From an early period Hooker had conceived the desire to explore the range of the Great Atlas, to become acquainted with its vegetation, and to ascertain whether this supplies connecting links between that of the Mediterranean region and the peculiar flora of the Canary Islands. This desire was increased during a journey in Syria, in 1860, made in company with Admiral Washington, the late Hydrographer of the Navy, one of the very few Europeans who had reached the flanks of the Great Atlas chain, when, as a young naval officer, he accompanied the late Sir John Drummond Hay on his mission to the city of Marocco in 1829. Maw had already made collections of living plants in the neighbourhood of Tangier, and had also visited Tetuan, where he had pushed his excursions farther than any but one preceding traveller. Ball had landed at Tetuan in 1851 with the hope of attaining some of the higher summits of the neighbouring Riff Mountains; but the disturbed state of the country in that year made it impossible to advance beyond the immediate outskirts of the city. From the moment when it seemed likely that the permission to visit the Great Atlas sought for by Hooker, through the intervention of our Foreign Office, would be accorded by the Sultan of Marocco, no time was lost in making the requisite preparations. Although everything was done within about a fortnight, our equipment was tolerably complete; and when, after the first excitement of departure had subsided, we thought it over on board ship, we found but one serious omission to deplore. Two mercurial barometers, provided by Hooker, had been entrusted to Crump, and were by him left behind at the last moment. Thus, in the important matter of determining heights, we were forced to rely upon aneroid barometers and boiling water observations. It was fortunate that Ball carried an excellent aneroid, by Secrétan of Paris, which has before and since been severely tested in the Alps with very satisfactory results, and whose indications during our journey agreed closely with those given by the thermometer in boiling water. Among the various preparations made for our journey there was none more important for our purpose than a manuscript catalogue of all the plants hitherto known or believed to have been found in the Empire of Marocco, which we owed to the kindness of our excellent friend M. Cosson, the eminent French botanist. Up to that date the information to be found in books was extremely scanty, and scattered throughout various systematic works, and the whole when summed up would have given a most incomplete account of the two or three districts partially explored by botanists. M. Cosson, by his unequalled knowledge of the North African flora, and by careful study of all the collections made in Marocco, many of which are in his exclusive possession, was the only person who could have supplied the materials which were so serviceable throughout our journey.