τ ΤΕΣ ΩΣ ἐν} 
; TRANSACTIONS OF THE SECTIONS. 187 
The author concludes his paper by enumerating many advantages which he con- 
ceives his invention to possess over the ordinary paddle-wheel motion. 
On the Rubble Bridge of Ashiesteel. 
By Joun Smitu. (L£xtract from a Letter to Sir David Brewster.) 
“Τὴ this bridge the object of my brother and myself was to dispense with every- 
thing costly that was not of essential service to the work, and with this view we 
used no materials but what could easily be managed without machinery ; there was 
not one stone in a hundred beyond what one man could easily lift. 
«« Whinstone has many properties to recommend it, particularly in bridge building. - 
No stone takes a firmer hold of lime or cement of any kind; it is a complete non-ab- 
sorbent, is harder and more durable than granite, and plentiful and easily procured 
in many localities. 
“This bridge, with the exception of the cornice and the coping- stones of the parapet 
walls, is entirely constructed of whinstone rubble, which was found in the immediate 
neighbourhood. 
«« My brother and I contracted with Gen. Sir James Russell of Ashiesteel to erect it 
opposite his house, as a private bridge. It was afterwards taken up by the road 
trustees, and the site fixed a little further down the river. 
“« Oar estimate was £1200 for the whole, Sir James furnishing the rough timber for 
the centre ; and had it not been for the centre giving way before the arch was balauced, 
the sum would have been sufficient. 
«The timber being mostly of spruce fir quite green, rather deceived us as to its 
strength. 
“To have done a bridge of the same dimensions in the same place, would have cost 
between three and four times the money. 
«The arch is not aregular semi-ellipse, but is formed of three curves, the two side 
ones being drawn to a radius of 24, and the centre one to 110 feet. 
“The breadth of the arch at the abutments is 19 feet, and it converges by curved 
lines towards the crown to 16 feet. 
“The thickness of the solid part of the arch is 23 feet, and is backed by ribs 24x 
24 feet; these ribs terminate at the distance of about 22 feet from the crown of the 
arch, and that part of it is built solid at the depth of about 4 feet. 
“The spandril walls rest upon the back of the ribs above-mentioned, and are so 
managed as to thickness and form as to be an exact counterpoise to the crown of the 
arch. 
“1 am not aware that anyarch of this extent was ever executed of Whinstone rubble, 
or that any stone arch of any description of the same span and radius, was ever built 
in Scotland.” 
On a new form of Equatorial Mounting now making for the Edinburgh 
Observatory. By Prof. Piazzi SmytTu. 
After briefly describing the two prevailing forms of equatorials, viz. the English 
with its long polar axis and two piers, and the German with its short polar axis and 
single pier, and pointing out their several defects and excellencies, the author then 
exhibited a model of a new construction, which appeared to combine the advantages 
of both the others, without their principal defects. The observatory having been 
already built, there was a necessity for the instrument being adapted to one central 
pier, as with the German ; but the violent wiads of the Calton hill rendered a much 
firmer stand necessary, especially in the power of the declination axis and frame to 
resist torsion. 
These requirements were obtained by making the polar axis in the form of a short 
cylindrical shell of cast iron; the axis of motion passing transversely through the 
middle of it, and being defined by small pivots at either end. i 
The declination axis, which is a cone of great breadth, passes through the cylinder 
in the direction of its axis, and one of its faces becomes the declination circle, and 
gives most powerful means for the firmest clamping in that direction. The tele- 
scope fixed at one end of the declination axis is certainly outside the cylinder, but is 
midway between the bearings of the polar axis diameter. 
