those districts where necessity urges them; and if so in these, how much less speedily will they be brought to light where the immediate want is not felt.

I have also shown that if only a very small proportion of the field contain workable ironstone, it will be sufficient to warrant the establishment of iron works; and I have shown the benefit to proprietors of minerals, to the North British Railway Company, and to the community at large, should these discoveries be realized.

In conclusion, I have to state that I am responsible only for the opinion laid before you, as to the corresponding position of the various strata of blackband. The extent and line of outcrop of the field under consideration are copied from the ordnance map, and from Mr. Milne's valuable paper; and should I have brought before the Society facts which will in any degree stimulate all or any interested in the search to make a vigorous attack on these hidden treasures, my writing will not have been in vain; and looking to their great value, it is to be hoped, should any one be induced thus to apply themselves, they will not, either in the first, or the second, or even the third effort, though unsuccessful, desist from the search, for the prize is well worth looking after.

Additional Remarks.*

It will be in the recollection of some now present, that in November last I read a paper on this subject, which I will shortly recapitulate, in order that my additional remarks may be more fully comprehended.

The coal measures in Scotland have been worked for hundreds of years; but it is not many years since the idea first occurred among practical men that the various fields were comparable with each other. Even now the idea is far from being generally known.† The writer attempted, by shewing the natural divisions of each into upper coal measures, limestones, and lower coal measures, to form comparative stratigraphical positions of the coals in Lanarkshire and Edinburgh, and deduced that the ironstones of the former might reasonably be expected to obtain in their proper position in the latter.

Making every allowance for blanks, which are common to all iron

* Read before the Society on 25th March, 1861.

Within the last five years, the most eminent mining engineer in Edinburgh held that the position of the Bo'ness coals was identical with the Redding or Grangemouth, an opinion which he does not hold now.

stone fields, it was thought probable that the field might contain sufficient ironstone to make fifty one millions of tons of cast iron; and that if only twenty furnaces were erected, they would have a supply of ironstone for 250 years; and which would benefit the community, the proprietors of minerals, and the North British Railway Company whose lines traverse the district.

In a short discussion which followed the reading of the paper it was admitted that the theory was correct, but at the same time it was well known that the ironstones had been unsuccessfuly sought for by the proprietors who hold lands where the position was to be found, except in the lower series where they had been attempted to be worked, but either owing to quality or deficiency in thickness, or above all, by the extraordinary quantity of water and cost of working, and also the cost of raising coal as compared with Lanarkshire, there was no chance of successful prosecution of iron making. The only furnace worked, (Gladsmuir) was an admitted failure, as had been predicted.

The objections stated are all worthy of notice, and are evidently fatal, if correct, and with the view of arriving at some correct data, I made inquiries, and otherwise endeavoured to inform myself as to them, and am in some measure in a position to speak to some of the points.

The possibility of finding the Lanarkshire blackband ironstone positions in the Edinburgh Coal Field was not generally known, and was certainly not generally known to the proprietors of minerals.

The trials that have already been made are not by any means conclusive, either as to the possibility of working the ironstone already discovered profitably, or to the existence of it generally throughout the field. Blackbands are notoriously irregular, and are not found uniform in thickness; for example, the Airdrie blackband occupies but a small portion of the space allotted to it in the Lanarkshire Coal Field. I have on the map delineated this space as far as it is yet ascertained, but it is possible a larger area may yet be found. A more notable example of caprice of blackband is to be found in the slatyband, which occurs occasionally in patches of irregular thickness, sometimes six inches and sometimes six feet in thickness, but there is always something to mark its position, either a coal or ironstone. Indeed all the ironstones in all the positions are erratic, they are presistent throughout in no field, yet it is a singular fact that we have in all the fields blackband ironstone.

With these facts before us, there does not appear to be any good reason for doubting the existence of ironstone in the field in question,

and it is highly probable my estimate is under, instead of over, the quantity to be realised.

But the quality may be inferior, it is said. It is well known that the quality does alter in a very short distance, but the average quality of any stone does not vary a great deal, comparing one square mile of a field with another. There does not appear to be much deficiency in the quality of the Penstone (or Gladsmuir) ironstone, and I have the best authority for stating, that with railway connection and a more favourable supply of coal, the Gladsmuir Iron Works could compete with those in Lanarkshire. As regards the cost of coal, the average cost of coal used in making iron in Lanarkshire is from 4s. 6d. to 5s. a ton at the furnaces, and looking at the number and thickness of the seams of coal which are in the coal field, it is difficult to see why coals cannot be raised in the Edinburgh Coal Field at even less than the sum mentioned.

As regards the quantity of water in the field, there can be no doubt of its having formed a serious drawback; but with the means which we now possess of raising water, this should be no great obstacle.

It is true, hitherto, the performance of our pumping engines, the best of them, are very much below the pumping engines in Cornwall, but when true economy must be sought for, we must either endeavour to make as good engines as theirs, or procure them from that county. With the view of ascertaining the "duty" of our Scotch pumping engines, I made some inquiries as to the performance of three which have been erected within the last four years on the most approved principles; also of a Cornish engine erected by Hocking and Loam, for Sir George Suttie, at Dolphingstone, near Prestonpans, in 1848. This latter engine lifts fifty millions of pounds of water, one foot high, by the consumption of 112 lbs. of coal; the others lift eighteen, fourteen, and twelve, respectively. This last, however, is a direct acting engine with the cylinder over the pit, and the pump rod attached to the pistonrod, high pressure, with condenser, but no air-pump. We have here, then, ample room for amendment. To further illustrate the superiority of the Cornish engine,-Suppose Sir George's engine were to require £1000 for fuel, the others, to do the same amount of work would require respectively £2670, £3570, and £3840 per annum, showing that by economy in this department, fields may be won which are not now attempted; and it is to be remarked that economy in fuel is economy in wear and tear, and in attendance as well; for example, I am informed that Sir George's engine has been constantly at work since 1848, yet the repairs on boilers have only cost £5.

25

On the Concrete used in the late Extension of the London Docks. By GEORGE ROBERTSON, C.E., F.R.S.E., Resident Engineer, New Works, Leith Docks.*

In the late extension of the London Docks at Shadwell, the point which appeared to me to be most worthy of, and to afford the greatest scope for enlargement, was that of hydraulic lime. Firstly, Because there were (in my mind at least) many unexplained difficulties and contradictory statements connected with its theory and practice; secondly, Because the manufacture of the lias lime used on these works being in the hands of the Company's engineers, afforded advantages for investigation which might never occur to me again on such an extensive scale. An investigation into the theory of hydraulic lime, and its manufacture into mortar, formed the subject of a paper read before the Institute of Civil Engineers in April 1858, to which the present one on Concrete is a sequel, completing the monograph on the lime used in this one work.

I also read a paper lately before the Royal Society here, upon the "Solidification of Limes and Cements," which was founded partly on the London Dock experiments and partly on a continuation of them at Leith Docks.

The new works made by the London Dock Company consist of a new basin thrown into one with the old Shadwell basin, and two large locks 350 feet long and 60 broad, parallel with the former small ones; one to lock vessels up if necessary from the River Thames to the Basin, and the other for vessels proceeding to the Eastern Dock, the water level of which is usually kept above Trinity high-water mark by a pumping engine.

Borings of the ground occupied by the new works showed how advantageously concrete could be used in their construction. Below the first eight feet of made ground and brick rubbish is a bed of brown clay some 6 or 7 feet thick; then a bed of peat, averaging 6 feet, but often much thicker, full

* Read before the Society, 10th December 1860.

VOL. VI.

D

of remains of beech, oak, hazel, and other trees. The lower part of this peat was full of veins and lumps of sesquiphosphate of iron, native Prussian blue. This made an excellent pigment when ground up with gum water, of a delicate smalt colour, which I used in tinting working drawings. Below the peat is a thin bed of clay, the bright blue colour of which was very likely due to this colouring matter in the overlying peat. Under the peat and clay is a thick bed of flint gravel, Thames ballast, which extended nearly over the whole area of the new works. In some places it was fine enough to form sharp clean sand for mortar, in other places coarse gravel well adapted for concrete. The chief material for concrete was therefore on the very site of the works ready for use, and the whole expense was saved likewise of barging it up to Battersea Park, where we were permitted to shoot out the excavations. Under the gravel, at an average depth of 30 feet below Trinity high-water, lies the solid London clay, into which, of course, most of the foundations had to be carried. The bed of sand and gravel was more than 12 feet thick at the two locks, but thinned out completely at the north wall of the basin. The ballast was in sufficient abundance to supply all the concrete required for foundations and counterforts, and to leave enough over to make it worth while using it for the dock walls themselves.

The subject naturally divides itself into two divisions—the manufacture and the application of the concrete.

The Manufacture.-The great mass of the concrete was made with naturally hydraulic lime, blue lias from Lyme Regis in Dorsetshire, which requires no artificial mixture with Pozzuolana or minion to render it capable of setting permanently under water. The word "concrete" in this paper implies, therefore, that made with blue lias lime, unless otherwise specified. The Dorsetshire lias was the only lime burned on the works; all lias from Warwickshire or Leicestershire was bought ready burned from the merchants. Lias requires much greater care in burning than richer limes, because any sudden or extra heat, which would do little harm to Dorking lime, greatly injures lias by forming a glass between the silica and the lime in the stone, instead of only driving off