The nodes of the Moon's orbit do not invariably occupy the same position, but move nearly uniformly, their position with regard to the Sun, Earth, and Moon being at any time approximately what it formerly was at a series of times separated by equal intervals from each other. Each interval comprises 223 lunations, or 18 years, 11 days, and 7 or 8 hours. The eclipses which occur within this interval are almost exactly repeated during the next similar interval. This period, called the "Saros," was well known to the ancients, who were enabled by its means to predict eclipses with some certainty. [PLATE III.--TOTAL ECLIPSE OF THE SUN. Observed July 29, 1878, at Creston, Wyoming Territory] A total eclipse of the Sun is a most beautiful and imposing phenomenon. At the predicted time the perfectly round disk of the Sun becomes slightly indented at its western limb by the yet invisible Moon. This phenomenon is known as the "first contact." The slight indentation observed gradually increases with the advance of the Moon from west to east, the irregularities of the surface of our satellite being plainly visible on the border of the dark segment advancing on the Sun's disk. With the advance of the Moon on the Sun, the light gradually diminishes on the Earth. Every object puts on a dull and gloomy appearance, as when night is approaching; while the bright sky, losing its light, changes its pure azure for a livid grayish color. Two or three minutes before totality begins, the solar crescent, reduced to minute proportions, gives comparatively so little light that faint traces of the Sun's atmosphere appear on the western side behind the dark body of the Moon, whose limb then becomes visible outside of the Sun. I observed this phenomenon at Creston during the eclipse of 1878. From 15 to 20 seconds before totality, the narrow arc of the Sun's disk not yet obscured by the Moon seems to break and separate towards the extremities of its cusps, which, thus divided, form independent points of light, which are called "_Baily's beads_." A moment after, the whole solar crescent breaks into numerous beads of light, separated by dark intervals, and, suddenly, they all vanish with the last ray of Sunlight, and totality has begun with the "second contact." This phenomenon of Baily's beads is undoubtedly caused by the irregularities of the Moon's border, which, on reaching the solar limb, divide the thin solar crescent into as many beads of light and dark intervals as there are peaks and ravines seen sidewise on that part of the Moon's limb.