PHYSICAL GEOGRAPHY. 99 



degrees ; it follows that fifteen degrees on the Equa- 

 tor will pass by any Celestial Meridian in one hour ; con- 

 sequently any place that is fifteen degrees eastward or 

 westward of a given place, will vary one hour in time, 

 and so in proportion. If the chronometer be regulated 

 for any particular meridian, it will always give the 

 time at that meridian ; then, by ascertaining the exact 

 time under any other meridian, which may be easily 

 done, the difference between the two, reduced to de- 

 grees, &c. will give the difference of Longitude between 

 the places. Thus, if the sun be observed to be on the 

 meridian of any place when it is eleven o'clock by the 

 chronometer, the difference of Longitude will be fifteen 

 degrees, and to the eastward; if on the meridian when it 

 is one o'clock, it will be fifteen degrees to the westward. 



Although it was long known that the earth was of a 

 spherical shape, yet its precise degree of ellipticity re- 

 mained undetermined, as also its magnitude. The ablest 

 mathematicians of which Europe could boast, particu- 

 larly during the last hundred years, have been directing 

 their attention to this subject, and as yet have been 

 unable to solve this problem as accurately as desired. 



The figure of the Earth is, however, known to be an ob- 

 late spheroid compressed towards the Poles; its equato- 

 rial diameter, as determined by M. Laplace, being 7924 

 miles, and its polar axis 7898 miles, while its circum- 

 ference is rather more than 24,OOO miles. 



DIVISION INTO LAND AND WATER ELEVATION OF 



SURFACE HIGHLANDS AND LOWLANDS. 



On considering the nature of the surface of the Earth, 

 its division into land and water will be the first circum- 

 stance that will attract the attention. 



The land consists of two continents, together with 

 innumerable islands ; the continents being separated by 

 those vast bodies of water, the Atlantic and Pacific 

 Oceans. It might be imagined that there was a time 

 when the whole land formed one immense mass, and 

 that some internal and most potent agency produced a 

 tremendous disruption, which separated it into conti- 

 nents, while the smaller pieces formed islands. 



