ASTRONOMICAL OBSEKVATOIUES. 387 



Tlie lar^e dome, built by the Union Iron Works, of San Francisco, is 

 75 feet in diameter and its moving- parts weigh 100 tons. It revolves 

 upon wheels wiiich run on hardened steel balls. A man merely pushing 

 against it can move the entire dome. The usual motive power is ob- 

 tained from a water engine which can rotate the dome 3(50 degrees in 

 less than nine minutes. Its diameter chang-es one-half inch in the e^c- 

 treme change of temperatur.', and its track is given a smooth and oi^ed 

 surface to slide upon. The observing slit is 9i feet wide, and the pintle 

 of the shutter is placed eccentrically. A practical device for carrying 

 the observer to the eye piece of the telescope, which at times is far above 

 the base of the dome, was planned by Sir JIoward Grubb, of Dublin. 

 This is an elevating floor (JL^ feet in diameter, weighing 50,000 pounds, 

 and is moved uj) and down'^through a space of 10 feet. It is highly 

 probable that the present systemWili not be sufticiently powerful to 

 raise the t^ooring rapidly enough, but iii this event the hydraulic system 

 can be altered, or steatn or electricity substituted. Tlie actual speed 

 required can oidy be determined after a series of experiments have 

 been made. 



The dome for the IL'-inch equatorial is 25 feet wide, weighs 8 tons, 

 and its observing- slit, wiiich extends beyond the zenith, is 3 feet wide. 

 The meridian-circle house is 43 by 38 feet. Its walls are double, the 

 outer frame of galvanized iron, the inyer one of California redwood. 

 Between these is an air space 21 inches wide, which encircles the build- 

 ings. There is also an air space above the ceiling, which communicates 

 with the room and with the air spaces of the walls, and on the west 

 there is a ventilating tower two stories in height, which connects with 

 the room ot the meridian-transit instrument. By these means thetem- 

 })erature of the building is kept the same as that of the extei nal air. The 

 transit house, adjoining the meridian-circle house, is l)uilt of iron, with a 

 wooden lining, and is arched by a curved shutter, which is controlled 

 by levers, planned by Sir Howard Grurb, The photographic observ- 

 atory, north of the transit house, is asnniU wooden building, with brick 

 foundation. The tube of the photoheliograi)li telescope enters tUis 

 house, and a brick pier supi^orts the photoheliograph. A room in the 

 second tioor of the main bnikling- is also titted for photography. 



The large telescope, which embodies the expresse<l object of the 

 $700,000 donatmu, was mounted by Warner & Swasey, of Cleveland. 

 The tube is nearly cylindrical, and is (JO feet in length. There are three 

 liuders, G, 4, and 3 inches in aperture, and in addition to these the 12- 

 inch equatorial can be quickly attached, as a pointer, for photograi)hic 

 work if the controlled driving- clock does not work satisfactorily. The 

 leusis36 inches clear aperture, being the largest object glassin tlie world, 

 and has a G78-inch focus. The flint disk was obtained from FEiL,in April, 

 1882, and after nineteen failures the crown-glass disk was cast snccess- 

 fully in September, 1885. The third photographic crown lens was pur- 

 chased from Feil in 1.S8G, and broke while in the hands of the Clarks. 

 The trustees of Yale University then otiered their 27-inch flint glass to 

 the Lick 01)Servatory, but this was too yellow, and in 1887 Mr. Alvan 

 G. Clark bought in Paris, from Feil, the crown glass which is worked 

 into a third lens. In addition to its nmgnifying power and its perfect 

 definition, i. e., neatness, accuracy, etc., this'telescope has great light- 

 gathering- ])ower, and stars may be seen through it which are thirty 

 thousand times fainter than the faintest seen by the naked e,ye, and the 

 moon will appear under the same conditions as if it were seen by the 

 naked eye at about 200 miles from the earth. 



The 12 inch refractoi-, which was originally made for Dr. Henry 



\ 



