390 THE PRINCIPLES OF SCIENCE. 



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. The details 

 of the construction and use of these instruments are im- 

 material to our immediate purpose. Now 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 temperature is in short the dif- 

 ference between what is received from the sun and lost by 

 radiation. But the latter quantity is capable of ready 

 determination ; we have only to shade the instrument 

 from the direct rays of the sun, while leaving it exposed 

 to the rest of the open sky, and we can observe how 

 much it cools in a certain time. The total effect of the 

 sun's rays will obviously be the apparent effect plus the 

 cooling effect in an equal time. By alternate exposure 

 in sun and shade during equal intervals the desired result 

 may be obtained with considerable accuracy c . 



Two quantitative effects were beautifully distinguished 

 in an experiment of John Canton, devised in 1761 for the 

 purpose of demonstrating the compressibility of water d . 

 He constructed a thermometer with a large bulb full of 

 water and a short capillary tube, the part of which above 

 the water was freed from air. Under these circumstances 

 the water was relieved from the pressure of the atmo- 

 sphere, but the glass bulb in bearing that pressure was 

 somewhat contracted. He next placed the instrument 

 under the receiver of an airpump, and on exhausting the 

 air, observed the water sink in the tube. Having thus 



c Pouillct, 'Taylor's Scientific Memoirs,' vol. iv. p. 45. 

 (1 Jmniii, '('ours <le Plrysujue,' vol. i. p. 158. 



