go SMITHSONIAN MISCELLANEOUS COLLECTIONS [vol. 45 



an inclined flared tube uniting with the main tube at the north end 

 close in front of the concave mirror. At intervals of about five feet, 

 five-inch ducts lead to air-mains 14 inches in diameter, which in 

 turn at length unite in two twenty-inch mains leading to the intake 

 and blast respectively of a twenty-nine-inch fan blower with direct 

 connected 2^ horsepower electric motor. It is so arranged that the 

 openings in the telescope tube communicate with the blast and suc- 

 tion of the blower alternately, so that the air within the tube is re- 

 peatedly carried through the system and churned over and over. 

 Thus the path of the beam from the coelostat to the focus of the mir- 

 rors is thoroughly stirred, but nothing has been done as yet to intro- 

 duce stirring between the ccelostat and the sun. It is possible that 

 an attempt will be made later to stir the path of the beam in the 

 eighty feet immediately above the coelostat, if it is found impossible 

 to get good enough definition with the present arrangements. 



It should be recalled that the conditions required for bolometric 

 work are quite different from those suited to direct eye vision or to 

 photography. Bolometric studies require unchanging transparency 

 of the air, else difference in the galvanometer deflection may be due to 

 alterations in transparency of the intervening medium and not to the 

 properties of the source of light. Thus those times when thin cirrus 

 clouds, fog, or smoke cover the sun, which are well known by solar 

 observers to be the times when " boiling " is apt to be diminished, 

 and which are the most favorable opportunities for visual and pho- 

 tographic observations, are quite unsuitable for bolometric work. 

 Indeed the best time for this is somewhat after noon on those clear 

 October days when " boiling " is apt to be at a maximum, but cloud- 

 iness at a minimum, and it is probable that the definition obtained 

 in such conditions will never be the best. 



Trials made thus far have demonstrated the great value of the 

 stirring apparatus, not only to diminish " boiling," but to preserve 

 a constant focal length and tolerable definition. " Boiling " is 

 still of course noticeable, because the long reach of air above the 

 coelostat is not stirred, but the image is far better than could be ob- 

 tained with the earlier appliances, and owing to the massive piers 

 and to the simplicity of driving mechanism it is less subject to jars 

 and wandering. 



In later communications it is expected to describe bolometric work 

 upon the solar image formed by the great horizontal telescope. 



