Page:Popular Astronomy - Airy - 1881.djvu/159

Rh. He was an old man at the time, nearly sixty years old. He explained what, as he said, would be a satisfactory method of solution, by observations of the "transit" (or "passage") of Venus over the sun's disc or face; and he pointed out the possibility of seeing the transits of Venus across the sun's disc, which were to occur in 1761 and 1769; and he bequeathed, as a task to posterity, the problem of ascertaining the distance of the sun from the earth. For understanding the following statement, it is important to remark, that the method of finding the distance of the sun by observation of the transit of Venus, requires that observations be made (as in the first method for the moon) at two Observatories at widely different positions on the earth. In 1761 a transit of Venus occurred, which was visible in many parts of Europe; it was necessary to observe it in other parts of the earth, and expeditions were sent out for that purpose; amongst others, Dr. Maskelyne was chosen by the Government to go to St. Helena, (where, however, clouds prevented any part of it being seen), and a Mr. Mason to the Cape of Good Hope. In 1769; another transit of Venus occurred, which was visible in the North of Lapland, but in no other part of Europe; it was necessary, in order to procure a good station for observations to be compared with those in Lapland, to send out an expedition to the Pacific Ocean. Captain Cook was sent out by the British Government to the South Seas, in the year 1769, in order to observe the transit of Venus in the island of Otaheite. I wish to mention with regard to this expedition, that so far as I can understand, the expenses incurred in that part