Sect. I X.I 



METEOROLOGY, 



271 



» 



and reading parts above and below accessible, and that 



* 



no more than is absolutely necessary. 



In choosing a barometer, select one in preference in 

 which the lower level (of the mercury in the cistern) is 

 adjustable to contact with a steel or ivory fiducial point, 

 and that not by altering the height of the mercurial su 

 face, but by depressing the steel point carrying doion with 

 it the whole divided scale, the zero-point of which is of 

 course the apex of the point itself. Care should be taken 

 that air have free but safe access to the lower surface. 



In transporting a compared barometer to its place of 

 destination great care is necessary. Carry it upright, or 

 considerably inclined, and inverted; and over all rough 

 roads in the hand, to break the shocks it would otherwise 

 receive. A " portable barometer " strapped obliquely 

 across the shoulders of a horseman travels securely and 

 well ; and with common care in this mode of transport 

 its zero runs no risk of change. If merely fastened to 

 any kind of carriage, and abandoned to its fate, it is 



almost sure to be broken. 



To make and reduce an Observation of the Barometer. 

 ■First read off and write down the reading of the 



attached thermometer. Then give a few gentle taps on 



the instrument to free the mercm7 from adhesion to the 

 glasB, avoiding to give it any violent oscillation. 



Adiust 



* For a permanently suspended or fixed barometer, the best tber- 

 mometer ^ould be one -svith a tubular bulb of equal bore and thickness 

 of glass with the barometer tube, and extending in length from the 

 cistern to the exposed face of the instrumexit, and as close to the baro- 

 metric column as is consistent with the structure of the upper \yorks. 

 Immersion of the ball of the attached thermometer in the cistern is the 

 worst arrangement of any. 



