MECHANICS AND USEFUL ARTS. 37 



nometricnl sections are derived from the measured base-line on Salisbury 

 Plain, and on the north shore of Lough Foyle, in the north of Ireland. This 

 most important branch of the work has been executed with the greatest 

 accuracy the difference between the measured lengths of the bases of veri- 

 fication and their computed lengths not exceeding two and one-half inches 

 in seven miles. The average length of the sides of the triangles in the 

 principal triangulation is about sixty miles, but many of the sides exceed 

 one hundred miles in length. The primary triangulation is next broken up 

 into smaller triangles, the sides of which are from five to ten miles in length, 

 and this secondary is again broken into triangles, the sides of which are 

 about one mile long, to form the tertiary or minor triangulation. The men 

 employed to make the detailed survey, then, actually measure the length of 

 each side of the minor triangles on the ground, noting in their " field-books " 

 every fence, stream, or other object they may cross; they then measure 

 cross lines from one side of the triangle to the other, and, by taking offsets 

 from the measured lines to every object on the face of the country, they ob- 

 tain in their field-books the data for plotting accurate plans upon any scale 

 which may be required. The length of every measured side of a triangle, 

 therefore, is checked by the computed trigonometrical distance, and the accu- 

 racy of the lines within each triangle is checked by the plotting, and thus no 

 errors can escape detection. By this method perfect accuracy is obtained, 

 not only in every part of the detail of the survey, but every object is in its 

 correct relative position to every other object, however distant. The levels 

 engraved on the plans are all given in relation to one datum level, that for 

 Great Britain being the level of mean-tide at Liverpool. 



The scales which have been adopted for the plans are as follows: Town 

 maps, 60 inches to the mile, or l-oOO of the actual linear measure; parishes, 

 25'334 inches to a mile, or 1-2-100 of the actual measurement ; counties, 6 

 inches to the mile; and the general map of the kingdom, 1 inch to the mile. 

 The parish plans are engraved upon zinc, and the remaining plans on cop- 

 per. Zincography is now generally adopted, instead of lithography, on ac- 

 count of the facility of handling zinc plates, rather than lithographic stones, 

 which are necessarily heavy, and are constantly liable to be broken. The 

 reduction of the scale of the one-inch plans from those of larger size, is done 

 by means of photography. The collodion process is employed for the pur- 

 pose of taking the negative copy. The lens of the camera used is a single 

 achromatic meniscus, three and one-half inches in diameter, with a principal 

 focal length of twenty-four inches. The plan to be reduced is attached to a 

 board, which can be adjusted by a screw to any height that may be required, 

 and turns upon a centre pivot. The camera is placed opposite to it on a 

 table which runs upon wheels upon a small tramway laid down on the floor 

 of the photographic room, and the required scale of the reduction is obtained 

 by tracing on the ground glass of the camera a rectangle corresponding on 

 the reduced scale to the rectangle of the plan to be reduced. The curvature 

 of the image, and the indistinctness of outline from spherical aberration, are 

 both remedied by reducing the diaphragm in front of the lens to a small 

 aperture. From the negative thus obtained on glass, as many positive copies 

 on paper as are required are then taken in the usual way. The introduction 

 of this method has greatly lessened the cost of reducing the plans, and also 

 saves an immense quantity of time and labor. The six-incn map is en- 

 graved in sheets three feet by two feet, the sheets of each county beiim- made 

 to fit together by the marginal lines, so as to form, if required, a single plan. 



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