found that the star (y Dracouis) on which they were principally made, appeared to describe annually a small ellipse, the transverse axis of which amounted to 40". This was an important determination ; for the ellipse afforded the means of computing at all times the aberration of any star whatever, whether in longitude, latitude, declination, or right ascension. Bradley also pointed out the physical cause of the aberration, and demonstrated that it resulted from the combination of the motion of light with the annual motion of the earth. This very remark
able and important discovery was made in 1728.Bradley, anxious to verify his ingenious theory, continued his observations, and soon felt the difficulty that had so much embarrassed Picard. The places of the stars, calcu lated according to his formula for the aberration, could not be reconciled with the observations. The errors continued to augment during nine years, after which they went on diminishing during the nine years following. This inequa lity, of which the period, like that of the nodes of the moon, was 18 years, was readily explained by supposing a slight oscillation of the earth s axis, occasioned by the action of the moon on the protuberant parts surrounding the equator of the terrestrial spheroid. After assiduously observing its effects during twenty years, Bradley found that the phenomena could be accurately represented by giving the pole of the equator a retrograde motion about its mean place in an ellipse whose axes are 18" and 16", and completing its revolution in the period of 18 years. This result was communicated to the Royal Society in 1 748. To these two grand discoveries of Bradley, the aberration and nutation, modern astronomy is greatly indebted for its accuracy and precision ; and as Delambre remarks, they assure to their author a distinguished place, after Hipparchus and Kepler, among the astronomers of all ages and all countries. For the biography see Bradley, James.
While England was witnessing the brilliant discoveries of Bradley, France produced a number of excellent astronomers, by whose successful labours every depart ment of the science was signally promoted. Among these Lacaille is distinguished, both by his scientific zeal and the importance of his observations. In 1751 he undertook a voyage to the Cape of Good Hope, the primary objects of which were to determine the sun s parallax, by means of observations of the parallaxes of Mars and Venus, while similar observations were made in Europe, and to form a catalogue of the southern circumpolar stars. No undertaking for the benefit of science was ever more successfully executed. In the course of a single year, Lacaille, without assistance, observed upwards of 10,000 stars, situated between the tropic of Capricorn and the pole, and computed the places of 1942 of them. The details of his observations were published in the Coclum Australe Stclliferum, which appeared in 17G3. During the same time he measured a degree of the meridian, and made numerous observations of the moon simultaneous with those of Lalande (who observed at Berlin), in order to determine the moon s parallax by means of direct observa tions made at the extremities of a meridional arc of upwards of 85. Astronomy is likewise indebted to Lacaille for a table of refractions, which he computed from a comparison of above 300 observations made at the Cape and at Paris. In 1757 he published his Astronomic? Fundamenta, in which he gave rules and tables for com puting the apparent motions of the stars, which continued to be employed till Lambert supplied the corrections de pending on the nutation, and Delambre those depending on the aberration.
The question of the figure of the earth furnished ample materials for the practical as well as the speculative astro nomer during the 18th century. The results of the measure ment of the meridian by Cassini were at variance with the theories of Newton and Huyghens : and the Academy of Sciences resolved on making a decisive experiment by the actual measurement of the lengths of two degrees, one at the equator, and another in as high a latitude as could be reached. In the year ] 735, three astronomers Godin, Bouguer, and La Condamine were commissioned by the French Government to accomplish the first of these objects in Peru ; and the year following, Maupertuis, Clairaut, Camus, and Lemonnier went, to Lapland to execute the second under the polar circle. Notwithstanding the greater difficulties they had to contend with, the first party were the more successful ; but the result of both operations established the compression of the earth at the poles. Bouguer pxiblishcd the details of the Peruvian measurement in an admirable work On the Figure of the Earth, in which he has also inserted an account of a great number of experiments made by him in the same country to determine the length of the seconds pendulum, and the effects of thn attraction of mountains on the plumb-line. Bouguer is likewise the author of an excellent treatise on light. The details of the labours of numerous other observers in the various countries of Europe will be found under their respective names. Here it is sufficient to indicate briefly the share each had in the development of the science.
Delisle formed a school of astronomy in Russia, and left a method of computing the heliocentric places of the sun s spots and of Mercury and Venus in their transits over the sun s disk, and likewise of determining, by means of the stereographic projection, the directions of their paths Then they enter and leave the disk. Wargentin, secretary of the Academy of Sciences of Stockholm, devoted himself specially to the correction of the tables of the satellites of Jupiter. The theory of the satellites was not then far advanced ; but when theory failed him, he profited by the remarks of others and by his own reflections, and endeavoured by repeated trials to find empirical equations capable of reconciling the tables with the best observations. By confining himself almost exclusively to this subject he acquired a high reputation, and was ranked among the first astronomers of an epoch which abounds in great names. His tables of the satellites have, on account of their superior accuracy, been employed in determining the masses and other elements, which serve as the basis of the analytical theories.
Bradley, was appointed, in 1761, to observe the transit of Venus at the island of St Helena, and endeavour to verify the existence of a small parallax of the star Sirius, which seemed to be indicated by the observations of Lacaille at the Cape. Unfortunately, the state of the weather prevented him from observing the transit ; and his observations on Sirius were abandoned in consequence of the discovery of a defect in the zenith sector which he had carried out with him for the purpose of making the observations in question. The main objects of his voyage were thus frustrated ; but some indirect advantages, notably the improvement of the sector by Ramsden, resulted from it, which compensated in some measure for the disappoint ment. At St Helena he made several interesting observa tions of the tides, the variation of the compass, the moon s horary parallaxes, tc. In going out and returning home he paid particular attention to the different methods of finding the longitude at sea, and practised that which depends on observations of the lunar distances from known stars, taken with Hadley s sextant, or some other reflecting instrument. In the year 1765 he was appointed astro nomer royal, and soon after recommended to the Board of
Longitude the general adoption in the navy of the lunar
