Churning the Air in a Telescopic Tube.
Astronomers suffer much from the inaccuracy of the images viewed in their telescopes in consequence of the disturbances in the atmosphere, common even in clear weather. Hence observatories have, of late years, been established at Arequipa, Peru, and at other stations where the atmosphere is calm and little disturbed by currents. On investigation Professor S. P. Langley, of Washington, discovered that a good deal of the perturbation of telescopic images arises from currents within the telescopic tube itself. As a remedy he adopted the heroic, yet simple, measure of thoroughly stirring up the air in the tube by a blower or other suitable means. Its air, thus brought to uniformity of condition, yielded images much clearer than those usually obtained. Especially convincing in this regard are capital photographs of artificial double stars whose beams were entirely confined within a horizontal tube in which they traveled to and fro through no less than 140 feet of churned air. These pictures showed that the disturbance within the tube itself appeared to be wholly eliminated by the device of vigorously stirring the air column.
This recalls a method of shipping pianos in refrigerator cars. The instruments are carefully brought to the temperature of the car, which is maintained at about zero, Centigrade. When the pianos arrive at their destination they are slowly warmed to the temperature of common air. No matter how long they have been cold, they suffer no hurt; for it is not cold, or moderate elevation of temperature, that does harm so much as uneasy fluctuations from one to the other.