16 BAROMETER. 



and thirty inches and one-tenth ; no line of the vernier also coin- 

 cides exactly ; but the line Y is a little above, the line 8 is a lit- 

 tle below, one of the lines of the scale; the fraction falls, then, 

 between seven and eight hundredths. Estimating in tenths the 

 distance the vernier passes over between the coincidence of seven 

 and that of eight, we thus obtain the tenths of an hundredth, or 

 the thousandths. In this latter case, the distance above seven 

 is less than the half; we shall then read 30.013. It will always 

 be easy to judge whether the top approaches nearer the upper 

 coincidence than the lower coincidence ; in the former case, the 

 fraction is greater than .005 ; in the latter it is smaller than 

 .005. The error which will be committed in this estimate will 

 remain less than .005; with practice and a little skill, it will 

 hardly ever exceed .002, always supposing the scale is well 

 graduated. For this reading, as well as for the others, it is par- 

 ticularly important to have the eye exactly at the height of the 

 line to be determined. 



The same process of reading is applied to the metrical scale ; 

 the vernier then gives tenths directly, and by estimate, the hun- 

 dredths of millimetres. In the English instruments, the inches 

 must be separated by a (.) and three decimals written, even when 

 the last is a zero ; e. g. 30.250, and not 30.25 ; the zero indicates 

 that the thousandths have been taken into account, but that 

 there are none. In the metrical scale put the (.) after the milli- 

 metres, and admit two decimals, e.g. 161.25.* 



During the whole time of the observation of the barometer, 

 the observer must endeavor to protect it as much as possible 

 from the heat which radiates from his body. But the best way 

 is to learn to observe rapidly. All the operations of which we 

 have just spoken take longer to describe than to execute ; one 

 or two minutes, if the instrument be in place, three minutes if it 

 is to be taken from its case and put back again, are sufficient for 

 a practised observer to make a good observation 



Altitude. — The height of the barometer above the ground, or 

 above some fixed point, which may serve as an invariable point 

 of reference, ought to be exactly determined. Such a point, for 

 instance, may be the base of a public edifice, the level of low 



* For the method of reading the vernier of Green's standard baro- 

 ^•^ter, see the description of the instrument, page 54. 



