THE PRINCIPLES OF SCIENCE. 



character were manifestations of the one same agent. To 

 observe with accuracy and convenience we must have 

 agents under our control, so as to raise or lower their 

 intensity, to stop or set them in action at will. Just as 

 Smeaton found it requisite to create an artificial and 

 governable supply of wind for his investigation of wind- 

 mills, so we must have, constant or governable supplies of 

 light, heat, electricity, muscular force, or whatever other 

 agents we are examining. 



It is hardly needful to point out too that on the earth's 

 surface we live under nearly uniform conditions of gravity, 

 temperature, and atmospheric pressure, so that if we are to 

 extend our inferences to other parts of the universe where 

 conditions may be widely different, we must be prepared 

 to imitate those conditions on a small scale here. We 

 must have intensely high and low temperatures ; we must 

 vary the density of gases from approximate vacuum up- 

 wards ; we must subject liquids and solids to pressures or 

 strains of almost unlimited amount. 



Mental Conditions of Correct Observation. 



Every observation must in a certain sense be true, for 

 the observing and recording of an event is in itself an 

 event. But before we proceed to deal with the supposed 

 meaning of the record, and draw inferences concerning the 

 course of nature, we must take care to ascertain that the 

 position and feelings of the observer are not to a great 

 extent the phenomena recorded. The mind of man, as 

 Francis Bacon said, is like an uneven mirror, and does not 

 reflect the events of nature without distortion. We need 

 not take notice of intentionally false observations, nor 

 of mistakes arising from defective memory, deficient 

 light, and so forth. Even where the utmost intentional 

 fidelity and care are used in observing and recording, 



