The labour of deducing an approximation to the earth’s mean density was undertaken by Dr. Hutton. By getting the best possible estimate of the materials of which Schehallien is composed, and comparing what we must call the weight of the plumb-line _towards the mountain_ with its weight towards the earth, it appeared that the mean density of the latter is about five times that of water. This, considered as a numerical approximation, alone and unsupported, would have been worth little, owing to the doubt which must have existed as to the correctness of the estimation of the mountain’s density. It would prove that there was attraction in the mountain, but would give no very great probability to the value of the earth’s density, as deduced. But a few years afterwards Cavendish made an experiment, with the same object, and by an entirely different method. By producing oscillations in leaden balls by means of other leaden balls, and by a process of reasoning wholly free from astronomical data, he inferred that the mean density of the earth was five and a-half times that of water. The experiment of Cavendish was published in 1798. It is much to be wished that the experiments of Cavendish should be repeated on a larger scale: but the expense of the apparatus will probably deter individuals from the attempt. The Schehallien experiment was carried on under many difficulties and privations; and its successful result places its author in the list of those who first opened the road to the determination of a fundamental element of the solar system. But brilliant as it must appear, it is by no means the most useful of Maskelyne’s labours. Excepting Bradley, he may almost be called the first who systematically directed his efforts to the attainment of the minutest accuracy in astronomical observation. His celebrated catalogue, A.D. 1790, consisted only of thirty-six of the principal stars, but the places of these, especially in right ascension, were determined with a degree of precision which was then believed to be hardly attainable. The means by which he accomplished his objects, such as taking the nearest tenth of a second instead of the nearest second, or half second, of time in his transit observations, the practice of uniformly observing all the wires of the instrument, instead of one; the introduction of the movable eye-piece, by which the several wires could all be viewed directly, instead of obliquely, and many little things of the kind, are the indications of a man who was familiar above his contemporaries with the sources of error, and who had formed at once a bold estimate of the extent to which they might be avoided, and a correct view of the means of doing it. It is difficult to say what portion of the present improved spirit of observation in these points may be attributed to Maskelyne, but it certainly was not small. Delambre, who knew at least as well as any man of his time what had been done and was doing, and who was never