enter; at the instant he suspects the contact to take place, he must note the time, and proceed to observe, in order to be certain that he was not mistaken. If he find that he was mistaken, he must continue to wait for it, always noting the time when he suspects it, in order that he may not miss it when it really happens; having entered on the sun's disc, wait for the internal contact and note the time; the same method is to be pursued for the internal and external contact when Mercury passes off the disc. At the transit of Venus, in 1761, some singular phenomena were observed. At Madras, a kind of penumbra was noticed, preceding the first external contact two or three seconds of time; this dusky shade was so remarkable, that the observer was assured that the contact was near, which happened accordingly. In the transit of 1769, Dr. Maskelyne was very attentive to observe if this circumstance took place, but he could perceive no such effect. When Venus was a little more than half immersed into the sun's disc, he saw its whole circumference completed, by means of a vivid, but narrow, ill-defined border of light, which illuminated that part of its circumference that was off the sun; but this disappeared about two or three minutes before the internal contact. In the same transit, an observer had warning of the approach of Venus to the contact, by the sudden appearance of a violent corruscation, ebullition, or agitation, of the upper edge of the sun, five or six minutes before the limb of Venus broke in upon the sun; this, it was supposed, might have been owing to the atmosphere of Venus. Some perceived at the first external contact a kind of watery pointed shadow, appearing to give a tremulous motion to that part of the sun's limb. Some astronomers observed a luminous crescent at the times of the ingress and egress which enlightened that part of Venus's circumference, which was off the sun, so that the whole circumference was visible. At the internal contact the limb of Venus seemed, to most observers, to be united to the sun's limb by a black protuberance, or ligament, which was not broken by the thread of light till some seconds after the regular circumference of Venus seemed to have coincided with the sun's. Others observed that the thread of light between the limbs did not break instantaneously, the points of the threads darting into each other, and parting again, in a quivering manner, several times before they finally adhered. The last transit of Mercury occurred 4th November, 1822, but invisible in this country: it was witnessed in India, and an account transmitted to Dr. Olinthus Gregory, by Major J. A. Hodgson, Revenue SurveyorGeneral of India; the communication was inserted in the Memoirs of the Royal Astronomical Society of London, and is as follows: Russell Street, Chouringhy, Calcutta, 4th November, 1822. The sun was frequently obscured by flying clouds at the commencement of the transit, that I thought it safest to use a large field of view until I saw the planet enter the sun. A cloud prevented my seeing the first external contact; I observed the first internal contact, but I cannot be certain to less than four or five seconds, as it was rather cloudy, and a power of 45 in the 42-inch telescope is not great. At the end of the transit I used the 60-inch telescope, but reduced the aperture of 3.7 to 2.5, as the sun's limb is better defined with a small than a large aperture. The magnifying power applied was 60. It would have been more desirable to use a greater magnifying power; but as clouds were flying about occasionally, and partially obscuring the sun, I was afraid of the inconveniences attending a small field. The observations of the preceding and following limbs of the planet, when leaving the sun, were made under favorable circumstances, and I am well satisfied with them, particularly with that of the following limb. No colored glass was used, the light of the sun was so faint that the eye could bear it; its limb appeared white and sharp, and the planet was dark, round, and well defined. I could not see it with the telescopes of the sextant, or reflecting circles, nor with a small Brewster's double object glass micrometrical telescope, though it showed the limbs of the sun perfectly well defined. The most important observations are these:— h. m. S. Internal contact.-Ingress or following limb .. 18 Ditto. At egress External ditto. At egress .... Mean time:-Estimated longitude Captain Herbert did not see the beginning of the transit. He was at the same place and observed the Internal contact at egress. External ditto. h. m. S. using the 42 inch telescope with a power of 80, and no colored shade to the eye piece. He also took several micrometrical measurements with a Dolland's parallel wire micrometer applied to the same telescope. The astronomical clock was by Earnshaw, No. 14. The transit was observed at the cantonment of Kurnaul, latitude north 29° 41′ 25′′, estimated longitude 5h. 8m. 1s. by Walter Ewer, Esq.; telescope by Dolland, 5 feet focal length, power about 100; dark glasses used in the eye piece. The young astronomer will avail himself of the useful hints suggested by this communication in the management of his instruments in viewing the present transit. A very pleasing method of observing the phenomenon, will be by transmitting the sun's image through a telescope into a darkened room; the image of the sun can be received on paper, and the whole transit observed without distressing the sight. The transits of Mercury, for many centuries to come, can take place only in the months of May and November. The first observation of this kind was made by Gassendi, in November, 1631, since which period they have been frequent. The following is a list of those that will happen till the end of the present century; those marked with an asterisk will be visible in this country. * 1835, November 7th. 1845, May 8th. * 1848, November 9th. A transit of Venus is a phenomenon of very rare occurrence, as will appear by the following list, which contains all those transits which have occurred since that which took place in December, 1639, inclusive, (the first that was ever known to have been seen by human being,) to the end of the 21st century. 1639, December 4th. 1761, June 5th. 1769, June 3rd. 1874, December 8th. *1882, December 6th. 2012, June 5th. The transits of Mercury and Venus can happen only when they are in their nodes, and when the earth is in the same longitude; for many centuries to come the transits of Venus can take place only in the months of June and December. No such useful results can be expected from the transits of Mercury as from those of Venus, namely, the ascertaining or confirming of the distance of the earth from the sun: owing to the greater distance of Mercury from the earth than Venus, the difference of their parallaxes is so little, as always to be less than the solar parallax that is sought, and therefore Mercury is considered ineligible for solving that interesting problem which, in 1769, was so satisfactorily derived from the transit of Venus. Mercury in aphelion on the 16th of this month. Stationary near 38 in Aries on the 17th. In conjunction with Venus on the 20th. Venus at her greatest south latitude on the 1st. In conjunction with o in Pisces on the 7th, difference of latitude 4'; with σ in Aries on the 21st, difference of latitude l'. Phases of Venus. The following are the proportions of the light and dark phases of Venus: May 1st.-Illuminated disc Dark part 11.0224 = 0.9776 |