214 ASTKONOMY. 



The hypothesis which is suggested as most probable is, that the more 

 recent changes in the system have been principally due to oceanic tidal 

 friction, and that the more ancient changes were produced by bodily 

 tidal friction. The initial state of the Moon, nearly in contact with the 

 Earth, and always opposite the same face of it, suggests that the Moon 

 was i^roduced by the rupture in consequence of vapid rotation or other 

 causes of a primeval i^lanet, wliose mass was made up of the present 

 Moon and Earth. It is a remarkable coincidence, that the shortest pe- 

 riod of revolution of a fluid mass of the same mean destiny as the Earth, 

 which is consistent with an ellipsoidal form of equilibrium, is two hours 

 and twenty-four minutes ; and that if the Moon were to revolve about 

 the Earth with this periodic time, the surfaces of the two bodies would 

 be almost in contact with one another. The theory gives an interesting 

 explanation of the rapid movement of the inner satellite of Mars. 



OBSERVATORIES. 



The buildings of the new observatory of the University of Strasburg 

 have been completed, and a description and plates of them made by the 

 director. Dr. Winnecke. The principal instruments are : 



1. A meridian circle by Eepsold ; aperture 1C2 millimeters (6.4 inches). 



2. An altazimuth by Repsold ; aperture 136 millimeters (5.4 inches). 



3. A refractor by Merz ; aperture 487 millimeters (20.2 inches). 



4. An orbit- sweeper by Eepsold ; aperture 163 millimeters (6.4 in- 

 ches). 



There are three buildings — one large one containing the residences of 

 the astronomers, and two smaller. 



On the main building is the large iron dome, some 39 feet in diameter. 

 There are two novelties in its construction: First, the shutters to the 

 dome are made in two parts, and extend from horizon to horizon. They 

 are opened symmetrically by screws, and leave the whole 180° open at 

 once. Secondly, the dome, which weighs about 93,000 pounds, is turned 

 by means of heavy weights, which .are wound up from time to time as 

 required. A touch from the observer will release these, and their fall 

 will turn the dome right or left as required. The working of this dome 

 will be attentively regarded, as most of the difliculties of such structures 

 appear to have been successfully avoided. In windy situations the 

 whole slit must not be opened at the same time, but the princiijle of 

 these shutters i)roperly modified will serve even in this case. The dome 

 of the new Naval Observatory at Washington is to be of the turret con- 

 struction, by which form the difficulties of shutters can be evaded. 



The 27-inch refractor for the Vienna observatory has been finished 

 by Mr. Grubb, and is now mounted, and is said to be satisfactory. An 

 elaborate description of it is published by Mr. Grubb as a reprint from 

 JEngineering. 



Mr. A. A. Common has mounted his new 36-inch silver-on-glass equa- 

 torial at Ealing, near London. It is described in " The Observatory," 



