387
The large dome, built by the Union Iron Works, of San Francisco , is75 feet in diameter and its moving parts weigh 1U0 tons. It revolvesupon wheels which run on hardened steel balls. A man merely pushingagainst it can move the entire dome. The usual motive power is ob-tained from a water engine which can rotate the dome 360 degrees inless than nine minutes. Its diameter changes one-half inch in the ex-treme change of temperature, and its track is given a smooth and oiledsurface to slide upon. The observing slit is S)| feet wide, and the pintleof the shutter is placed eccentrically. A practical device for carryingthe observer to the eye--piece of the telescope, which at times is far abovethe base of the dome, was planned by Sir Howard Grubb, of Dublin .This is an elevating door 61^ feet in diameter, weighing 50,000 pounds,and is moved up and down through a space of 16 feet. It is highlyprobable that the present system will not be sufficiently powerful toraise the flooring rapidly enough, but in this event the hydraulic systemcan be altered, or steam or electricity substituted. The actual speedrequired can only be determined after a series of experiments havebeen made.
The dome for the 12-inch equatorial is 25 feet wide, weighs 8 tons,and its observing slit, which extends beyond the zenith, is 3 feet wide.The meridian-circle house is 43 by 38 feet. Its walls are double, theouter frame of galvanized iron, the inner one of California redwood.Between these is an air space 24 inches wide, which encircles the build-ings. There is also an air space above the ceiling, which communicateswith the room and with the air spaces of the walls, and on the westthere is a ventilating; tower two stories in height, which connects withihe room ot the meridian-transit instrument. By these means the tem-perature of the building is kept the same as that of the external air. Thetransit house, adjoining the meridian-circle house, is built of iron, with awooden lining, and is arched by a curved shutter, which is controlledby levers, planned by Sir Howard Grubb . The photographic observ-atory, north of the transit house, is a small wooden building, with brickfoundation. The tube of the photoheliograph telescope enters thishouse, and a brick pier supports the photoheliograph. A room in thesecond floor of the main building is also fitted for photography.
The large telescope, which embodies the expressed object of the$700,000 donation, was mounted by Warner & Swasey, of Cleveland .The tube is nearly cylindrical, ami is 60 feet in length. There are threefinders, 6, 4, and 3 inches in aperture, and in addition to these the 12-incli equatorial can be quickly attached, as a pointer, for photographicwork if the controlled driving clock does not work satisfactorily. Thelens is 36 inches clear aperture, being the largest object glassiu the world,and has a 678-inch focus. The flint disk was obtained from Feil, in April,1882, and after nineteen failures the crown-glass disk was cast success-fully in September, 1885. The third photographic crown lens was pur-chased from Feil in 1886, and broke while in the hands of the Clarks.The trustees of Yale University then offered their 27-inch flint glass tothe Lick Observatory, but this was too yellow, and in 1887 Mr. AlvanG. Clark bought in Paris , from Feil, the crown glass which is workedinto a third lens. In addition to its magnifying power and its perfectdefinition, i. e., neatness, accuracy, etc., this telescope has great light-gathering power, and stars may be seen through - it which are thirtythousand times fainter than the faintest seen by the naked eye, and themoon will appear under the same conditions as if it were seen by thenaked eye at about 200 miles from the earth.
Tb© 12-iuch refractor, which was originally made for Dr. Henry