YIO REPORT — 1891. 



ment, quite recently added to the apparatus of tlie surveyor, is the photographic 

 camera, converted for his especial benefit into a photofframmeter. This instrument 

 can perhaps never be utilised for ascertaining the relative positions of celestial bodies, 

 but has already done excellent service in ordinary surveying, especially when it is 

 required to portray the sides of inaccessible mountains. 



But the full fruits of these inventions could be enjoyed only after Bradley had 

 discovered the aberration of light (1728) and the nutation of the earth 's axis (1747) ; 

 Domenique Cassini had furnished trustworthy tables of the refraction of light ; and 

 the complicated movement of the moen had been computed by Euler (1746), Tobias 

 Mayer (1753), Bradley (1770), and, more recently by Hansen. 



Positively novel methods for determining the latitude and longitude of a place 

 can scarcely be said to have been proposed during this period, but many of the 

 older methods only became really available after the improvements in the instru- 

 ments indicated above had taken place, and the computations had been freed 

 from the errors which vitiated them formerly. 



Eeal progress, however, has been made in the determination of altitudes. 

 Formerly they could be ascertained only by trigonometrical measurement, or by a 

 laborious process of levelling, but since physicists have shown how the decrease of 

 atmospheric pressure with the altitude, and the boiling-point of water depending 

 upon this decrease, afforded a ready means of determining heights, the barometer, 

 aneroid, and boiling-point thermometer have become the indispensable companions 

 of the explorer, and our knowledge of the relief of the land has advanced rapidly. 



Equally rapid have been the improvements in our instruments for measuring 

 the depth of the ocean, since a knowledge of the configuration of its bed was 

 demanded by the practical requirements of the telegraph engineers. 



And in proportion as the labours of the surveyoi's and explorers gained in pre- 

 ciseness, so did the cartographer of the age succeed in presenting the results achieved 

 in a manner far more satisfactory than had been done by his predecessors. His 

 task was comparatively easy so long as he only dealt with horizontal dimensions, 

 though even in the representation of these a certain amount of skill and judgment 

 is required to make each feature tell in proportion to its relative importance. 

 The delineation of the inequalities of the earth's surface, however, presented far 

 greater difficulties. The mole-hills or serrated ridges, which had not yet quite 

 disappeared from our maps in the beginning of this century, failed altogether in 

 doing justice to our actual knowledge. The first timid attempt to represent 

 hills as seen from a bird's-eye view, and of shading them according to the steep- 

 ness of their slopes, appear on a map of the Breisgau, published by Homann in 

 1718. We find this system fully developed on La Condamine's map of Quito, 

 published in 1751, and it was subsequently popularised by Arrowsmith. In this 

 crude system of hill shading, however, everything was left to the judgment of the 

 draughtsman, and only after Lehmann (1783) had superimposed it upon a ground- 

 work of contours, and had regulated the strength of the hatching in accordance 

 with the degree of declivity to be represented did it become capable of conveying 

 a correct idea of the configuration of the ground. 



The first to fully recognise the great importance of contours was Philip 

 Buache, who had prepared a contoured map of the Channel in 1737, and suggested 

 that the same system might profitably be extended to a delineation of the relief 

 of the land ; and this idea, subsequently taken up by Ducarla of Vabres, was for 

 the first time carried into practice by Dupain-Triel, who published a contoured 

 map of France in 1701. Up to the present time more than eighty methods of 

 showing the hills have been advocated, but it may safely be asserted that none 

 of these methods can be mathematically correct unless it is based upon horizontal 

 contours. 



The credit of having done most towards the promotion of cartography in the 

 course of the eighteenth century belongs to France. It was France which fii-st 

 equipped expeditions to determine the size of the earth ; France which produced 



