Page:Popular Science Monthly Volume 7.djvu/508

492 book opens with an account of the transits of the seventeenth century, when, as a consequence of the establishment of the Copernican theory of the solar system, astronomers perceived that the inferior planets, Mercury and Venus, must from time to time appear to cross the face of the sun. Kepler calculated and announced, in 1627, that in the year 1631 both Mercury and Venus would pass over the sun's face—Mercury on November 7th, and Venus on December 6th; and that in 1761 Venus would again pass across the face of the sun. As the first occasion on which the transit of an inferior planet was ever witnessed, the transit of Mercury in 1631 has an interest resembling that which attaches to the first observation of a transit of Venus, eight years later, the one contemporaneous with Mercury not having been observed, as it took place in the night-time; and, as to this one which took place eight years later, Kepler had calculated that, while in inferior conjunction December 4, 1639, Venus, though near the sun, would pass below its disk, and there would be no transit, which calculation happily was found to be a miscalculation, and therefore a transit would really occur. The first observation of a planet's transit, that of Mercury, was made by Gassendi, of Paris. Through a small aperture in a shutter the solar light was admitted into a darkened room, and an image of the sun, some nine or ten inches in diameter, was formed upon a white screen. A carefully divided circle was traced upon this screen, and the whole was so arranged that the image of the sun could be made to coincide exactly with the circle. As he had no trustworthy clock with which to ascertain exactly the moment of ingress, which he was anxious to do, he determined that the altitude of the sun should be carefully estimated during the progress of the transit. For this he needed an assistant, whom he placed, with a large quadrant, in a room above him, instructing him to observe the height of the sun as soon as he heard Gassendi stamp upon the floor of the room beneath. With these preparations Gassendi began to watch for the transit two days before its appointed time. To make a long story short, by the evening of November 7, 1831, the first transit bad been observed, and in the manner here described. The first observed transit of Venus (to which planet Mr. Proctor gives his whole attention from this point) followed that of Mercury, and was calculated and observed by Horrocks, a young minister of Hoole, in Lancashire, who was a prodigy for his skill in astronomy. He was but twenty years old when he calculated this transit, and died two years afterward. He possessed a telescope, "the recent and admirable invention," which he used in the observation. The transit came on Sunday. He had watched two days, and Sunday until the hour for divine service. Returning from this at fifteen minutes past three in the afternoon, he found Venus just entered on the sun. Sunset cut him short with half an hour, but Venus had been seen in the act of transit. The transits of 1769, together with the methods suggested, during the interval since the last transit, for utilizing them in determining the solar parallax, are next dwelt on. A long and instructive chapter on transits and their conditions is then introduced, after which the subject of the coming transits is taken up. As the book was put to press just before the late transit, it is of course included among the latter.

briefly announced this work in the June It has been for some time anticipated with an earnest interest by some, and a vague curiosity by others, as rumor made it the joint production of two eminent savants; and it was expected that a crushing double shot would be poured into—somebody. It has been since stated that the book is due to Prof. G. P. Tait, the eminent mathematician of Edinburgh, and Prof. Balfour Stewart, of Owens College, Manchester, author of various works on physics, among which is the little volume on the "Conservation of Energy," published in the "International Scientific Series." These are strong men; the subject is one of profound interest, and is certainly handled in an original way, and the volume, besides, is cheap, and in excellent type. The best analysis that we have