OBSERVATION. 



experiment. When the earliest astronomers simply noticed 

 the ordinary motions of the sun, moon, and planets upon 

 the face of the starry heavens, they were pure observers. 

 But astronomers now select precise times and places for 

 important observations of stellar parallax, or the transits of 

 planets. They make the earth's orbit the basis of a well 

 arranged natural experiment, as it were, and take well 

 considered advantage of motions which they cannot control. 

 Meteorology might seem to be a science of pure 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 telegraphic wires, we gather 

 such strong currents during periods of disturbance as to 

 render them capable of easy observation. 



The most well arranged and assiduous systems of ob- 

 servation, however, would fail to give us a large part of 

 the facts which we now possess. Many of the processes 

 which are continually going on in nature may be so slow 

 and gentle in operation that they would for ever escape 

 our powers of observation. Lavoisier remarked that the 

 decomposition of water must have been constantly pro- 

 ceeding in nature, although its possibility was unknown 

 till his time . No substance is wholly destitute of mag- 

 netic 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. 

 Passive and accidental observation long ago impressed 

 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 



c Lavoisier's 'Elements of Chemistry/ transl. by Kerr, 3rd ed. p. 148. 



B 2 



