404 Prof. Bessel's Remarks on Mr. Hassler's flan for an 



dolite of two feet, not constructed for repetition, appears to me 

 to possess a peculiarly good construction. From a hexagonal 

 centre-piece emanate six horizontal conical arms, whose bases 

 are three inches, and ends one inch and a half in diameter. 

 Upon these arms the two-feet horizontal circle is made fast ; 

 three of these cones are longer ; these contain at their ends the 

 screw-work for the stands by which the instrument rests upon 

 three vertical cones of brass, fastened to the wooden stand of 

 the instrument; between this and the six horizontal conical 

 arms there is room for the verification telescope, which has 

 precisely the arrangement of a transit, and hangs in its Ys, 

 which are fastened underneath to two opposite radii. This 

 telescope has no lateral motion, but the wires in the focus are 

 directed by means of a screw, to the object which is taken as 

 the point of comparison during an observation. In the same 

 hexagonal centre-piece is fastened the vertical axis, eleven 

 inches long, and two inches in diameter. Upon this revolves 

 a drum nine inches in diameter, and five inches and a half high ; 

 upon the upper surface of this, stand two columns bearing the 

 Ys for the transit telescope by which the observations are made ; 

 this is a complete transit, and the columns are sufficiently ele- 

 vated to allow its passage through the zenith. The horizontal 

 angles are measured by the revolution of this upper part of 

 the instrument upon the vertical axis, and are read off by three 

 microscopes, which are fixed at the end of as many conical 

 arms, coming from the central drum, each having a microme- 

 ter screw. The illumination is made through the axis of the 

 telescope, the one side of which is perforated, the other has an 

 altitude circle of six inches diameter. The axis is about twelve 

 inches long, which is more than the interval between the co- 

 lumns. Its supports are therefore set upon pieces of brass, 

 elevated above the columns, and extending outwards; they 

 have the same kind of vertical adjustment as the large transit 

 described above. 



In relation to the observations with this instrument, Mr. 

 Hassler properly remarks : that the eccentricity is equally cor- 

 rected by means of three equidistant readings, as by two, four, 

 or so on ; he also shows that when the vertical axis is not per- 

 pendicular to the plane of the horizontal circle, the errors of 

 the angle will be corrected if the position of the instrument's 

 place is alternately changed to the three truncated cones of 

 the stand, so as to give the three regularly succeeding posi- 

 tions of a full revolution. These three observations, each made 

 in the two diametrically opposite positions of the telescope, and 

 by a half revolution of the instrument, give a mean which is 



free 



