THE EXACT MEASUREMENT OF PHENOMENA. 341 



scale are naturally added together without the slightest 

 error. There is no difficulty in the precise juxtaposition 

 of two grammes, but the juxtaposition of two metre mea- 

 sures can only be effected with tolerable accuracy, by the 

 use of microscopes and many precautions. Hence, the 

 extreme trouble and cost attaching to the exact measure- 

 ment of a base line for a survey, the risk of error entering 

 at every juxtaposition of the measuring bars, and indefatig- 

 able attention to all the requisite precautions being neces- 

 sary throughout the operation z . 



Measurements by Natural Coincidence. 



In certain cases a peculiar conjunction of circumstances 

 enables us to dispense more or less with instrumental 

 aids, and to obtain the most exact numerical results in 

 the simplest manner. The mere fact, for instance, that 

 no human being has ever seen a different face of the moon 

 from that familiar to us, conclusively proves that the 

 period of rotation of the moon on its own axis is equal 

 to that of its revolution round the earth. Not only have 

 we the repetition of these movements during 1000 or 

 2000 years at least, but we have observations made for 

 us at very remote periods, free from instrumental error, 

 no instrument being needed. We learn that the seventh 

 satellite of Saturn is subject to a similar law, because its 

 light undergoes a variation in each revolution, owing to 

 the existence of some dark tract of land ; now this failure 

 of light always occurs while it is in the same position 

 relative to Saturn, clearly proving the equality of the 

 axial and revolutional periods, as Huyghens perceived a . 



2 Herschel's, ' Familiar Lectures on Scientific Subjects/ p. 184. 

 a ' Hugenii Cosmo theoros,' pp. 117-18. Laplace's ' Systkme/ trans- 

 lated, vol. i. p. 67. 



