xv.] ANALYSIS OF QUANTITATIVE PHENOMENA. 337 



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 

 make 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 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 to the expan- 

 sion of the mercurial column by heat.' The effects may 

 be discriminated, if, instead of one barometer tube we have 

 two tubes containing mercury placed closely side by side, 

 so as to have the same temperature. If one of them be 

 closed at the bottom so as to be unaffected by the atmo- 

 spheric pressure, it will show the changes due to tempera- 

 ture only, and, by subtracting these changes from those 

 shown in the other tube, employed as a barometer, 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 other cases a quantitative effect will be the difference 

 of two causes acting in opposite directions. Sir John 

 Herschel invented an instrument like a large thermometer, 

 which he called the Actinometer, 1 and Pouillet constructed 

 a somewhat similar instrument called the Pyrheliometer, 

 for ascertaining the heating power of the sun's rays. In 

 both instruments the heat of the sun was absorbed by a 

 reservoir containing water, and the rise of temperature 

 of the water was exactly observed, either by its own 

 expansion, or by the readings of a delicate thermometer 

 immersed in it. But in exposing the actinometer to the 

 sun, we do not obtain the full effect of the heat absorbed, 

 because the receiving surface is at the same time radiating 

 heat into empty space. The observed increment of tem- 

 perature is in short the difference between what is received 

 from the sun and lost by radiation. The latter quantity is 

 capable of ready determination ; we have only to shade the 

 instrument from the direct rays of the sun, leaving it 



1 Admiralty Manual of Scientific Enquiry, 2nd ed. p. 299, 



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