150 
DE. T. E. EOBINSON AIM'D ME. T. G-EXJBB’S DESCEIPTION OF 
a small brass cap f\ with a pair of feathers on the inside which slide into two of the 
teeth of the nut.] 
The declination-axis has also its three sets of counterpoises ; but their function is of 
a more complicated character. The weight of itself, with the telescope and counterpoises, 
acts in the vertical passing through their centre of gravity, and may be conceived as the 
resultant of three rectangular components, one parallel to the polar axis, one parallel to 
the declination-axis, and one perpendicular to both. The first of these is constant in 
magnitude and direction, the others vary from 0 to a maximum at an inverse rate. The 
two first tend to make the axis shift in its Y’s transversely and leave its bearings, the 
third to jam it against the cube of the polar axis. 
[The second and third components are relieved by the mechanism shown in Plates VII. 
& VIII. figs. 25, 26, 27, and 28. It consists of two nearly semicircular cast-steel rings of a 
strong section. These surround the declination-axis at the point where its axis intersects 
that of the polar axis, and where it is shaped as in Plate VIII. fig. 26. Between the jaws 
of these rings are bolted the gun -metal blocks c, carrying each three rollers, two of which, 
d, d, roll on the trued rings/', f of the declination-axis, while the third, e, acts at right 
angles to the other two in a groove formed for the purpose in the declination-axis. The 
lower semiring has a steel stud g, which enters into a cavity prepared for it at the bottom 
of the polar axis, and forms the fulcrum of the lever. The upper ring is prolonged into 
a steel bar stiffened with a cast-iron jacket. This bar passes up through the polar axis, 
and projects at the upper end, where it is acted upon by a small sector and lever which 
produce a sufficient upward thrust (see Plate VI. fig. 19 e). By this means any desirable 
portion of the resultant of the second and third component can be relieved. 
Plate VII. fig. 25 is an end view of this apparatus in its position embracing the decli- 
nation-axis. 
N.B. A portion of the axis is broken away to show the roller e working in the groove. 
Plate VIII. fig. 26 is a plan of the rollers and declination-axis, 
Plate VII. fig. 27 is a vertical section of the steel ring and rollers. 
Plate VII. fig. 28 a side elevation of the same. It should be mentioned that, for the 
efficient working of this apparatus, it is necessary that both polar and declination-axis 
should be balanced “ inter se ” round their point of intersection. This is not the case 
in some instruments which have the counterpoise of the telescope hanging from the 
opposite side of the polar and not the declination-axis. 
When this adjustment has been completed, then, and not till then , the first component 
may be relieved by the apparatus which is partly seen at O, Plate XII. fig. 15, and more 
fully in Plate VIII. figs. 29 & 30. Fig. 29 is an elevation of the larger, and fig. 30 
half elevation, half section of the lesser roller-frames, scale 1*5 inch to 1 foot. That 
represented in fig. 29 is applied to the larger or telescope-end of the axis, immediately 
outside the larger bearing, between it and the cradle. The smaller, fig. 30, is applied 
immediately outside the smaller bearing, and can be seen at P in Plate XII. fig. 15. 
At a, a' are seen the ends of the two steel levers which bear up the roller-frames. The 
