39 Mr. Nixon on the [JaK. 



(better) yellow paper behind the barometer, or let the guide 

 interpose his hand held at the distance of about a foot. 



Having completed the adjustment of the rim of the vernier to 

 the height of the column, note immediately the indication of the 

 detached thermometer, and the time of observation, adding 12 

 hours to the limes, p.m. 



In reading off the pressure, write down first the inches and 

 tenths the next below the zero of the vernier, to which affix the 

 remaining fractional parts as indicated by the vernier. To read 

 these off correctly, commence at the zero of the latter, — terming 

 it '050 when a twentieth of an inch intervenes between it and 

 the tenth of an inch already registered, — and count the lines 

 upwards, calling each '002 inch, until one of the lines of the 

 vernier coincides in height with, or appears to be a prolongation 

 of, some one of the adjoining lines of the fixed scale of inches. 



Having completed the requisite observations at the summit 

 station, the altitudes of several of the surrounding mountains, 

 &c. may be determined a^'ter the following method on making 

 the descent to the reference station.* Selecting the loftiest of 

 the objects visible below the apparent horizon, steadily bisect its 

 summit from time to time with the intersection of the cross wires 

 of the telescope until the bubble of the level is observed by the 

 guide to remain in or near the middle. Then fix the tripod for 

 a support, and continue to alter its height until the bubble of the 

 telescope resting upon it, and accurately pointed at the object, 

 remains steadily between the marks. Here suspend the baro- 

 meter, and make the requisite observations of the pressure, 

 temperature, &.c. allowing for the height of the tripod above the 

 cistern. With these data, in addition to those obtained at the 

 reference station, we learn by calculation the elevation in feet 

 of the level point above the latter. To this difference of level 

 add the square of the distance, in miles of the observed object, 

 from the level point multiplied by 0*676 foot,t and we have the 

 correct altitude of the object above the level of the reference 

 station. The telescope being small, and the distance taken 

 merely from the map, the observations should not extend to 

 mountains, &.c. at a greater distance than eight or ten miles. 



In mountainous districts, consisting of rocks regularly strati- 

 fied under a moderate dip, the depths and inclination of the 

 principal beds may be ascertained with tolerable precision by 

 barometrical measurements made in the ravines, beds of torrents, 

 or on the precipitous declivities of the mountains. The first 

 object in an undertaking of this description will be to determine 

 with great care the relative heights of a sufficient number of 



• De Luc 



/o^" 52'' \ 

 + = 5280 feet x tang. (~ — T^ I • -A- mile (5280 feet) subtends an arc of 52"; 



the refettction is considered to be 1.15th of the arc. 



