18 INVESTIGATION of certain THEOREMS 



20. In the part of England, where the meafures we are now 

 treating of have been taken, the ftrata are of chalk, and though 

 of great extent, are bordered, on all the fides that we have accefs 

 to examine by ftrata much denfer and more compact. Toward 

 the weft the chalk is fucceeded by limeftone, and that limeftone 

 by the primitive fchiftus and granite of the weft of Devonfhire 

 and of Cornwall. On the eaft we may fuppofe-that fomething of 

 the fame kind takes place, though the fea prevents us from ob- 

 ferving it, as the chalky and argillaceous beds extend in this di- 

 rection to the coaft, and probably to fome diftance beyond it. 

 Now the meridian of Greenwich may be confidered as dividing 

 the tract: of country, occupied by thefe lighter ftrata, into two 

 parts, in fuch a manner, that the plummet being carried to a 

 diftance from it, either eaft or weft, approaches to the denfer 

 ftrata, and is of courfe attracted by them, fo that the zenith is 

 forced back, as it were, to the meridian of Greenwich, and does 

 not recede from it, in the heavens, at fo great a rate as the plum- 

 met itfelf does, on the earth. Hence the longitudes from this 

 meridian, eftimated by the arches in the heavens, intercepted be- 

 tween the zenith and the faid meridian, wall appear lefs than 

 they ought to do ; and too much fpace on the furface of the 

 earth will of confequence be affigned as the meafure of a degree. 

 In this way D' is made too great ', and we may fuppofe the cir- 

 cumftances fuch that D, on going north or fouth, is not enlar* 



ged in the fame proportion ; hence — jj; — will be augmented, 



and of courfe - will be reprefented as too great. This explana- 

 tion may perhaps appear very hypothetical, and it is certainly 

 propofed merely as a hypothefis. It is a hypothefis, too, that 

 lays claim only to a temporary indulgence, as it is propo- 

 fed at the very moment when it may be brought to the trial, 

 and when, by a further continuation of the furvey toward the 



norths 



