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errors have been determined. Moreover, from the nature of things 

 we can not determine these corrections except by comparison with a 

 standard barometer, and the question might properly be asked. How 

 do we know the standard barometer is right? We will answer this 

 by saying that the standard barometer ought to be a normal barom- 

 eter. So few understand clearly the distinction between these words 

 " standard " and " normal " in the present connection that some 

 explanation is necessary. In the first place, the expression " normal 

 barometer " is used a great deal by the Weather Bureau and meteorol- 

 ogists in general when, strictly speaking, the expression should be 

 normal barometric . pressure, by which is meant the average of a 

 great many years' observations of atmospheric pressure at a single 

 station. In the present case the word " normal " has an entirely 

 different meaning. 



A standard barometer need not necessarily by anything more than 

 an instrument which has been pronounced to be correct by some 

 special authority. For instance, the Congress of the United States 

 might say that the indications of such and such an instrument 

 represent the true atmospheric pressure and that the particular 

 barometer in question is the standard of the nation. Such an instru- 

 ment, although formally pronounced to be a standard, might, never- 

 theless, possess little more than the average accuracy and its indica- 

 tions still be more or less erroneous. Since the several errors to 

 which barometers are subject can not, in the majority of cases, be 

 determined except by comparison with an instrument whose errors 

 are all known, a standard based only on the dictum of some authority 

 can not necessarily be regarded as giving true indications. A normal 

 barometer, however, is one the construction of which is such that the 

 instrument, fundamentally and independent of all other similar in- 

 struments, gives a true measure of the pressure of the air. 



Standard barometers should therefore generally be also normal 

 barometers. It must not be understood that a normal barometer is 

 absolutely without any error. The construction, however, is such 

 that those errors which can not be wholly eliminated can yet be 

 ascertained from the indications of the instrument itself. The error 

 for capillary action, for example, is wholly eliminated by employing 

 a tube of very large diameter. On the other hand, if the vacuum is 

 not sufficiently perfect, the error from this cause can still be ascer- 

 tained, for the barometer will be constructed so that readings can be 

 made when the vacuum chamber is large, and again when it is many 

 times smaller and the pressure of the remnant of air therein propor- 

 tionately increased; from such readings the desired corrections can 

 be computed. So, also, other errors are either eliminated or are 

 ascertained by special investigations, and the reading of the barom- 

 eter after all known corrections are made is regarded as funda- 

 mentally correct. 



Barometers of this type are generally elaborate of construction 

 and will not be described here. Several of the European normals 

 are fully described by Professor Abbe in the annual report of the 

 Chief Signal Officer, 1887, Part II. 



