338 THE PRINCIPLES OF SCIENCE. [CHAP 



exposed to the 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 phis 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. 1 



Two quantitative effects were beautifully distinguished 

 in an experiment of John Canton, devised in 1 76 [ for the 

 purpose of demonstrating the compressibility of water. 

 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 air-pump, and on exhausting the 

 air, the water sank in the tube. Having thus obtained a 

 measure of the effect of atmospheric pressure on the bulb, 

 he opened the top of the thermometer tube and admitted 

 the air. The level of the water now sank still more, partly 

 from the pressure on the bulb being now compensated, and 

 partly from the compression of the water by the atmo- 

 spheric pressure. It is obvious that the amount of the 

 latter effect was approximately the difference of the two 

 observed depressions. 



Not uncommonly the actual phenomenon which we wish 

 to measure is considerably less than various disturbing 

 effects which enter into the question. Thus the compres- 

 sibility of mercury is considerably less than the expansion 

 of the vessels in which it is measured under pressure, so 

 that the attention of the experimentalist has chiefly to be 

 concentrated on the change of magnitude of the vessels. 

 Many astronomical phenomena, such as the parallax or the 

 proper motions of the fixed stars, are far less than the 

 errors caused by instrumental imperfections, or motions 

 arising from precession, nutation, and aberration. We 

 need not be surprised that astronomers have from time to 

 time mistaken one phenomenon for another, as when Mam- 

 steed imagined that he had discovered the parallax of the 

 Pole star. f 



1 Poniiiet, Taylors Scientific Memoirs, vol. iv. p. 45. 



2 Baily's AKvunt of the Rev. John Flatnstced, p. 58. 



