xviii.] OBSERVATION. 401 



observation, because we cannot possibly govern the changes 

 of weather which we record. Nevertheless we may ascend 

 mountains or rise in balloons, like Gay-Lussac and Glaisher, 

 and may thus so vary the points of observation as to render 

 our procedure experimental. We are wholly unable either 

 to produce or prevent earth-currents of electricity, but 

 when we construct long lines of telegraph, we gather such 

 strong currents during periods of disturbance as to render 

 them capable of easy observation. 



The best arranged systems of observation, however, would 

 fail to give us a large part of the facts which we now 

 possess. Many processes continually going on in nature 

 are so slow and gentle as to escape our powers of observa- 

 tion. Lavoisier remarked that the decompositibn of water 

 must have been constantly proceeding in nature, although 

 its possibility was unknown till his time. 1 No substance 

 is wholly destitute of magnetic or diamagnetic powers ; 

 but it required all the experimental skill of Faraday to 

 prove that iron and a few other metals had no monopoly 

 of these powers. Accidental observation long ago im- 

 pressed upon men's minds the phenomena of lightning, 

 and the attractive properties of amber. Experiment only 

 could have shown that phenomena so diverse in magnitude 

 and character were manifestations of the 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 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 constant conditions of gravity, 

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

 extend our inferences to other parts of the universe where 

 conditions are 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 



1 Lavoisier's Elenients of Chemistry, translated by Kerr, 3rd ed. 

 p. 148. 



