148 
DE. T. E. EOBINSON AND ME. T. GrETTBB’S DESCEIPTION OF 
there described for slow motion in Al. E is the finder, 4 inches aperture, fitted with 
eyepieces of large field.] 
The equatorial which carries this huge telescope is not less remarkable. It will be 
seen in Plate XII. fig. 15 that the polar axis is inverted, so that the declination-axis, 
and of course the centre of gravity of all the moving parts, is near the ground. So also 
are the circles ; which gives extreme facility in setting or reading the instrument. The 
axes are exceedingly massive; but from the beautiful arrangement of counterpoises 
which is adapted to them, their motion is very easy. One of these is seen at K (fig. 15); 
it relieves the upper pivot of the polar axis from its lateral friction to any required 
amount. Another, F, does the same thing for the lower pivot, and the peculiar one, L*, 
takes off the end pressure on the lower pivot. The result of their combined action is, 
that the instrument is turned round this axis by a force of 5 lbs. at a leverage of 20 feet. 
[Plate VI. fig. 19 is an end elevation, and fig. 20 is a cross section of the upper end 
of the polar axis resting on its bearing ; scale 1^ inch to the foot. 
The axis in this part, 12 inches diameter, rests on two blocks of gun-metal inserted in 
cast-iron wedge-shaped pieces, b , b', sliding in a horizontal groove, and acted upon by the 
screws a, a', by the combined action of which the axis is adjusted in the direction of the 
plane of the meridian, or at right angles to it. At c (Plate VI. fig. 19) is seen the roller, 
8 inches diameter, by which the Y supports are relieved of any desired portion of the 
weight of the axis, & c. This roller is acted upon by a pair of steel levers of the propor- 
tion of 3 to 1, to which weights are attached hanging down the eastern side of the polar 
pier. The fulcra of these levers are capable of adjustment, so that the roller may be 
brought exactly under the centre of the axis, as it is adjusted by the screws and wedges 
a a!, b b'. At d (Plate VI. fig. 19) is seen a sector acted upon in like manner as the roller 
by a lever and weights hanging down the western side of pier. This exerts an upward 
pressure on the bar e, which is connected with the relief of the friction of the declination- 
axis (see fig. 25, where this apparatus is described). 
Plate VI. fig. 21 is a side elevation of the lower bearing of the polar axis, showing 
the plummer block in section ; scale 1^ inch to the foot. 
a is the lower pivot of the polar axis terminated by a piece of chilled cast iron, b, 
polished flat on its lower face. This under face revolves in contact with a piece of bell- 
metal c, fiat on its upper side, and partially spherical on its lower, bearing in a corre- 
spondingly shaped spherical annulus formed to receive it in the bottom of the plummer 
block. This arrangement enables the bell-metal cushion to take its own position, as the 
axis is adjusted by the screws and wedges in the upper bearing (see a a!, b V , Plate VI. 
figs. 19 & 20), and so ensures an equable bearing throughout the whole of the surfaces 
in contact of the pieces b and c. Although the component of the weight of the telescope 
* Tlie chain connected with the crank arm L is peculiar. It is made of circular iron disks connected hy 
semicircular links of steel. Each link can twist a little on its partner with very little friction. They are 
hardened at their point of action ; their ends pass through holes in the disks where they are secured hy nuts, 
and can easily he removed for repair. No chain of the common construction would answer here. 
