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irresistible evidence, as a universal law of the whole material creation.

Thus, in this and the preceding chapter, I have given a brief sketch of the history of the verification and extension of Newton's great discovery. By the mass of labour and of skill which this head of our subject includes, we may judge of the magnitude of the advance in our knowledge which that discovery made. A wonderful amount of talent and industry have been requisite for this purpose; but with these, external means have co-operated. Wealth, authority, mechanical skill, the division of labour, the power of associations and of governments, have been largely and worthily applied in bringing astronomy to its present high and flourishing condition. We must consider briefly what has thus been done.

CHAPTER VI.

THE INSTRUMENTS AND AIDS OF ASTRONOMY DURING THE NEWTONIAN PERIOD.

Sect. 1.-Instruments.

1. SOME instruments or other were employed at all periods of astronomical observation. But it was only when observation had attained a considerable degree of delicacy, that the exact construction of instruments became an object of serious care. Gradually, as the possibility and the value of increased exactness became manifest, it was seen that everything which could improve the astronomer's instruments was of high importance to him. And hence in some cases a vast increase of size and of expense was introduced; in other cases new combinations, or the result of improvements in other sciences, were brought into play. Extensive knowledge, intense thought, and great ingenuity, were requisite in the astronomical instrument maker. Instead of ranking with artisans, he became a man of science, sharing the honour and dignity of the astronomer himself.

1. Measure of Angles.-Tycho Brahe was the first astronomer who acted upon a due appreciation of the importance of good instruments. The collection

of such at Uranibourg was by far the finest which had ever existed. He endeavoured to give steadiness to the frame, and accuracy to the divisions of his instruments. His mural quadrant was well adapted for this purpose; its radius was five cubits: it is clear, that as we enlarge the instrument we are enabled to measure smaller arcs. On this principle many large gnomons were erected. Cassini's celebrated one in the church of St. Petronius at Bologna, was eighty-three feet (French) high. But this mode of obtaining accuracy was soon abandoned for better methods. Three great improvements were introduced about the same time. The application of the micrometer to the telescope, by Huyghens, Malvasia, and Auzout; the application of the telescope to the astronomical quadrant, by Picard; and the fixation of the centre of its field by a cross of fine wires placed in the focus. We may judge how great was the improvement which these contrivances introduced into the art of observing, by finding that Hevelius refused to adopt them because they would make all the old observations of no value. He had spent a laborious and active life in the exercise of the old methods, and could not bear to think that all the treasures which he had accumulated had lost their worth by the discovery of a new mine of richer ore.

The apparent place of the object in the instrument being so precisely determined by the new methods, the exact division of the arc into degrees

and their subdivisions became a matter of great consequence. A series of artists, principally English, have acquired distinguished places in the lists of scientific fame by their performances in this way; and from that period, particular instruments have possessed historical interest and individual reputation. Graham was one of the first of these artists. He executed a great mural arc for Halley at Greenwich; for Bradley he constructed the sector which detected aberration. He also made the sector which the French academicians carried to Lapland; and probably the goodness of this instrument, compared with the imperfection of those which were sent to Peru, was one main cause of the great difference of duration in the two series of observations. somewhat later', (about 1750,) divided several quadrants for public observatories. His method of dividing was considered so perfect, that the knowledge of it was purchased by the English government, and published in 1767. Ramsden was equally celebrated. The error of one of his best quadrants (that at Padua) is said to be never greater than two seconds. But at a later period, Ramsden constructed mural circles only, holding this to be an instrument far superior to the quadrant. He made one of five feet diameter, in 1788, for M. Piazzi at Palermo; and one of eleven feet for the observatory of Dublin. Troughton, a worthy successor of the artists we have

1 Mont. iv. 337.

Bird,

mentioned, has invented a method of dividing the circle still superior to the former ones; indeed, one which is theoretically perfect, and practically capable of consummate accuracy. In this way, circles have been constructed for Greenwich, Armagh, Cambridge, and many other places; and probably this method, carefully applied, offers to the astronomer as much exactness as his other implements allow him to receive; but the slightest casualty happening to such an instrument, or any doubt whether the method of graduation has been rightly applied, make it unfit for the jealous scrupulosity of modern astro

nomy.

The English artists sought to attain accurate measurements by bisection and other aliquot subdivision of the limb of their circle; but Mayer proposed to obtain this end otherwise, by repeating the measure till the error of the instrument is unimportant, instead of attempting to make an instrument without error. This invention of the repeating circle was zealously adopted by the French, and the relative superiority of the rival methods is still a matter of difference of opinion.

2. Clocks. The improvements in the measures of space require corresponding improvements in the measure of time. The beginning of anything which we can call accuracy, in this subject, was the application of the pendulum to clocks, by Huyghens, in 1656. That the successive oscillations of a pendulum occupy equal times, had been noticed by Galileo;

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