492 



THE POPULAR SCIENCE MONTHLY 



book opens with an account of the transits 

 of the seventeenth century, when, as a con- 

 sequence of the establishment of the Coper- 

 nican theory of the solar system, astron- 

 omers 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 inter- 

 est 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 shut- 

 ter the solar light was admitted into a dark- 

 ened 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 dur- 

 ing the progress of the transit. For this he 

 needed an assistant, whom he placed, with 

 a large quadrant, in a room above him, in- 

 structing 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, i\\& 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) fol- 

 lowed 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 ad- 

 mirable 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, to- 

 gether with the methods suggested, during 

 the interval since the last transit, for utiliz- 

 ing 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. 



The Unseen Universe ; or, Physical Spec- 

 ulations ON A Future State. 212 

 pages. New York : Macmillan & Co. 

 1875. Price, $1. In paper, 60 cents. 



We briefly announced this work in the 

 June Monthly. 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 emi- 

 nent 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 Col- 

 lege, Manchester, author of various works 

 on physics, among which is the little vol- 

 ume on the " Conservation of Energy," 

 published in the " International Scientific 

 Series." These are strong men ; the sub- 

 ject is one of profound interest, and is 

 certainly handled in an original way, and 

 the volume, besides, is cheap, and in excel- 

 lent type. The best analysis that we have 



