346 THE PKINCIPLES OF SCIENCE. [CHAP. 



3. Method of Correction. 



Whenever the result of an experiment is affected by an 

 interfering cause to a calculable amount, it is sufficient to 

 add or subtract this amount. We are said to correct 

 observations when we thus eliminate what is due to 

 extraneous causes, although of course we are only sepa- 

 rating the correct effects of several agents. The variation 

 in the height of the barometer is partly due to the change 

 of temperature, but since the coefficient of absolute 

 dilatation of mercury has been exactly determined, as 

 already described (p. 341), we have only to make cal- 

 culations of a simple character, or, what is better still, 

 tabulate a series of such calculations for general use, and 

 the correction for temperature can be made with all desired 

 accuracy. The height of the mercury in the barometer is 

 also affected by capillary attraction, which depresses it by 

 a constant amount depending mainly on the diameter of 

 the tube. The requisite corrections can be estimated with 

 accuracy sufficient for most purposes, more especially as 

 we can check the correctness of the reading of a barometer 

 by comparison with a standard barometer, and introduce 

 if need be an index error including both the error in the 

 affixing of the scale and the effect due to capillarity. But 

 in constructing the standard barometer itself we must take 

 greater precautions ; the capillary depression depends 

 somewhat upon the quality of the glass, the absence of air, 

 and the perfect cleanliness of the mercury, so that we 

 cannot assign the exact amount of the effect. Hence a 

 standard barometer is constructed with a wide tube, some- 

 times even an inch in diameter, so that the capillary effect 

 may be rendered almost zero. 1 Gay-Lussac made baro- 

 meters in the form of a uniform siphon tube, so that the 

 capillary forces acting at the upper and lower surfaces 

 should balance and destroy each other ; but the method 

 fails in practice because the lower surface, being open to 

 the air, becomes sullied and subject to a different force of 

 capillarity. 



In mechanical experiments friction is an interfering 

 condition, and drains away a portion of the energy in- 



1 Jevons, Watts' Dictionary of dhemittry, vol. i. pp. 513 5 1 5. 



