NEVIL MASKELYNE was born in London, October 6, 1732. He was educated at Westminster, and in time proceeded to Catherine Hall, Cambridge, from whence he migrated to Trinity College. He took the degree of Bachelor of Arts, with honours, in the year 1754. In 1755, he was ordained to a curacy near London. He had previously turned his attention to astronomy, to which he was led by the solar eclipse of 1748; and he now formed an acquaintance with Bradley, an astronomer of unequalled merit, whether in discovery or practical excellence in observation, whom he assisted in calculating his table of refractions. It is no wonder that, under such instruction, Maskelyne should have distinguished himself afterwards as an observer. From this period (A. D. 1750) Delambre dates the commencement of really good observations.

In 1758 Maskelyne was elected Fellow of his college; in 1759 he became Fellow of the Royal Society. In 1761 he went to St. Helena, to observe the transit of Venus, and also to collect such observations as might, if possible, enable him to detect the parallax of the fixed stars. He failed in both objects; in the first from cloudy weather, in the second from faulty instruments, as he supposed, though the quantity in question is so small that its existence has not yet been detected; but he was enabled to correct the principal errors of those instruments in a considerable degree, and also to make very good observations on various other points. In his voyage out and home he applied himself to perfect the method of observing lunar distances, and deducing the longitude from them. In 1764 he sailed to Barbadoes, to make a trial of Harrison's time-keeper; and in 1765 he was appointed Astronomer Royal, on the decease of Mr. Bliss. He was then only thirty-three

years of age, and had enjoyed a rapid career of celebrity. He had published enough in the British Mariner's Guide,' A. D. 1763, to require honourable mention of his name and methods in every work of navigation.

As soon as he had obtained the post of Astronomer Royal, he began to call the attention of the Commissioners of Longitude to the practicability of the method of lunar distances, and proposed to them to establish a Nautical Almanac, which should contain such an ephemeris of the moon's path as would make the object in view attainable. The memorial on this subject was presented February 9, 1765, and the evidence of various officers of the East India Company's service was taken as to the success of the method. The lunar tables of Mayer furnished the proposed materials for the moon's places; and upon the adoption of the scheme of Maskelyne, a parliamentary reward of 30007. was given to Mayer's widow. To Maskelyne we are thus indebted for a work which has more than any other contributed to the advancement of navigation, in the removal of the great difficulty of finding the longitude. It is true that this first effort could hardly then be expected to give the longitude within a degree; but this was a great improvement, when it is considered that the reckoning of a ship might be out several degrees, and that chronometers had not yet been introduced. But the Nautical Almanac' must be considered as a work addressed to astronomers as well as seamen, from its earliest commencement. Maskelyne saw the importance of saving the observer the trouble and risk of error which would attend his reductions without such assistance, and contemplated the continual improvement of the lunar tables. It is not one of the least obligations which astronomical science owes to Maskelyne, that since his time a very slender portion of mathematical knowledge will enable a diligent observer to turn his means to good account in the promotion of sidereal and even of planetary astronomy. Without saying that the observer, as such, is employed about the highest department of the science, or in any way recommending the lover of observation to stop his career at that point, we may remind him that, with the assistance of an ephemeris, such as the 'Nautical Almanac' of Maskelyne did, still more as that of the present day, he can never want the means of turning his amusements to useful purpose.

The first Nautical Almanac was published in 1767, and was continued by Maskelyne to the end of his life. The requisite tables, intended to accompany that work, were first published by him in

1781.

With the exception of attending the meetings of the Royal Society, Maskelyne hardly ever quitted his observatory. His life is therefore difficult to describe, except by its results. But in 1772 he went to Scotland, to pursue his celebrated experiment for the discovery of the earth's density. The Newtonian doctrine of attraction, in the general form, that all matter attracts all other matter, could hardly be said to be finally established, except as a point of strong probability. That a planet, considered as a whole, attracts a planet, might be thought to be demonstrated, but there was no proof of matter being the agent of attraction upon matter, on the earth, except in the case of magnetised or electrified bodies. The notion that the attraction of a mountain, if it existed, would cause a slight deviation of the plumb-line, which should be perceptible in its effect on the observed position of the stars, had been entertained, and the effect even suspected, but without being reduced to absolute proof. To give an idea of the minuteness of the angle of deviation which was to be looked for, we may state that a pendulum ten thousand inches long, vibrating through an angle of ten seconds, would only move through half an inch at the end farthest from the point of suspension, and that ten seconds was, as it turned out, nearly double of the angle in question. Maskelyne chose the mountain Schehallien, in Perthshire, as the scene of his operations. By observing well-known stars with an instrument depending on a plumb-line, both north and south of the mountain, he determined the difference of latitude of two stations, subject of course to an error if the plumb-line were affected in its position by the attraction of the mountain. He then measured the difference of latitude of his stations by a trigonometrical survey, which gave their relative position by a method independent of the plumb-line and its errors. He thus found that his north and south plumb-lines were inclined to each other at an angle of about eleven seconds and a half more than they should have been from their difference of position on the earth, and that the direction of their inclination was towards the mountain. He deduced his results from those among his observations which he considered as the best, being about one out of ten of the whole; but it is much to his credit as an observer, that Baron Zach afterwards found that all his observations, good and bad, gave the same average result as those he had selected. Zach also established the same fact by his observations in the neighbourhood of Marseilles, namely, that the vicinity of a mountain affects the level, which was the instrument he used, and not the plumb-line.

The labour of deducing an approximation to the earth's mean density