Mer de Glace of Chamounix and its tributaries, which, in some of the following years, he extended by further observations, so as to include the Glacier des Bossons." The map produced from this survey was worthy of its author; and subsequent explorers of the region he investigated have been able to detect only trivial inaccuracies in his work.
The district surveyed by Forbes remained a solitary bright spot in a region where all besides was darkness until the year 1861. Praiseworthy attempts were made by different hands to throw light upon the gloom, but the efforts were ineffectual, and showed how labour may be thrown away by a number of observers working independently, without the direction of a single head.
In 1861, Sheet xxii. of Dufour's Map of Switzerland appeared. It included the section of the chain of Mont Blanc that belonged to Switzerland, and this portion of the sheet was executed with the admirable fidelity and thoroughness which characterise the whole of Dufour's unique map. The remainder of the chain (amounting to about four-fifths of the whole) was laid down after the work of previous topographers, and its wretchedness was made more apparent by contrast with the finished work of the Swiss surveyors.
Strong hands were needed to complete the survey, and it was not long before the right men appeared.
In 1863, Mr. Adams-Reilly, who had been travelling in the Alps during several years, resolved to attempt a survey of the unsurveyed portions of the chain of Mont Blanc. He provided himself with a good theodolite, and starting from a base-line measured by Forbes in the Valley of Chamounix, determined the positions of no less than 200 points. The accuracy of his work may be judged from the fact that, after having turned many corners and carried his observations over a distance of fifty, miles, his Col Ferret "fell within 200 yards of the position assigned to it by General Dufour!"
In the winter of 1863 and the spring of 1864, Mr. Reilly constructed an entirely original map from his newly-acquired data. The spaces between his trigonometrically-determined points he
