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VOL. XXXIX.] PHILOSOPHICAL TRANSACTIONS. 6l 



At first he took the exact length of the rods, in their natural temper. Then 

 he heated them well in a smith's tire, from end to end, nearly to a flaming 

 heat; by which means they were lengthened -i^ o( an inch. Then he 

 quenched them in cold water; which made them y^ of an inch shorter than in 

 their natural state. 



Then he warmed them to the temper of his body; by which means they were 

 about -j-i-5- of an inch longer than in their natural temper. 



Afterwards he cooled them in a strong frigorific mixture of common salt and 

 snow, which shortened them -pf-j- parts of an inch. 



Afterwards he measured these rods, when heated in a hot sun, which length- 

 ened them -T-*Tr parts of an inch more than their natural temper. 



All these experiments seem to concur in resolving the phenomenon of pen- 

 dulum clocks going slower under the equator than in the latitudes from it; but 

 yet he has too good an opinion of Sir Isaac Newton's notion of the spheroidal 

 figure of the earth, easily to part with it; and therefore he leaves it to the con- 

 sideration of others, how far the figure of the earth, and how far heat and 

 cold, and the rarity and density of the air, are concerned in that pheno- 

 menon. 



The Construction and Use of Spherical Maps, or such as are delineated on Por- 

 tions of a Spherical Surface. By Mr. John Colson, M. A. F. R. S. N° 440, 

 p. 204. 



Geographical maps, and hydrographical charts, though representations of a 

 convex spherical surface, were first delineated on planes, as being the most easy 

 and obvious, though not the most natural and accurate representations; and 

 they are sufiiciently near the truth, when the part of the earth or seas described, 

 is not of a very large extent. Such as these have been usually called chorogra- 

 phical and topographical maps ; but when the map is any thing general, or is to 

 contain any large tract of the earth or seas, suppose, for instance, one of the 

 four quarters of the world, then, when they are projected, or represented on a 

 plane, the parts must necessarily be distorted, one way contracted beyond the 

 truth, another way dilated, so as to give no just idea of the whole. Nor can 

 this distortion be possibly avoided, when any considerable part of a spherical 

 surface, by any projection whatever, is to be represented on a plane. This 

 distortion is indeed always regular, and according to certain laws; so that 

 knowing the nature of the projection, it may tolerably well be allowed for. 

 But to do this scientifically, requires much skill and accuracy in the maker, as 

 well as proficiency and experience in the peruser; and therefore not so proper 

 for an introduction to learners, in the rudiments of geography. Young minds 



