OP THE UNITED STATES. 357 



Peculiarities of the Fire Feet Tvanait Instruments destined fm- 



the Observatories. 



The axes of the transits are generally made of considera- 

 ble length ; but there is probably more lost by this in solidity 

 than can l)e gained by the nicety of the adjustments. 



In each of the transit instruments made by Mr. I'roughton 

 for the two observatories wliich were intended to be built, 

 the axis is thirty-three inches long between the supports. 

 The two truncated cones which form it meet in the middle 

 upon a spherical piece al)out nine inches in diameter, which 

 receives also tlie two parts of the telescope ; exactly in the 

 same manner in all respects as the transit telescope of the 

 two-feet theodolite. 



The bell metal ends of the axis, which are about three 

 quarters of an inch in diameter and one inch and a quarter 

 in length, rest on supports which are screwed to the flat top 

 of tlie stone pillars, the transit not being hung to the inner 

 side of the pillars, as was formerly the custom. They are 

 of the following form : — 



In Plate IX. fig. 8 ^ 9, a, a, a, a is a plate of brass al)out 

 lialf an inch thick and six indies and a half square. Four 

 strong screws b. b, b, b, fastened in the top of the stone by 

 gypsum, receive it, and it is secured by four mother screws 

 pressing it close to the stone. 



In the middle of this plate is elevated at right angles a 

 strong piece c, c, about three inclies high and one inch tliick, 

 in the shajie of a l)ridgc, which slides in a runner, cut in the 

 plate parallel to the telesco|)c. This piece bcim; moved by 

 means of the screw c, e. will adjust the telescope to the me- 

 ridian. In tiie uppermost part of this piece is the rectangular 

 incision, forming the Y's upon which tiie axis revolves. Be- 

 low it, this piece is cut out in the form of a segment of a cir- 

 cle, whicli is subtended l)y a perpendicular sricw in tlic rnid- 



VOL. U. .3 A 



