ANALYSIS OF QUANTITATIVE PHENOMENA. 389 



The apparent motion of the spots on the sun, is the 

 algebraic sum of the sun s axial rotation, and of the 

 proper motion of the spots upon the sun s face ; hence 

 the difficulty of ascertaining by direct observations the 

 period of the sun s rotation. 



We cannot obtain the weight of a portion of liquid 

 in a chemical balance without weighing it with the 

 containing vessel. Hence to have the real weight of 

 the liquid operated upon in an experiment, we must 

 have a separate weighing of the vessel, with or without 

 the adhering film of liquid according to circumstances. 

 This is likewise the mode in which a cart and its load 

 are weighed together, the tare or weight of the cart 

 previously ascertained being deducted. The variation 

 in the height of the barometer is a joint effect, partly 

 due to the real variation of the atmospheric pressure, 

 partly due to the expansion of the mercurial column by 

 heat. The effects may be discriminated, if, instead of 

 one barometer tube we have two tubes placed closely 

 side by side, so as to have exactly the same temperature. 

 If one of them be closed at the bottom so as to be 

 unaffected by the atmospheric pressure, it will show 

 the changes due to temperature only, and, by subtracting 

 these changes from those shown in the other tube, we 

 get the real oscillations of atmospheric pressure. But 

 this correction, as it is called, of the barometric reading, 

 is better effected by calculation from the readings of 

 an ordinary thermometer. 



In a great many other cases a quantitative effect will be 

 the difference of two causes acting in opposite directions. 

 The late Sir John Herschel invented an instrument like a 



large thermometer which he called the Actinometer b , and 



, 



M. Pouillet constructed a somewhat similar instrument 

 b Admiralty Manual of Scientific Enquiry, edited by Sir John 

 Herschel, 2nd ed. p. 299. 



