226 TENTH ANNUAL REPORT OP 



case, tlie distance above seven is less than the half; we shall then read 

 30.073. It will always be easy to judge whether the top approaches 

 nearer the npper 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 re- 

 main 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 particularly 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 hundredths 

 of millimetres. In the English instruments^ the inches must be sepa- 

 rated 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 is none. In the metrical 

 scale put the (.) after the millimetres, and admit two decimals, e. g. 

 •761.25. 



During the whole time of the observation of the barometer, the ob- 

 server 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 pla-^e, 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 water of a neighboring 

 river, the ordinary level of the surface-water of a canal, the upper 

 part of a wharf in mason-work, &c. If the barometer has changed 

 place, it is again necessary to measure exactly its height above the 

 same point of reference ; the latter will serve to fix the height of the 

 barometer and of the station above the level of the ocean ; this datum 

 being of the greatest importance. Every change of this nature should 

 be carefully noted in the journal. 



It is greatly to be desired that the place of the barometer, once 

 determined, should not be changed, either from one story to another, 

 or from one house to another. If circumstances compel this to be 

 done, we should begin, before taking it from its place, by raising the 

 mercury in the cistern by means of the screw, so as to fill the cistern 

 and the tube ; it must then be gently taken from the hook, turned upside 

 doicn, and carried with the cistern up, taking great care not to strike 

 it against anything. If it were transported without these precautions, 

 even from one chamber to another, great risk would infallibly be run 

 of breaking it, or letting in air, and thus rendering it useless. 



Verification. — From time to time the barometer should be so inclined 

 as to cause the mercury to strike gently against the top of the tube. 

 If it gives a dry and clear sound, it is free from air, and the instru- 

 ment is in good condition. If the sound is flat and muffled, there is 

 a little air ih. the barometric vacuum ; and the fact should be noticed 

 in the journal. Every occasion should be seized to compare it anew 



