ON TELESCOPES AND EQUATORIAL MOUNTINGS. 199 
It will be seen, from this short mention of the several varieties of equato- 
rial mountings, that I give a most decided preference to the general form or 
principles of construction of the German instrument, in which preference it 
cannot be supposed that there is any undue element of partiality, as none of 
the forms are of my own devising. The British Association, it will be re- 
collected, long since impressed with the importance to science of having a 
powerful instrument sent to a southern latitude, urged the British Govern- 
ment to contribute to the work, and appointed a committee to examine and 
select the most effective form of instrument, &c. &c. Anxious to be a suc- 
cessful competitor in such an undertaking, I applied myself to remove the 
only apparent objection to the German form of instrument, and also to devise 
such modification of its details as would suit it pre-eminently for a large re- 
| fleeting telescope. Both these objects were accomplished with entire success. 
My plan, as also estimates, had the honour of being approved of by the 
Committee before referred to; and although it would now appear as if, from 
some occult cause, the distinction of being the constructor of the proposed 
instrument is likely to remain an honorary one, still, as the improvements 
spoken of are applicable to instruments of much smaller dimensions, it may 
not be unprofitable to lay the general principles and results of these before 
the Association. 
That which the German form of equatorial seemed alone to require, was 
a system of equipoise inter se, unobjectionable in itself, and which would 
reduce the nature of the pressures of the declination axis on its bearings to 
the same which these would be if the polar axis were vertical instead of in- 
clined to the co-latitude of the observatory. The declination axis of a large 
instrument of the German type has necessarily great diameter, and its bear- 
ings, if ground in to fit without shake, as they may be in a small instrument, 
would have too much friction; it is therefore desirable that the bearings 
. should be in effect Ys, and in order that such bearings shall be as admissible 
as in a meridional instrument, the pressure of the axis on each side of its MG 
bearing should be equal in every position of the instrument; moreover, it is 
desirable that the end pressure of the declination axis, in all positions of the 
instrument out of the meridian, shall be neutralized. These important condi- 
tions are perfectly fulfilled by a system of internal counterpoise, which, being 
applied, then permits of an external system of anti-friction rollers, relieving 
the Y bearings of all but a fractional portion of the remaining pressure of 
_ the declination axis and its appendages (viz. the telescope and its counter- 
_ poise). The resuit of such arrangement may be readily anticipated; an 
achromatic telescope of 12 inches aperture and 20 feet focus, so mounted 
and with perfect steadiness, is moved by a force of about one pound applied 
at the eye end. 
Secondly, and for the equatorial mounting of the largest reflecting instru- 
ments up to 6 feet diameter (the size of Lord Rosse’s), if required, I modify 
the German type, as shown in the drawings exhibited, by placing the decli- 
nation axis within instead of beyond the larger end of the polar, and I invert 
_ the whole. Thus the previous steadiness is rather increased ; the bearings of 
. the polar axis may both be of minimum size, and the centre of gravity of the 
whole instrument is brought as close to the ground-level as can be desired, 
_ instead of being considerably aloft; also, the settings and readings of the 
_ largest instrument are rendered most convenient, and the observer is gene- 
rally close to the ground, and never more than a few feet from it. 
The subjoined Table contrasts the weights to be moved and power required 
for the same, in the second English and improved German forms of equa- 
torials, for telescopes of several sizes, viz.— 
s 
