477 



3. Notice of a Collection of Maps. By A. K. Johnston, Esq. 



In this paper the author reported the progress made by the commit- 

 tee appointed to select and purchase a series of chartagraphic works for 

 the library of the Royal Society, and the means adopted for their 

 arrangement and classification. The collection already comprises 

 534 separate sheets of the best existing maps, chiefly of the several 

 countries of Europe, but embracing the survey of India, in so far as 

 published. The maps are placed in cases resembling volumes, so 

 indexed as to admit of being indefinitely extended, and easily con- 

 sulted. Specimens of the diff^erent works were exhibited, and the 

 author presented a rapid sketch of the progress of surveying and 

 mapping, from the sixteenth century to the present time. He 

 showed that modern improvement in this important branch of science 

 dates from the middle of the 18th century, when, in 1750, Cassitii 

 de Thury, under the auspices of the French Academy of Sciences, 

 constructed a map of France on astronomical principles. In 1784 

 the French triangulation was extended to London, and formed the 

 basis of the trigonometrical survey of Gi'eat Britain. The surveys 

 of Belgium, the Netherlands, Prussia, and Sardinia, have also been 

 based on that of France. The different methods adopted to repre- 

 sent relief of the surface by contour lines and hill shading, were 

 then referred to, and examples of the effects produced by vertical and 

 oblique lights were exhibited. It was shown that the method which 

 supposes the light to fall vertically on the model, casting the 

 shadow in all directions, gives the most exact idea of the ine- 

 qualities of the ground, and that it is adopted in nearly all the 

 great survey maps now in progress. As an example of the time and 

 labour necessary to produce a good map, it was explained that in the 

 great survey of France, now nearly completed, a single sheet re- 

 quires, for reduction and drawing, at least two years, and for en- 

 graving, five to eight years. Thus, between the termination of the 

 field-work of the surveyor and the publication, seven to ten years 

 must necessarily elapse. Mr Keith Johnston concluded his remarks 

 by referring to the economical advantages of the electrotype process 

 in reproducing copies of original plates, thus reducing the price of 

 the publication ; and to an ingenious application of this process, 

 recently adopted at the Depot de la Guerre, Paris, by which 

 erasures made in the work, for correction, are filled up by a fresh 

 deposit of copper, leaving a surface ready for being re-engraved. 



VOL. III. 2 Q 



