ASTRONOMICAL OBSERVATORIES. 229 



wires and one horizontal wire. There is also another 

 diaphragm, movable by a micrometer screw, and fur- 

 nished with five horizontal and equidistant wires, by 

 which the distance of any star from the fixed horizontal 

 wire is measured as it passes through the field. The 

 magnifying power ordinarily employed is 125. The cost 

 of the mural circle was $3,550. 



The meridian circle was made by Ertel and Son, of 

 Munich. Its object-glass has an aperture of four and a 

 half inches, with a focal distance of 58 inches. This 

 instrument is provided with a circle 30 inches in 

 diameter, divided into arcs of three minutes, and reads 

 by four, microscopes to single seconds. The clock in the 

 east wing has a mercurial pendulum, and was made by 

 Charles Frodsham. 



The transit in the prime vertical was made by Pistor 

 and Martins, of Berlin. The object-glass of the telescope 

 has a clear aperture of five inches, with a focal length 

 of 78 inches. The eye-tube carries a system of 2 hori- 

 zontal and 15 vertical stationary wires, with one movable 

 vertical wire. This instrument is mounted at one end 

 of its axis and outside of its supports. It is reversed 

 from one side to the other twice during every observa- 

 tion; and though it weighs upwards of 1000 pounds, 

 so perfect is its system of counterpoises and the reversing 

 apparatus, that a child can lift it from its supports, reverse 

 and replace it in them, in less than one minute. The 

 cost of this instrument was $1750. The clock in this 



