A few years ago not many persons knew from personal knowledge how extremely inaccurately the chain of Mont Blanc was delineated. In the earlier part of the century thousands had made the tour of the chain, and before the year 1860 at least _one_ thousand individuals had stood upon its highest summit; but out of all this number there was not one capable, willing, or able, to map the mountain which, until recently, was regarded the highest in Europe. Many persons knew that great blunders had been perpetrated, and it was notorious that even Mont Blanc itself was represented in a ludicrously incorrect manner on all sides excepting the north; but there was not, perhaps, a single individual who knew, at the time to which I refer, that errors of no less than 1000 feet had been committed in the determination of heights at each end of the chain; that some glaciers were represented of double their real dimensions; and that ridges and mountains were laid down which actually had no existence. One portion alone of the entire chain had been surveyed at the time of which I speak with anything like accuracy. It was not done (as one would have expected) by a Government, but by a private individual,—by the British De Saussure,—the late J. D. Forbes. In the year 1842, he “made a special survey of the 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. 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. 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!”