15 



too low, and by placing it below, it would be too high. 

 This difference in the manner of reading off is called 

 error from parallax. It is indispensable that a reading of 

 the attached thermometer be made simultaneously with 

 the observation of the height of the mercuiy. Accuracy 

 is the spirit of observation. A careful reading of inches, 

 tenths and hundredths produces excellent results : the 

 ToW pl ace is better left to the skill of the old observer 

 who is usually obliged to estimate it, scarcely any baro- 

 meter being graduated with sufficient precision to trust 

 to the divisions for so small a quantity. 



The barometer is slightly affected periodically during 

 the twenty-four hours : at 9 A.M. and 9 P.M. it stands 

 higher, and at 3 A.M. and 3 P.M. it stands lower ; the 

 mean annual difference amounts nearly to '03 of an inch. 

 These four periods of the day have been recommended 

 for observation by the Committee of Physics of the 

 Royal Society. It is usual, for the sake of comparison, 

 to reduce the observations to 32 of Fahrenheit. 



in. 



Ex. If barom. stood at 29-900 therm, attached 54, 



Correct for temp. '057 (by table), 



Height of barom. at~l OQ Q , Q 

 temp, of 32 J* 



The wheel-barometer, from its construction, cannot 

 be trusted to for correct heights ; it merely shows if the 

 mercury be in a rising or falling state : it may rather 

 be considered as an ornamental piece of furniture than 

 as having the slightest pretensions to a scientific instru- 

 ment. 



