272 Measurement of an inacccssibh Line. 



tallic silver and decomposes a part of the sulphuret of lead, 

 iodide of silver is precipitated on which the nitric acid has no 

 action, we are l^d to believe that the iodine is combined 

 with the silver, Moi'eover, this opinion appears fully con- 

 firmed by the circumstance that a certain quantity of iodine 

 may be extracted from the silver ore by boiling it for some 

 time with ammonia : besides, it is known that iodine, like chlo- 

 rine, has a great affinity for silver. 



I shall deposit the remainder of this specimen in the Ca- 

 binet at the Jardin du.Roi, to serve for comparison with others 

 in case its locality should be discovered at some future time. 



XL. -<4 Method of Measuring an inaccessible Line : with Ob- 

 servations on Mr. Newton's Cross for Surveying. 



To the Editor of the Philosophical Magaziiie and. Journal. 



Sir, 

 TTAVING many years since occasion to measure a long 

 -*•-'■ line which fell across a pond of considerable breadth, 

 the following method of obtaining the distance occurred to me 

 at the time; and as it has, I believe, never been published, nor 

 have I ever seen the method used by any other person, if you 

 think it deserving of notice, its insertion in your Journal will 

 oblige, sir, Yours, &c. 



Oct. 4th, 1825. Leander. 



Measure a straight line from a 

 to c and from d to e, making b c 

 =■ b a and b e = b d; then by si- . ^ 

 milar triangles c c is = to a d; • '•.. 

 whence by measuring c e we get i 

 the distance a d as was required, j 



In Number 326 of the Phil, j "^'■, 



Mag. is given a description of an j 

 improved ci'oss for the use of land- j 

 surveyors by Mr. Isaac Newton. j / 



Now I cannot exactly agree y^ 

 with this gentleman as to the ad- 

 vantage to be derived from the 

 use of this new instrument. Sup- 

 pose, for instance, a diagonal line 

 to run due north and south, and 

 the staff A B D G (see vol. 65, page 431) to be fixed in 

 it, the cross C revolving on the arm F H can only give the 

 perpendicular correct when due north and south of tlie staff, 

 or when it coincides with the diagonal line, as in revolving 



round 



