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XLVI. On Plans in Relief. By A Correspondent. 



To the Editor of the Philosophical Magazine andjcmrnal. 

 Sir, 

 CUCH of your readers as have seen the plan of the "Swiss 

 ^ Mountains" in relief, with a similar plan of the city of 

 " Edinburgh," will readily admit their interest as a matter of 

 taste, and their political and geological value. The " Pyra- 

 mids " with the sphynx and neighbouring country may be 

 seen in the royal library at Paris; and I believe that a plan 

 of "Gibraltar" in relief has been executed with great minute- 

 ness. Evelyn, the author of the Sylva, mentions in an early 

 part of the Philosophical Transactions that he saw similar 

 plans on the continent, and particularizes an island which 

 gave him great satisfaction. 



The want of such plans has been felt in the original pro- 

 jection of important drainages, roads, canals, rail-ways, and all 

 other works depending upon levels. Shading and sections are 

 imperfect substitutes ; and an actual plan in relief, except of a 

 mountainous country, is almost useless from the small propor- 

 tion which the elevations bear to the horizontal plane. But 

 why should not plans in relief be formed, such that the alti- 

 tude of any point above the horizontal plane shall always be 

 increased in ajixed ratio to the true altitude ? If the altitude, 

 for instance, be increased {n) times, it will in all parts of the 

 plan bear this fixed proportion to the base line, and will truly 

 exhibit the di^-ection of the inclination, and its comjyarative 

 magnitude. No ordinary map whatever is mathematically 

 true ; when the lines in longitude are correct, the latitudinal 

 distances must be incorrect, and conversely : the various pro- 

 jections have always been formed on some conventional prin- 

 ciple, never accurately exhibiting the true horizontal distance. 



To the artist I leave the mode of multiplying such plans 

 in plaster, copper, or otherwise ; and the contrivances neces- 

 sary for forming a series into a volume, folded up like a back- 

 gannnon board. 



Septimus. 



XLVI I. On the Locality of Rain. By Mr. James Stockton. 

 To the Editor of the Philosophical Magazine and Journal. 

 Sir, 

 T N Dr. Burney's valuable j^aper inserted in your last Maga- 

 -^ zine, relative to the great dillerence of rain caught at different 

 places in the British isles, he makes mention of this place, and 



states 



