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THE PROGRESS OF SCIENCE.

paper at the Washington meeting of the Astronomical and Astrophysical Society of America, in which he said that if there were no atmosphere, there would be no twilight, and the brightness of midday would be succeeded, the moment after sunset, by the darkness of midnight. Twilight may be said to last until the last bit of illuminated sky disappears from the western horizon. In general it has been found that this occurs when the sun has sunk about 18° below the horizon. The duration of time which the sim takes in reaching this position is very different at different latitudes. At the North Pole one would have about six months of daylight, followed by nearly two months of decreasing twilight, followed in turn by more than two months of night. In summer, at latitudes greater than 50°, twilight lasts from sunset to sunrise. There is no night there during this season. In the temperate zones the duration of twilight ranges from an hour and a half to more than two hours. Within the tropics the sun descends nearly or quite vertically; but even here the time required for the sun to reach a point 18° below the horizon is more than an hour. There seems to be no reason, therefore, in the general theory, for the widespread belief that the duration of the tropical twilight is extremely brief. This idea is found not only in current popular literature, but also in some of the best text-books on general astronomy. Young's 'General Astronomy,' says: "At Quito and Lima it (the twilight) is said to last not more than twenty minutes." 'The Heavens Above,' by Gilbert and Rolfe, remarks: "Within the tropics, where the air is pure and dry, twilight sometimes lasts only fifteen minutes." Since Arequipa, Peru, lies within the tropics and has an elevation of 8,000 feet, and the air is especially pure and dry, the conditions appear to be exceptionally favorable for an extremely short twilight. On Sunday, June 25, 1899, the following observations were made at the Harvard Astronomical Station, which is situated there: The sun disappeared at 5:30 p.m., local mean time. At 6:00 p.m., 30 m. after sunset, I could read ordinary print with perfect ease. At G:30 P.M. I could see the time readily by an ordinary watch. At 6:40 p.m., 70 m. after sunset, the illuminated western sky was still bright enough to cast a faint shadow of an opaque body on a white surface. At 6:50 p.m. the illumination was faint, and at 6:55 p.m., 1 h. and 25 m. after sunset, it had disappeared. On August 27, 1899, the following observations were made at Vincocaya. The latitude of this place is about 16° south, and the altitude 14,360 feet. Here it was possible to read coarse print 47 m. after sunset, and twilight could be seen for an hour and twelve minutes after the sun's disappearance. It appears, therefore, that while the tropical twilight is somewhat shorter than occurs elsewhere, and is still further lessened by favorable conditions, such as great altitude and a specially pure air, it is never less, and generally much longer, than an hour.

A GENUS OF GREAT ANTIQUITY.

The number of genera which came into existence in early geological times and have persisted until the present day is very small, and a study of the recent representatives of such genera is always of interest. The gasteropod genus, Pleurotomaria, made its appearance during the lower Cambrian period and is represented in the seas of to-day by at least four species, P. Quoyana, P. Adansoniana, P. Beyrichii and P. Rumphii. These species were founded on the characters of the shells alone, and the animal remained unknown until 1871, when Professor Louis Agassiz dredged a specimen of P. Quoyana off the Barbadoes in about 100 fathoms. Additional specimens of both Quoyana and Adonsoniana were obtained by the 'Blake' under the direction of