ANALYSIS OF QUANTITATIVE PHENOMENA. 401 



dilations 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 

 ah 1 desired accuracy. The height of the mercury in the 

 barometer is also affected by capillary attraction, which 

 depresses it by a constant amount depending 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 perfect standard 

 barometer, and introduce if need be an index error 

 including both the error in the affixing of the scale 

 arid the effect due to capillarity. But in constructing 

 the standard barometer itself we must take greater pre- 

 cautions ; 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 

 with confidence assign the exact amount of the effect. 

 Hence a standard barometer is constructed with a wide 

 tube, sometimes even an inch in diameter, so that the 

 capillary effect may be rendered of little account 1 '. 

 Gay Lussac made barometers in the form of a siphon so 

 that the capillary forces acting equally 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 a great many mechanical experiments friction is 

 an interfering condition, and drains away a portion of 

 the energy intended to be operated upon in a definite 

 manner. We should of course reduce the friction in the 

 first place to the lowest possible amount, but as it cannot 

 be altogether prevented, and is not calculable with cer- 

 tainty from any general laws, we must determine it 

 r Watts' ' Dictionary of Chemistry,' vol. i. pp. 513-15. 

 Dd 



