that, 

 almost without the necessity of writing at all, I have caused 

 the following short table to be engraven on the scale of the 

 barometer. It expresses the value of the difference of the 

 tenth of an inch in the height of the mercury at the tempe- 

 rature of freezing, in English feet. 



Feet. 



Q2 ' 



91 



90 



89 



SB 

 S7 

 86 

 85 

 84 

 83 

 82 

 81 



The method of using it is as follows : — 1st, Add the two 

 observed heights of the barometer, and halve the sum to 

 obtain the mean height. 2d, Subtract the lesser height 

 from the greater, the remainder is of course the difference 

 of height in tenths, &c. of an inch. 3d, Enter the table 

 with the mean height, and take out the feet answering to it, 

 making a proportion, if the mean heiglu does not exactly 

 answer to a foot. (This proportion may be made bv head.) 

 Multiply the number thus obtained by the tenths. Sec. of 

 an inch of difference of height. The result will be nearly 

 the nunibcr of feet, answering to the difference of height be- 

 tween the two barometers at the temperature of freezing. 

 When the lower barometer stands between 29 and 30 inches, 

 and the elevation docs not exceed 1500 feet, this rule will 

 give the height wiihin one foot of the result from the loga^ 

 rithniic method. When the elevation is about 3000 feet, 

 the error will be nearly three feet, and at heights greater 

 than 2000 feet, the error increases in a higher ratio. It is 

 ^Iwavs in defect. In this country, however, such eleyations 



do 



