usually greatest, upon the whole, in planes parallel to bedding or lamination, the transit-rate of shock is most generally fastest in the line of the beds or lamination, rather than across them.

Or, at least, this latter condition may interfere with the former to the extent of partial, complete, or more than complete obliteration.

I am not aware that any experiments have previously been made upon the compressibility, &c., of the slate- and quartz-rocks of Holyhead; and as these rocks are being employed there upon a vast scale for submarine building works, it may not be out of place to draw a few conclusions of a character useful to the practical engineer from the data that have been obtained. Some conclusions may be drawn which are applicable to all classes of laminated rocks in the hands of the engineer.

It is a very prevalent belief that slate-rock (for example), in the form of the sawed roofing-slate of Anglesea or of Valentia (Ireland), will bear a much greater compressive load when the pressure is in the direction of the lamina, than in one across them. This the preceding experiments prove to be wholly a mistake-one that has very probably arisen from some vague notion of an analogy with timber compressed the end-way of the grain.

It is now certain that Silurian slates and quartz-rock, and probably all sedimentary laminated rocks, whether with cleavage or not, are much weaker to resist a crushing force edgeways to the lamina, than across the same, and that the range of compressibility is much greater, for equal loads, in the former direction.

The facts now ascertained as to the great relative compressibility of laminated rock in the direction of the lamina also points out the reason of the great bearing power to sustain impulsive loads, which the toughest and most cohesive examples of slate-rocks, such as the slates of Caernarvonshire, present; for there can be no grounds to doubt that the high compressibility of rocks of this structure in the plane of the lamina is also accompanied with a high coefficient of extensibility, although probably confined within much narrower limits as to inceptive injury to perfect continuity.

My experiments point out, that the Silurian slate of Holyhead (the mean both of the hard and the soft) is crushed by a load across the lamina of about 1250 tons per square foot, and that its molecular arrangement is permanently injured at a little more than 1000 tons per square foot.

The quartz-rock (the mean of both hard and soft) is crushed by a load, applied in the same manner, of 1630 tons per square foot, and its molecular arrangement is permanently injured at less than 1000 tons per super foot. The quartz-rock gives the highest measure of ultimate resistance, but it is the less trustworthy material when loaded heavily.

Neither of these sorts of rock, if loaded so as to be pressed in the direction of the lamina, would sustain more than about 07 of the above loads at the crushing-point and at that of permanent injury, respectively. From the extreme inequality found within narrow limits in both rocks as quarried, neither should be trusted for safe load in practice with more than about th of the mean load that impairs their molecular arrangement, as ascertained from selected specimens, or (say) not to more than 50 tons per square foot for passive or 25 tons per square foot for impulsive loads.

The high relative compressibility of laminated rocks in the direction of the lamina might probably be made advantageous use of, where they are employed as a building material, for the construction of revetment or other walls of batteries exposed to the stroke of cannon shot, by building the work (under suitable arrangements to obviate splitting up) with the planes

of the lamina in the direction of the line of fire, i. e. perpendicular to the faces of the work; for on inspecting the last column in Table XII. which contains the values of T, under the several conditions of rock and of compression, it is at once apparent how much greater is the "work done" in crushing the slates and the quartz in their toughest and most compressible direction, i. e. in the direction of the lamina. Twice as much work is, upon the average, consumed in crushing the rock in this direction, that suffices to destroy its cohesion in the one transverse to the lamina; and the proportion in the two, in the case of the softest quartz (Nos. 5 and 8), is as much as about five to one.

It would be unsuitable, however, to the present memoir to pursue further here such practical deductions suggested by the results obtained experi mentally.

On the Explosions in British Coal-Mines during the year 1859. By THOMAS DOBSON, B.A., Head Master of the School Frigate "Conway," Liverpool.

In my Report "On the Relation between Explosions in Coal-mines and Revolving Storms," read at the Meeting of this Association, at Glasgow, in 1855, I have given my reasons for thinking that the freedom of the atmosphere of a mine from noxious gases, and the occasional abundant issue of such gases into a mine, are in a great measure dependent upon certain conditions of the pressure and temperature of the external atmosphere. This dependence is, indeed, a consequence so direct and obvious of the first principles of pneumatics, that we may speak with certainty of the kind of influence exerted by the atmosphere in restraining or augmenting the flow of inflammable gases into a mine; and we have only to inquire whether this influence is ever exercised to such a degree as to charge a mine up to the point of explosion.

It is, I think, now generally admitted that a high atmospheric pressure tends to check the issue of gases into the workings of a mine, and that a low pressure favours their copious effusion from the broken coal and deserted goaves.

It is also evident that a low temperature of the external air makes the ventilation of a mine brisk and effective, while a high temperature of the air above renders the ventilation sluggish, and causes the gases to accumulate below.

I have compared the dates of all the fatal explosions in British coal-mines, as given in the Reports of the Government Inspectors of Mines, with the corresponding barographical and thermographical records for several years, and find that this comparison tends to confirm in a very striking manner the conclusions arrived at in my Report of the year 1855.

Were the Government Inspectors to give in their Reports the dates of all explosions of gases in mines, whether fatal or not, and also the dates of days when mines have been in a dangerous state from the abundance of gas, but explosion avoided, the evidence of atmospheric influence would soon be placed beyond doubt. Seeing that the great atmospheric disturbances with which we are here concerned generally extend nearly simultaneously over Britain and the adjacent countries of the Continent, I have been at some pains to obtain the dates of all the great explosions in the coal-mines of France and Belgium; but I was told at the École des Mines, in Paris, that they had no

such record, and a communication with the director of the mines of Belgium was also fruitless.

The dates for the year 1859 of all the fatal explosions in the coal-fields of England, Scotland, and Wales are marked in the meteorological diagram (Plate V.), in which one day is represented by a horizontal space of onetwentieth of an inch, and 20° Fahr. by a vertical height of one inch.

For the meteorological data I am indebted to the kindness of Mr. Milner, the surgeon of Wakefield Prison, where the instruments are read every six hours, night and day. The portion of the diagram for the months of October and November, showing the state of the atmosphere during the passage of the Royal Charter' storm, has been compared with observations made at Oxford, Kew, Stonyhurst College, Lancashire, and the Bishop's-rock Lighthouse, Scilly Isles; and the general agreement fully warrants the selection of the Wakefield curves as a fair type of the state of our atmosphere during the year 1859.

The curve of mean temperature is from results in a paper by Mr. Glaisher in the Transactions of the Royal Society' for 1850.

If there were no connexion whatever between the weather and the conditions that favour an explosion in a coal-mine, it would be found that the 70 or 80 vertical lines that denote fatal explosions would be scattered, as if by chance, over the whole diagram, without any apparent reference to the great depressions in the barometric curve, or to the great and sudden rises in the thermometric curve. But this is not the case in any of the years that I have examined. On the contrary, it is found that the lines of explosion have a very decided tendency to group themselves about the few great atmospheric perturbations of each year; and to leave a very conspicuous and highly significant blank in spaces, of a whole month's duration occasionally, where the pressure has been uniformly high and the temperature moderate.

In the 68 explosions of 1859 are found three dense groups and a number of equally instructive blanks.

The first group falls between the 11th of January and the 17th of February, during which period the diagram shows that even the nocturnal temperature was considerably above the mean daily temperature, and the barometric curve exhibits a succession of deep indentations marking the passage of a series of storms.

The dates and localities of the explosions forming this group are :—

January 11, Bewdley.

12, Atherstone.

15, Huddersfield.

17, Ayr, Scotland.

19, Wigan.

25, Stevenston, Scotland.

29, Burslem, Staffordshire.

January 29, Aberdare, S. Wales.
February 2, Dudley.

3, Coatbridge, Scotland.

9, Willenhall.

12, Wednesbury.

17, Wigan.

Two cases of death from suffocation by gas fall within this

On February 1, at St. Helen's, and

18, at Tiviotdale, Rowley Regis.

group, viz.,

An interval of a fortnight follows, with a high atmospheric pressure, and no fatal accidents in mines from gas.

During March, and the first week of April, the temperature is far above the mean, and two well-characterized cyclones send the mercury in the barometer at Wakefield down to 28-83 on the 14th, and to 28·91 on the 28th.

The second great group of 14 explosions falls in this interval; 8 explosions happening within 8 consecutive days-exactly coinciding with one of the