Page:Philosophical Transactions - Volume 054.pdf/210

 The clouds greatly obstructed Mr. Maskeline's observations; he could make but one that was useful. At 7$h$ 31′ 07″ in the morning, apparent time, the bodies both of the Sun and of Venus being perfectly well defined, the distance of the nearest limbs was, by means of an object-glass Micrometer adapted to a reflecting telescope according to Mr. Dollond's invention, found to be 1′ 44″ $3⁄4$. I have computed that by allowing 10″ for the horizontal parallax of the Sun, from that phasis to the internal contact of the limbs, there must have passed 34′ 52″ at St. Helena; I have even assured myself, by some other calculations, that, by an alteration of one or two seconds in the parallax, and of several minutes of time in the longitude of St. Helena, the interval which I have just now determined would suffer an increase or diminution but of very few seconds. The internal contact of the limbs must therefore have happened at St. Helena at 8$h$ 05′ 59″ in the morning. One can't suppose it to have happened later, because 17′ after, or at about 8$h$23′, the skies having cleared again, nothing more could be seen, and the external contact was over. This observation being compared with that of Tobolsk, would give 11″ for the horizontal parallax, which is a little too much. Mr. Maskeline observes, that tho' Venus's limb and the Sun's appeared as defined as could be desired, yet when the artificial internal contact of Venus's limb with the Sun's was made by means of the object-glass micrometer, Venus's limb dilated and contracted itself alternately, getting and losing a small space within the Sun's limb. He adds, that he endeavoured to take it in the middle of this vibration, but dares not affirm that he