1 8 T^E EUR CO UNTR Y. ing that the Greenwich astronomer should have been chosen for the mission we are about to describe, which involved results so interest- ing for selenographic science. We know that during a total eclipse of the sun the moon is surrounded by a luminous corona. But what is the origin of this corona ? Ig it a real substance 1 or is it only an effect of the diffrac- tion of the sun's rays near the moon 1 This is a question which science has hitherto been unable to answer. As early as 1706 this luminous halo was scientifically described. The corona was minutely examined during the total eclipse of 1715 by Lonville and Halley, by Maraldi in 1724, by Antonio de' Ulloa in 1778, and by Bonditch and Ferrer in 1806 ; but their theories were so contradictory that no definite conclusion could be arrived at. During the total eclipse of 1842, learned men of all nations — Airy, Arago, Keytal, Langier, Mauvais, Otto, Struve, Petit, Baily, &c. — endeavoured to solve the mystery of the origin of the phenomenon ; but in spite of all their efforts, " the disagreement," says Arago, ** of the observations taken in different places by skilful astronomers of one and the same eclipse, have involved the question in fresh obscurity, so that it is now impossible to come to any certain conclusion as to the cause of the phenomenon." Since this was written, other total eclipses have been studied with no better results. Yet the solution of the question is of such vast importance to selenographic science that no price would be too great to pay for it. A fresh opportunity was now about to occur to study the much-discussed corona. A total eclipse of the sun — total, at least, for the extreme north of America, for Spain and North Africa — was to take place on July 18th, 1860. It was arranged between the astronomers of different countries that simultaneous observations should be taken at the various points of the zone where the eclipse would be total. Thomas Black was chosen for the expedition to North America, and was now much in the same situation as the English astronomers who were transported to Norway and Sweden on the occasion of the eclipse of 1851. It will readily be imagined that Thomas Black seized with avidity the opportunity offered him of studying this luminous halo. He was also to examine into the nature of the red prominences which appear on different parts of the edge of the terrestrial satellite when the totality of the eclipse has commenced ; and
