THE EXACT MEASUREMENT OF PHEXOMEXA. 347 



minute difference in the lengths of the waves makes a 

 very perceptible difference in the position of the point 

 at which two rays will interfere and produce darkness. 



M. Fizeau has recently employed Newton's rings in an 

 inverse manner, to measure small amounts of motion. By 

 merely counting the number of rings of sodium mono- 

 chromatic light passing a certain point where two glass 

 plates are in close proximity, he is able to ascertain with 

 the greatest accuracy and ease the change of distance 

 between these glasses, produced, for instance, by the ex- 

 pansion of a metallic bar, connected with one of the glass 

 plates 11 . 



Nothing excites more admiration than the mode in 

 which scientific observers can occasionally measure quan- 

 tities, which seem beyond the bounds of human obser- 

 vation. We know the average depth of the Pacific 

 Ocean to be 14,190 feet, not by actual sounding, which 

 would be impracticable in sufficient detail, but by noticing 

 the rate of transmission of earthquake waves from the 

 South American to the opposite coasts, the rate of move- 

 ment being connected by theory with the depth of the 

 water 1 . In the same way the average depth of the 

 Atlantic Ocean is inferred to be no less than 22,157 feet, 

 from the velocity of the ordinary tidal waves, A tidal 

 wave again gives beautiful evidence of an effect of the 

 law of gravity, which we could never in any other way 

 detect. Newton estimated that the moon's force in mov- 

 ing; the ocean is onlv - part of the whole force of 



2,871, 4 oo L 



gravity, which even the pendulum, used with the utmost 

 skill, would fail to render apparent. Yet the immense 

 extent of the ocean allows the accumulation of the effect 

 into a very palpable amount ; and from the comparative 



h 'Proceedings of the Royal Society,' 3oth November, 1866. 

 i Herschel, ' Physical Geography,' 40. 



