12 ON THE FIGURE OF THE EARTH. 



and river fand, to which a veryfmall quantity of common brown, 

 or yellow iron ochre, fliould be added, and well incorporated 

 therewith. 



Obfervations 

 upon Profeffor 

 Play fair's 

 memoir on tlie 

 figuie of the 

 earth. 



III. 



On the Figure of the Earth. By Peregrinus Proteus. 



To Mr. NICHOLSON. 

 SIR, 



An fome of your late Journals I obferve a paper on the figure 

 of the earth, by Mr. John Play fair, profeflbr of mathematics 

 in the Univerfity of Edinburgh, containing feveral new the- 

 orems, and ingenious remarks, on a fubjedt which has engaged 

 the attention of the firft mathematicians of Europe fince the 

 days of Newton. On reading it, I was led to examine the 

 properties of fpheroidal triangles, and to inveftigate the pro- 

 blem^ propofed by the author, for determining the dimenfions 

 of the earth from the length of the firaight line or chord join- 

 ing two places whofe geographical fituations are given. Thefe 

 are intended to form the principal fubje6l of this letter; but, 

 before I proceed to them, I beg leave to make a few obferva- 

 tions on that paper, without any view to cavil, or detra6t 

 from its real merits. 



After taking notice of the difagreement in the compreffions 

 of the terreftrial fpheroid, which refult from the comparifon of 

 different meafurements, he afligns, as the principal reafon for 

 this inconfiflency, the local irregularities in the direction of 

 gravity, arifing in fome fituations from the attradion of moun- 

 tains, and in others from the unequal denfity of the materials 

 under, and not far from, the fur face of the earth. That the 

 firfl has a fenfible efFedl on the plumb-line has been proved by 

 accurate and undeniable experiments ; the fecond is an ingeni- 

 ous and probable conje6iure, which the furveys carrying on in 

 Great Britain and France will afford daia to refute or confirm. 

 But though the former may operate in the general furvey of a 

 country, where the obferver has not his choice of ground, it 

 has always been avoided as much as pofTible in meafurements 

 made for the exprefs purpofe of determining the figure of the 

 earth ; and though the latter may produce fome perceptible 



difference 



