40 VIIE PRINCIPLES OF SCIENCE. 



In the science of radiant heat, early inquirers were led 

 to the conclusion that radiation proceeded only from the 

 surface of a solid, or from a very small depth below it. 

 But they happened to experiment upon surfaces covered 

 by coats of varnish, which is highly athermanous or 

 opaque to heat. Had they properly varied the character of 

 the surface, using a highly diathermanous substance like 

 rock salt, they would have obtained very different results > T . 



One of the most extraordinary instances of an erroneous 

 opinion due to overlooking interfering agents is that con- 

 cerning the increase of rainfall near to the earth's surface. 

 More than a century ago it was observed that rain- 

 guages placed upon church steeples, house tops, and other 

 elevated places, gave considerably less rain than if they 

 were on the ground, and it has very recently been shown 

 that the variation is most rapid in the close neighbourhood 

 of the ground 2 . All kinds of theories have been started to 

 explain this phenomenon ; but I have attempted to show a 

 that it is simply due to the interference of wind, which 

 deflects more or less rain from all the guages which are 

 at all exposed to it. 



The great magnetic power of iron renders it a constant- 

 source of disturbance in all magnetic experiments. In 

 building a magnetic observatory great care must therefore 

 be taken that no iron is employed in the construction, and 

 that no masses of iron are near at hand. In some cases 

 magnetic observations have been seriously disturbed by the 

 existence of masses of iron ore in the neighbourhood. In 

 Faraday's experiments upon feebly magnetic or diamag- 



ix. pp. 433, 455; Swan, ibid. [1856] vol. xxi. p. 411.; 'Philosophical 

 Magazine,' 4th Series, vol. xx. p. 173, [Sept. 1860] ; Roscoe, 'Spectrum 

 Analysis,' Lecture III. 



y Stewart, ' Elementary Treatise on Heat,' p. 192. 



z British Association, Liverpool, 1870. 'Report on Rainfall,' p. 176. 



a 'Philosophical Magazine,' Dec. 1861, 4th Series, vol. xxii. p. 421. 



