Astronomy. 295 
passages 10 feet wide, crossing each other at right angles. The diam- 
eter of the great pier at the base is 15 feet, and it is solid to a height of 
104 feet, where the diameter is 12 feet. Upon this is erected a conical 
pier of hard burned brick, the diameter at the base being 12 feet, the 
height 28 feet; diameter at top 7 feet, and walls 3 feet thick. The 
pier is capped by New York flagging stone, on which rests the pedestal 
of the equatorial. 
On the east and west sides of this edifice are wings, each 26 feet 
long, 21 feet wide and 18 feet high. There is also a wing to the south, 
2h feet long, with the same breadth and height as the others. In the 
_ west wing is the meridian transit instrument, and the mural circle; in 
the east wing it is intended to place a meridian circle and a portable 
transit instrument, and in the south wing is the transit instrument in the 
prithe vertical. There will also be aclock to each wing. The piers 
are of granite from Maryland. The pedestal for the great telescope is 
one block weighing about 74 tons; that for the prime vertical transit 
weighs 114 tons; and the axis of the mural circle rests upon a block 
whose weight is 3 tons. The dome weighs about 4 tons; rests on six 
_ 32 pound balls, and may be revolved by a power of ten pounds. 
The magnetical observatory is in the form of a cross, the arms being 
70 feet long, 10 feet wide, and 10 feet high, and is entirely under 
ground, the roof being 5 feet beneath the soil. Four marble pillars are 
placed at the extremities of the cross, for the support of the instruments, 
and three others near the center for the reading telescopes and scale— 
the observer occupying a seat in their midst. No iron or steel was 
used in the construction of any part of this building. 
The great refractor was made by Merz and Mahler, of Munich, and 
reached Washington in May, 1844. The object glass is 9;% inches in 
diameter, with a focal length 154 feet, and the time occupied in per- 
fecting it was nearly two years. The finder has an object glass 2,8, 
inches aperture, with a focal length of 32 inches. The hour circle is 
15 inches in diameter, and reads by two verniers to 2*; the declination 
~ circle is 21 inches in diameter and reads by four verniers to 4”. The 
telescope is moved by a clock which may be detached at pleasure. 
The instrument is furnished with a filar-micrometer, having eight eye- 
pieces with magnifying powers from 100 to 1000. - It is so arranged 
that the observer may have bright lines and a dark field, or bright field 
and dark lines; and the quantity of light is controlled at pleasure. 
There are two annular micrometers, and five ordinary eye-pieces, with 
powers from 150 to 750 times. 
The cost of this telescope was $6000; its object glass alone being 
valued at $3,600. 
