9 



78 



METEOROLOGY. 



[Sect. IX. 



i ■ 



I 



i 



f 



i 



< 



4 



i 



* 



Tl 

 ^ 



sensed height to the standard temperature of the mercury 

 at 32^ Fahr. 



After these the index correction should be applied. 

 This is the amount of difference between the particular 

 instrument and the readings of the Royal Society's flint- 

 glass barometer when properly corrected, and is generally 

 known as the zero. It is impossible to pay too much 

 attention to the determination of this point. For this 

 purpose, w^hen practicable, the instrument should *be im- 

 mediately compared with the Royal Society's standard, 

 and the difference of the readings of both instruments, 

 when corrected as above, carefully noted and preseryed. 

 Where, however, this is impracticable, the comparison 

 should be effected by means either of some other standard 

 previously so compared, or of an intermediate portable 

 barometer, the zero-point of which has been well deter- 

 mined. Suspend the portable barometer a.s near as con- 

 venient to the ship's barometer, and after at least an 





hour's quiet exposure, take as many readings of both 

 instrumxTxts as may be necessary to reduce the probable 

 error of the mean of the differences below O'OOl inch. 

 Under these circumstances the mean difference of all the 

 readings will be the relative zero or index error, whence, 

 if that of the intermediate barometer be known, that of 

 the other may be found. As such comparisons will 

 always be made when the vessel is in port, sufficient time 

 can be allowed for making the requisite number of ob- 

 servations : hourly readings would perhaps be best, and 



they would have the advantage of forming part of the 

 system when in operation, and might be accordingly used 

 as such. 



) 



K 



I 







r 



t 



a 



I 



^i 



