x l v i INTRODUCTION TO TUB 



is bolted to one of the rafters, serves as a leader for the pulley to elevate the cap. This may 

 be thrown beyond the vertical, if necessary, and a single cord fastened to the strengthening 

 bars of tin across its base, serves to pull it down after the inclined door has been first closed. 

 Although of like materials and construction as the weather-boarding, a canvass cover was accu- 

 rately fitted to the roof for additional security. This was drawn tightly over the planks, and 

 when securely nailed down and painted, the portion obstructing the door-way was cut away. 

 There is a small window opposite the door of entrance. 



The foundation-wall was sufficiently wide to support the ends of the" floor-joists, also. These 

 timbers were framed entirely free of the masonry-pier in the centre, and were of such width 

 that the floor was on a level with the upper surface of the curb. To prevent the building from 

 wabbling, there are three horizontal, equi-distant, and adjustable rollers secured to the joists, 

 the space between them and the curb not exceeding one-fourth of an inch. 



The building was first erected at Washington, and its several portions carefully numbered 

 before taken down for transportation. As it was extremely light, no machinery was required 

 to turn it, slight pressure by the hand being quite sufficient to move it in either direction. 

 Altogether, it may be ranked among the most economical and easily-managed rotary observa- 

 tories ever built. 



THE EQUATORIAL. 



The equatorial telescope was made by Mr. William J. Young, of Philadelphia ; its object- 

 glass, from French materials, by Henry Fitz, jr., of New York; and its micrometer by Mr. 

 William Wurdeman, at Washington. As will be seen in the opposite plate, the construction of 

 its stand is that devised by Frauenhofer, and since so successfully built by Messrs. Merz & Mahler 

 for observatories in many parts of the world. The diameter of its object-glass is 6.4 French 

 inches, with a focal length of 103. *7 inches. Its polar and declination axes are frustums of steel 

 cones, the former twenty-one and a half inches long, with diameters of two and a half and one 

 and three quarters inches at its two extremities, and the latter twenty-two inches long by three 

 and a half and two and a half inches diameter, respectively. 



The hour-circle, with a diameter of nine and a half inches, is divided on a band of inlaid 

 silver into spaces each of four minutes of time, which are read by two opposite verniers to four 

 seconds, and by estimation may be easily subdivided to one second. The declination-circle has 

 a diameter of twelve and a half inches, divided, also on a silver band, into spaces of ten minutes 

 of arc, divisible by two verniers into ten seconds. It is numbered from to 3GO, and reads 

 when the telescope, pointed to the equator, is on the west side of the stand. The finder has an 

 object-glass, 1.75 inch in diameter, with a focal length of eighteen inches. This latter object- 

 glass and the eye-pieces to the micrometer are somewhat imperfect, and there is some little play 

 to the declination axis ; but, except these defects, the instrument is admirably constructed. 



The supporting-stand is of solid and well-seasoned black walnut, dried in a kiln for more 

 effectual security against shrinkage in the dry summer atmosphere of Chile. It rests on four 

 heavy steel screws, by means of which the adjustments for polar elevation and transverse level 

 may be effected. The azimuth is controlled by horizontal screws, working through iron clamps 

 let into and firmly fastened to the pier at the two meridional foot-screws. A driving-clock is 

 fitted to the instrument ; but dust accumulated so rapidly in summer, that its use was never 

 attempted after the first few nights. 



The repeating-micrometer has a divided circle of four inches diameter, which may be read 

 by one vernier to 30". Constructed on the English plan, its counting-scale is within the box, 

 each five teeth of the comb being separated by a deep indentation. It has five permanent 

 transit-wires, and two parallel movable wires perpendicular thereto. One of the latter is pro- 

 vided with a register-head, divided into one hundred equal parts ; the other is only a zero-wire. 

 Its eye-lenses, with magnifying powers from one hundred and fifty to five hundred times, may 

 be moved parallel with the register-wire by means of a rack and pinion and clove-tailed slide. 



