THE PROGRESS OF SCIENCE. 



571 



paper at the Washington meeting of 

 the Astronomical and Astropliysical 

 Society of America, in which he said 

 that if there were no atmospliere, there 

 would be no twilight, and the bright- 

 ness 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 differ- 

 ent at different latitudes. At the 

 North Pole one would have about six 

 months of daylight, followed by nearly 

 two months of decreasing twilight, fol- 

 lowed in turn by more than two 

 months of night. In summer, at lati- 

 tudes greater than 50°, twilight lasts 

 from sunset to sunrise. There is no 

 night there during this season. In the 

 temperate zones the duration of twi- 

 light 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 re- 

 quired 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 wide- 

 spread 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 astron- 

 omy. 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 Har- 

 vard 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 G: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 alti- 

 tude 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 dis- 

 appearance. It appears, therefore, that 

 while the tropical twilight is some- 

 what shorter than occurs elsewhere, 

 and is still further lessened by favor- 

 able 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 appear- 

 ance 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 



