36 3Ir J Nixon on the [Jan. 



tolerable accuracy. In order to ascertain the rate of the fall or 

 rise of the barometer, the pressures must be first reduced to one 

 and the same temperature ; preferably that of the mercury at 

 the first observation at the reference station. 



One datum more is wanting, — the temperature at the base 

 contemporaneous with that observed at the summit. To recur 

 to interpolation, as in the case of the pressure, would evidently 

 be purely conjectural, and most generally incorrect, the 

 observer being engaged at the upper station in the heat of the 

 day. A register of the thermometer at brief intervals must 

 consequently be kept either at the reference station, or some- 

 where in the immediate vicinity nearly on the same level. The 

 thermometer being suspended the height of the eye in a northern 

 aspect, to which the wind may have free access, any one of 

 moderate capacity (furnished with a watch) will succeed in 

 noting its indications to the accuracy of at least a degree, 

 whence the temperature at the base for any given instant may 

 be readily deduced. 



Unfurnished with a similar register, substitute in the calcula- 

 tion the double of the temperature at the upper station for the 

 sum of the detached thermometers, and increase the resulting 

 altitude by the square of -^J— th part thereof. When the differ- 

 ence of level does not exceed 36U0 feet, the augmentation may 

 be found in Table V. 



The method of making and registering the observations 

 remains to be indicated. 



The mercury being forced by means of the screw pressing 

 against the leathern bag of the cistern to within a quarter of an 

 inch, but on no account to the very summit of the interior of the 

 tube (which would force part of the mercury through the pores 

 of the leather), the barometer may be carefully inverted, and 

 carried, lightly grasped in the hand, with the cistern always 

 uppermost, at an angle of about 45° with the horizon. It should 

 be kept at some distance from the body, and, when practicable, 

 on the shaded side, the suspension end projecting in i'ront, 

 except on climbing steep acclivities, when it will be more safe 

 to have it in the rear. On making a halt, avoid placing the 

 instrument on damp ground, or on rocks exposed to the rays of 

 the sun. 



Arrived at the station, erect the tripcd, the legs being firmly 

 fixed in the ground, or rendered steady by piling heavy stones 

 around them. A wall, rock, tree, &c. being near the station, 

 make use of the iron crook in preference to the tripod, and drive 

 it firmly into the shaded side of the object at the proper height. 

 When the situation is open to the direct rays of the sun, or when 

 the shaded side of the rock, wall, &.c. is so much exposed to the 

 fury of the wind that the crook must be inserted on the sunny 

 side, it will then be necessary to shade the barometer by one of 



