1826.] Measurement of Heights by One Barometer^ 39 



reference stations judiciously selected on the banks of the prin- 

 cipal streams irrigating the district. The absolute elevation of 

 the upper surfaces of the different strata above the loicest of these 

 stations being computed from the barometrical observations, 

 the height of some one of the principal or best identified beds, 

 at the various places where sections have been measured, may- 

 be marked on a map, accurately laid down on a large scale. 

 To find the dip, draw the direction line through a number of 

 section-stations where the stratum is on the same level, or 

 equally elevated above the lowest reference station, and is at its 

 maximum height. From the remaining section-stations draw- 

 lines intersecting the direction line at right angles, and, measure 

 the distances in feet on the scale of the map. Dividing the 

 distance by the difference of altitude of the stratum at the 

 section-station and at the direction line, we obtain the tabular 

 tangent of the dip in the vicinity of that section. 



As the exactness of the barometrical measurements of the 

 sections will depend almost entirely on the correct observation 

 of the height and temperature of the mercurial column, the 

 observer must so habituate himself to the accurate adjdstment 

 of the ring of the vernier to the proper height that the pressures 

 may be read off true to -002 or -OU.i inch. To meet the other 

 more serious difficulty, the observations should be made exclu- 

 sively in calm cloudy weather, between noon and three, p.m. at a 

 temoerature little differing from the mean annual one. 



riaving completed the observations at the reference station, 

 proceed to the torrent, and slowly ascend to the summit, mark- 

 ing with piles of stones the heights of the planes of contact of 

 the strata ; cautiously avoiding to mistake an accumulation of 

 bowlders for an alternation of th'e beds. When the contact is 

 indiscernible in the ravine, &.c. deviate to the right or left on the 

 same level, when it will seldom fail to be recognized in a scar, 

 chff, &c. or identified by the gushing out of numerous springs at 

 the same altitude. In most formations, the change of beds is 

 generally intimated by the varied verdure of the vegetation ; the 

 appearance of different plants, &c. Descending to the reference 

 station, suspend the barometer at every pile of stones, noting 

 its height and that of the interior thermometer, after a lapse of 

 time, preciseli/ the same at every observation, or about ten 

 minutes. When the beds are only a few feet in thickness, it 

 will be better to measure them with the telescopic level, and a 

 light staff divided into mches, than to recur to the barometer. 



Construction of the Tables. 



The reduction of the pressures at the reference station to one 



temperature, the determination of the rate per hour of the fall or 



rise of the barometer, together with the interpolation of the 



pressures and temperatures to the times at the summit, present 



