74 Things not generally Known. 



of TWO of a second daily. At the equator the moon's attrac- 

 tion changes the weight of a body only ?000 oo6 of the whole ; 

 yet this force is sufficient to produce the vast phenomena of 

 the tides ! 



It is no slight evidence of the importance of analysis, that 

 Laplace's perfect theory of tides has enabled us in our astro- 

 nomical ephemerides to predict the height of spring-tides at 

 the periods of new and full moon, and thus put the inhabitants 

 of the sea on their guard against the increased danger attending 

 the lunar revolutions. 



MEASURING THE EARTH BY THE MOON. 



As the form of the Earth exerts a powerful influence on the 

 motion of other cosmical bodies, and especially on that of its 

 neighbouring satellite, a more perfect knowledge of the motion 

 of the latter will enable us reciprocally to draw an inference 

 regarding the figure of the earth. Thus, as Laplace ably re- 

 marks : " an astronomer, without leaving his observatory, may, 

 by a comparison of lunar theory with true observations, not 

 only be enabled to determine the form and size of the earth, 

 but also its distance from the sun and moon ; results that other- 

 wise could only be arrived at by long and arduous expeditions 

 to the most remote parts of both hemispheres." The compres- 

 sion which may be inferred from lunar inequalities affords an 

 advantage not yielded by individual measurements of degrees 

 or experiments with the pendulum, since it gives a mean 

 amount which is referable to the whole planet. Humboldt's 

 Cosmos, vol. i. 



The distance of the moon from the earth is about 240,000 

 miles ; and if a railway-carriage were to travel at the rate of 

 1000 miles a-day, it would be eight months in reaching the 

 moon. But that is nothing compared with the length of time 

 it would occupy a locomotive to reach the sun from the earth : 

 if travelling at the rate of 1000 miles a-day, it would require 

 260 years to reach it. 



CAUSE OF ECLIPSES. 



As the Moon is at a very moderate distance from us (astro- 

 nomically speaking), and is in fact our nearest neighbour, while 

 the sun and stars are in comparison immensely beyond it, it 

 must of necessity happen that at one time or other it must 

 pass over, and occult or eclipse, every star or planet within its 

 zone, and, as seen from the surface of the earth, even some- 

 what beyond it. Nor is the sun itself exempt from being thus 

 hidden whenever any part of the moon's disc, in this her tor- 

 tuous course, comes to overlap any part of the space occupied 



