TRANSIT OF VENUS. 337 



planet, and he called upon his assistant to mark the Sun's attitude, but that indi^ 

 vidual had deserted his post, and it was only within a few minutes before its go- 

 ing down, and in spite of the unfortunate circumstances, that an important addi- 

 tion was made to our knowledge of the motions of the innermost planet of the 

 solar system. Although successful in observing Mercury Gassendi was deprived 

 of the pleasure of seeing the transit of Venus, as it took place in the night. 

 Transits of Venus can occur only in June and December, and as the two transits 

 of a pair always happen in the same month, if we start from a June transit the 

 intervals between consecutive transits will be eight years, one hundred and five 

 and a half years ; eight years, one hundred and twenty-one and a half years ; eight 

 years, one hundred and five and a half years, and so on. This is the order which 

 exists now. The path of Venus across the Sun is not the same in the two transits 

 of a pair. For a pair of June transits, the path at the second one is sensibly par- 

 allel to, and about twenty minutes north of, that at the first, while for a pair of 

 December transits the parallelism still holds, but the path of the second one is 

 about twenty-five minutes south of that of the first. The paper went on relating 

 all the efforts made by Lansberg, the Belgian astronomer, and others, and the 

 fact that about the most valuable information on the subject had been discovered 

 by Horrocks, a young curate of twenty, who resided near Liverpool in 1631, 

 The years sped rapidly by, and as the transit of 1761 approached, Halley's paper 

 of 17 16 was not forgotten, but his plans were taken up and discussed. Delisle 

 was the first to point out the exact conditions of the transit and the circumstances 

 upon which the success of the observations would depend. The Sun causes 

 Venus to cast a shadow which has the form of a gigantic cone, its apex re>ting 

 upon the planet and its diameter continually increasing as it recedes into space. 

 All the phenomena of transits are produced by the passage of this shadow cone 

 over the eartli, and as each point of the cone corresponds to a particular phase of 

 a transit, any given phase of a transit will encounter the earth and first become 

 visible at some point where the Sun is just setting, and will leave the earth and 

 therefore be last visible at some point where the Sun is just rising. Between, 

 these two points it will traverse nearly half the earth's circumference and in so 

 doing will consume about twenty minutes. The transit of 1761 was visible 

 throughout Europe and was well observed by astronomers. England sent expe- 

 ditions to St. Helena and to the Cape of Good Hope; and English astronomers 

 observed at Madras and Calcutta. French astronomers went to Tobolsk, Rodrig- 

 uez and Pondicherry, and Russians to Tartary and China, and Swedes to Lap- 

 land. No less than 117 stations were occupied by 176 observers, and of these 

 137 published their observations. The various experiences of several astrono- 

 mers were then fully discussed and the transit of 1882 was referred to. Astrono- 

 mers met in Paris last year from Europe, and after discussion the system of photog- 

 raphy, which had been suggested, was condemned as being useless. The United 

 States were not represented at this conference and therefore the experience from 

 America was not obtained. He then referred to the efforts which were being 

 made by the United States Transit of Venus Commission in this regard, and 



