accurate Survey of the United States of America. 403 



Troughton ; a clock by Hardy ; and an eighteen-inch repeating 

 circle; there were also to be placed in it, finally, a zenith sector 

 and a meridian (mural) circle, &c. I cannot describe the 

 building in detail, but I will remark that it was to be surrounded 

 by a ditch, in order the better to avoid the oscillations of the 

 ground, by the passage of waggons, &c. The jnllars of the 

 instruments were to be placed upon solid bases six feet thick, 

 standing in a cellar of five feet depth, and to pass through the 

 floor of the observatory, which was to be supported indepen- 

 dent of them. The axis of the transit is thirty-three inches 

 long, which also corresponds to the views of Reichenbach, who 

 considers long axes as not advantageous; the cylindrical parts 

 are of bell-metal, as usual with the English artists. The sup- 

 ports are not between the pillars, but upon them ; a strong 

 metal plate is fixed upon the middle of the pillar, bearing the 

 parts which move the Ys, and these are moved in the direction 

 of ihe meridian by screws, by which the adjustment to that 

 direction is made ; the usual vertical screw is not in the ar- 

 rangement; instead of this, the piece bearing the Ys is formed 

 like an arch, the middle of which is supported by a screw, the 

 higher or lower position of which elevates or depresses it by 

 the different degree of tension of the metal which is produced 

 hy the action of the screw and its own elasticity. This me- 

 thod promises to secure complete stability : but it is supposed 

 that the two pillars have the same altitude, and also that no 

 remarkable change should take place in them. The counter- 

 poising apparatus is placed about five inches from the end, 

 and consists of springs, which press rollers under the axis, pei'- 

 forming what Reichenbach effects by levers and weights. By 

 Mr. Hassler's arrangement, this counterpoising apparatus 

 occupies the place on the pillars, which the supports formerly 

 did; this arrangement, likewise, appears to me good: whether 

 it would be apjilicable to very heavy instruments, remains still 

 to be tried "*'. The two conical axes are not joined by a cube 

 in the centre, but by a zone of a sphere of eight inches diame- 

 ter, to which the two parts of the telescope tube are screwed ; 

 this arrangement is made with a view to greater stability. 



Of the other instruments of Mr. Hassler it will not be pos- 

 sible to give an adequate description without drawings, but 

 I may however indicate some of their peculiarities. The theo- 



* The transit of the observatory at Greenwicli is acljiistcd in this man- 

 ner; and as it is ten feet in length, the doubt whetiicr the plan be ariplicuble 

 to laigc instruments is settled by actual cx[)eriencc. — [The transit instru- 

 ment at Greenwich is not counterpoised.— liuir. Phil. Mag.] 



.3 F 2 dolite 



