REPORT OF COMMITTEE ON OBSERVATORIES 103 



carries the settled cold air out of the valley and raises the tempera- 

 ture. 



' ' The southwest margin of this desert plateau is a ridge called in 

 part the Mogollon mesa. It follows the line of a great fault through 

 Arizona from northwest to southeast. This ridge acts like a dam to 

 the rising warm air by day and the down flowing cold air at night. 

 Flagstaff is at one of the low points on this dam, and therefore gets 

 a particularly pronounced effect from this interchange of air between 

 the high and low desert regions. Its winds are probably stronger 

 than those anyw^here else along the ridge and they begin earlier and 

 last later. 



' ' Now as the cause of unsteady seeing is the multitude of irregu- 

 arities of density in the air, produced by contact with heated or 

 chilled surfaces, and as, also, these irregularities are brought over the 

 telescope by aid of the wind, we may generalize and say that the 

 badness of seeing increases with wind and the opportunity that wind 

 has of contact with heated or chilled surfaces. We have seen that 

 Flagstaff" is particularly subject to strong winds as compared with 

 other parts of Arizona ; we have now to study the opportunity that 

 wind has of contact with heated or chilled surfaces. It is perfectly 

 evident that hills and mountains, reaching up into the great mass of 

 moving air, present the best opportunity for such contact. On ex- 

 amination we find that the entire horizon of Flagstaff from north- 

 northwest to east-northeast is occupied by a mountain range, rising 

 from 2,000 to more than 5,000 feet above the town, and distant 

 from it between 4 and 1 2 miles. The winds that pass these peaks 

 are not only diverted in a vertical direction, which causes change in 

 their densit}-, but also, by contact with their warmed or chilled sur- 

 faces, suffer small irregularities in density throughout their mass. 

 Hence, whenever the wind blows from a northerly direction the see- 

 ing becomes poor in proportion to the velocity of the wind and its 

 directness of passage over the mountains. Therefore in winter, when 

 the northeast wind predominates, the seeing is generally poor. 



' ' This explanation of the bad seeing in winter has been frequently 

 verified by testing on the same night the atmosphere at Flagstaff 

 and at points as little as 12 miles east, out of the lee of the moun- 

 tains. Even when the seeing at Flagstaff was frightfully bad, Eros 

 showing a diameter of 13 seconds, the seeing awaj' from the moun- 

 tains was perfectly fine and steady. 



' ' Thus the times of best seeing at Flagstaff are at sunset and sun- 

 rise in the non-winter months, and the poor seeing at other times 

 results from the topographic features and the prevailing winds. 



' ' It should be remembered that this criticism of Flagstaff seeing 

 is derived from a minute comparison of it with other parts of the 

 southwest arid regions. Compared with eastern localities I regard 

 its qualities as very superior." 



I took with me to Flagstaff one of the Draper thermographs be- 

 longing to the equipment of this expedition, and during the week 



