534 The Neutral Point of Barometers. [June, 



4. Hence it will follow that, 



1st, When h is greater than H ; P=A+ (h— H) c ; 

 2nd, When h is less than H, P=A— (H— h) c. 



5. Both cases are included in the following rule. 



Rule. Multiply the difference between the true height of the mercu- 

 rial column (ascertained by a correct barometer) and the approximate 

 height as found by the instrument of which the neutral point is sought, 

 by the reciprocal of the proper fraction expressing the ratio of the 

 capacities of the latter instrument ; the product being added to or sub- 

 tracted from the approximate height, according as that is greater or 

 less than the true height, will give the neutral point that was required. 



Example. Barometer. Temp. 



Standard Instrument, 29.979 85 



Barometer No. 36, 29.970 91 



Barometer No. 36 observed 29.970 



Capillarity, +.0291 +0011 



Correction for -f-6° temp. — .018 J 



Approximate height, 29.981 



True height by Standard, 29.979 



Difference 0.002 



No. 36, Capacities Jj ; Reciprocal, 48 



Product 0.096 



Approximate height, 29.981 



Neutral point of No. 36, 30.077 



6. The Neutral point of No. 36, as marked by the maker, is 30.075. 

 The result here shown is a proof of the correctness of both the instru- 

 ments used. The standard is that in the Surveyor General's office; 

 No. 36 is a portable barometer by Newman, belonging to Colonel 

 Ouseley. 



7. As the Neutral point of portable barometers is liable to material 

 alteration by the accidental escape of a small portion of mercury from 

 the cistern, the above simple rule may be found useful, either for 

 verification or discovery. 



