I02 CARNEGIE INSTITUTION 



Close pair, companion of £1718. 

 8.5 and n.o, Est. i // . 

 A close pair. 



While we were at the Lowell Observatory the wind during the 

 day was usually light and always from the southwest. Late in the 

 afternoon it would die down and remain quiet for some hours. This 

 is said to be the normal summer condition. Mr. Lowell states that 

 at Flagstaff the best seeing of the day is usually at this turn of the 

 tide — in the afternoon when the wind is dying down and at the cor- 

 responding period in the morning before it rises. This accords with 

 the experience of Mr. A. E. Douglass, who has very kindly placed at 

 my disposal the following statement concerning the seeing at Flag- 

 staff from his forthcoming article on the ' ' Selection of observatory 

 sites ' ' : 



"The seeing at Flagstaff, Arizona, is dependent on the general 

 and local topographic features and the prevailing winds. The town 

 is situated on the southwest margin of the great Rocky Mountain 

 plateau. On account of the great size, elevation, and barrenness of 

 that plateau, insolation and radiation are stronger than on the other 

 desert regions touching it on the southwest. There is, therefore, 

 an indraft toward this plateau from the southwest in the daytime, 

 stronger in the afternoon owing to the insolation, and an overflow 

 from the desert at night as the cold air settles down upon it, owing 

 to radiation, reaching a maximum in the late night. The normal 

 winds at Flagstaff, therefore, are southwest in the day and north- 

 east at night. In summer, when the days are longer, the day con- 

 dition prevails, and the prevailing clear-weather wind is southwest 

 (changed to southeast when bringing the summer rains) ; in winter, 

 with its length of night, the night condition dominates, and the 

 prevailing clear- weather wind is northeast (changed to southwest 

 in a storm area). It is the sunset and sunrise lulls between these 

 day and night winds that give us the periods of best seeing. 



" This explanation of the good seeing at sunset and sunrise has 

 been verified in many ways. During the observations of Eros at 

 the Lowell Observatory, when it was necessary to observe at least 

 twice during the night, once early and once late, the late seeing on 

 clear nights was always worse than the early, and it was usually 

 accompanied by a current of air from the northeast. This is the 

 overflow from the desert, as already explained. This overflow, 

 coming on rather suddenly late in the night, is also the explanation 

 of the sudden rise in temperature in the valley of Flagstaff in the 

 summer, at three or four o'clock in the morning, frequently found 

 on the thermograph records. Previous to this hour the quiet air 

 in the valley of Flagstaff has been growing steadily colder ; but 

 when the overflow occurs in the form of a fresh northeast wind it 



