Scr X1H. WEIGHTS AND MEASURES 85 



lost in the vicissitude of human affairs, both may be 

 recovered ; since they are derived from natural standards 

 presumed to be invariable. The length of the pendu- 

 lum would be found again with more facility than the 

 metre. But as no measure is mathematically exact, an 

 error in the original standard may at length become 

 sensible in measuring a great extent, whereas the error 

 that must necessarily arise in measuring the quadrant of 

 the meridian is rendered totally insensible by subdi- 

 vision in taking its ten-millionth part. The French 

 have adopted the decimal division, not only in time but 

 also in their degrees, weights, and measures, on account 

 of the very great facility it affords in computation. It 

 has not been adopted by any other people, though 

 nothing is more desirable than that all nations should 

 concur in using the same standards, not only on account 

 of convenience, but as affording a more definite idea of 

 quantity. It is singular that the decimal division of the 

 day, of degrees, weights, and measures, was employed 

 in China 4000 years ago ; and that at the time Ibn Tunis 

 made his observations at Cairo about the year 1000 of 

 the Christian era, the Arabs were in the habit of em- 

 ploying the vibrations of the pendulum in their astro- 

 nomical observations as a measure of time. 



SECTION XIII. 



Tides Forces that produce them Three kinds of Oscillations in the Ocean 

 The Semidiurnal Tides Equinoctial Tides Effects of the Declina- 

 tion of the Sun and Moon Theory insufficient without Observation 

 Direction of the Tidal Wave Height of Tides Mass of Moon obtained 

 from her Action on the Tides Interference of Undulations Impossi- 

 bility of a Universal Inundation Currents. 



ONE of the most immediate and remarkable effects of 

 a gravitating force external to the earth, is the alternate 

 rise and fall of the surface of the sea twice in the course 

 of a lunar day, or 24 h 50 m 28 s of mean solar time. As it 

 depends upon the action ofthe sun and moon, it is classed 

 among astronomical problems, of which it is by far the 

 most difficult and its explanation the least satisfactory. 

 The form of the surface of the ocean in equilibrio when 

 revolving with the earth round its axis, is an ellipsoid 

 H 



