560 THE PRINCIPLES OF SCIENCE. [cHAr. 



far have better claims to attention. Of course we must 

 never estimate the probability of an hypothesis merely by 

 its accordance with a few results only. Its general analogy 

 and accordance with other known laws of nature, and the 

 fact that it does not conflict with other probable theories, 

 must be taken into account, as we shall see in the next 

 book. The requisite condition of a good hypothesis, that 

 it must admit of the deduction of facts verified in observa- 

 tion, must be interpreted in the widest manner, as including 

 all ways in which there may be accordance or discordance. 

 All our attempts at reconciliation having failed, the only 

 conclusion we can come to is that some unknown cause of 

 a new character exists. If the measurements be accurate 

 and the theory probable, then there remains ^residual phe- 

 nomenon, which, being devoid of theoretical explanation, 

 must be set down as a new empn^ical fact worthy of further 

 investigation. Outstanding residual discrepancies have 

 often been found to involve new discoveries of the greatest 

 importance. 



Accordance of Measurements of Astronomical Distances. 



One of the most instructive instances which we can 

 meet, of the manner in which different measui'emeuts con- 

 tirm or check each other, is furnished by the determination 

 of the velocity of light, and the dimensions of the planetary 

 system. Eoemer first discovered that light requires time 

 to travel, by observing that the eclipses of Jupiter's satellites, 

 although they occur at fixed moments of absolute time, are 

 visible at different moments in different parts of the earth's 

 orbit, according to the distance between the earth and 

 Jupiter. The time occupied by light in traversing the 

 mean semi-diameter of the earth's orbit is found to be 

 about eight minutes. The mean distance of tiie sun and 

 earth was long assumed by astronomers as being about 

 95,274,000 miles, this result being deduced by l^essel from 

 the observations of the transit of Venus, which occurred in 

 1769, and which were found to give the solar parallax, or 

 which is the same thing, the apparent angular magnitude 

 of the earth seen from the sun, as equal to 8""578. 

 Dividing the mean distance of the sun and earth by the 



