Chemical and Physical Notes 103 



of uncertainty in barometric readings. After what has been 

 said about the term of cooling of a thermometer, the reader 

 will easily see where the fault lies and how to remedy it. In 

 so far as the effect of temperature on the length of the 

 mercurial column is concerned, the barometer is a thermo- 

 meter with a bulb of the volume and thermal mass of the 

 column of mercury. The term of cooling of this mass must 

 be very much greater than that of the attached thermometer ; 

 so that if at any epoch the two chanced to be at exactly the 

 same temperature, and the temperature afterwards went through 

 the usual diurnal changes, they would never again have the 

 same temperature, except for a moment of time when their 

 thermal paths happen to cross. The only adequate " attached 

 thermometer" is one which has a bulb which is a copy of the 

 barometer, has the same thermal mass and the same term of 

 cooling, and is exposed to exactly the same condition as the 

 barometer. In important central stations the standard baro- 

 meter should be accompanied by such a thermometer. In 

 standardising stations nothing but absolute uniformity of 

 temperature should be admitted. The need of correction is 

 the confession of imperfection. In exact work no correction 

 should be admitted which it is mechanically possible to 

 exclude. 



In Europe and North America, and generally in countries 

 where for a part of the year dwellings have to be artificially 

 heated, the condition of uniformity of temperature is not 

 secured of itself, and it may be troublesome to provide it. 

 The Paris observatory has its classical cellars of invariable 

 temperature. What could be artificially provided centuries 

 ago can be provided now. For a central national standard- 

 ising institution such a chamber is indispensable, if its certi- 

 ficates are to have the value which ought unquestionably to 

 belong to them. 



In tropical, and still more in equatorial regions, where 

 the diurnal variation of temperature is small and its rate 

 slow, uniformity of temperature, sufficient for a first-class 

 meteorological station, is easily obtained in any well- 

 constructed building, and consequently perfection is more 



