270 HISTORY OF ASTRONOMY. 



illuminated in a dark field, or the reverse, at the option 

 of the observer. The granite piers for this instrument 

 are of the T form, gradually diminishing upward to 

 within 10 inches of the top. They are 30 inches apart, 

 and rise 5 feet above the floor. They rest on a granite 

 foundation, which is based on the solid rock below. 



The comet-seeker, by Merz and Sons, has an aperture 

 of about 4 inches, and a focal length of 32 inches. It is 

 mounted on an equatorial stand, and has 4 eye-pieces, 

 magnifying from 12 to 40 times. 



The sidereal clock is furnished with Mahler's compen- 

 sation pendulum, and runs a month without winding. 

 The library contains about 500 volumes of the most 

 valuable books pertaining to theoretical and practical 

 astronomy. 



The cost of the building, exclusive of the lot and 

 its embellishments, was about $4,500. The cost of the 

 equatorial telescope, together with the sidereal clock, 

 was about $2,300. The. comet-seeker ' cost $180 ; and 

 the meridian circle cost .275 in London. 



MR. VAN ARSDALE'S OBSERVATORY, NEWARK, NEW JERSEY. 



This observatory was built in the spring of 1850. It 

 is a circular wooden building 13 feet in diameter, resting 

 on a brick foundation, underlaid with stone to the 

 depth of two feet. The circular brick foundation is 

 covered with wood-work three inches in thickness, and 

 upon this are placed the wheels upon which the whole 

 building turns. The wheels are eight in number, of cast- 



