322 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[September, 



AVhen it was considered that the superficial area of these boilers was about 

 GO square feet ; that the pressure of the atniosphere was uearly 1 ton per 

 square foot, and that the weight of the boilers was only 8 or 10 tons, it 

 would be apparent that the cause was quite ailequate to the effect, with a 

 very partial vacuum or inequality of atmospheric pressure. The case was 

 analogous to those in which light bodies were raised into the air by wbirl- 

 trinds. 



lie referred also to two cases of an equally uncommon nature, which had 

 lately come under his notice professionally, and which lie considered to arise 

 chielly from inequality ef atmospheric pressure. 



The first occurred at the I'lymouth Breakwater during the great storm in 

 the month of February, 1838, when several of the largest granite lilocks, 

 weighing from 3 to H tons each, composing the surface or pavement of the 

 breakwater, which, although squared and dove-tailed into the structure, and 

 embedded in excellent cement to the extent of their whole depth, and thus 

 forming a solid mass, were torn from their positions, and projected over the 

 breakw.iter into the Sound. He attributed tliis to the hydrostatic pressure 

 exerted beneath the stones, at the moment when the atmospheric pressure 

 above had been disturbed by the masses of water suddenly and rapidly thrown 

 upon the surface of the breakwater. Ulocks of stone were tlius often carried 

 to a great distance, not so much by the waves lifting them, as by the vacuum 

 created above them by the motiou of the water, which exerted at the same 

 tinie its full pressure from below. 



The other rase occurred during a storm in the year 1840, when the sea 

 door of the Eddystone Lighthouse was forced outv\ards, and it strong iron 

 bolts and binges broken by the atmospheric pressure from within. In this 

 inst.mces be conceived that the sweep of the vast body of water in motion 

 round the lighthouse had created a partial and momentary, though eft'eetual, 

 Tacuuni, and thus enabled the atmospheric pressure within the building to 

 act iipon the only yielding part of the structure. 



Signals for Railways, 



A letter was read from Mr. Edward Alfred Cowper, describing some ex- 

 periments on the use of maroons as signals on railways. 



The maroons are either small tin cases, or cartridges of brown paper, charged 

 with from \ oz. to2oz. of gunpowder, mingled with which are 4 of " Jones's 

 Prometheans," which are small glass tubes, each containing a drop of sul- 

 phuric acid ; the tubes are surrounded with chlorate (hyper-oxy-muriate) of 

 potassa, and are each enveloped in a strip of paper. 



In the event of an accident occurring, which renders it necessary to give 

 notice that an approaching train should be stopped at a given point, two or 

 more of these maroons are fastened upon the upper surface of the rail by the 

 strips of lead attached to them. The wheels of the engine, in passing over 

 them, crush the glass tubes of the " Prometheans," the sulphuric acid in- 

 flames the chlorate of potassa, and causes an explosion of the gunpowder, 

 which is distinctly heard by the engine driver, who immediately shuts off the 

 Steam, and puts down tlic break. 



Mr. C. H. Gregory bad permitted several trials to be made with these 

 maroons on the Croydon Railway. 



An engine was driven at full speed with a number of empty wagons attach- 

 ed to it, and with the steam blowing off to create as much noise as possible, 

 yet the explosion of even half a drachm of gunpowder was distinctly per- 

 ceived : he considered the invention to be practically useful. 



BRITISH ASS0CI.A.T10X FOR THE ADVANCEMENT OF SCIENCE. 



Eleventh Meeting, 1841. 



Desei-ij/fion of the existing and contemplated Rnihrays and Machinery con* 

 nevted vith the Granite Quarry on Dartmoor, and of the mode of working 

 them. By William Johnson, Grosvenor Wharf, Al'cstminstcr. 



Dartmoor is an extensive granitic table-land occupying the heart of Hie 

 county of Devon, and in which many of the rivers of this county originate. 

 It is in length from north to south 22 miles, and in breadth from east to 

 west 14 miles. The heiglit of the table-land above the level of the sea is 

 from 1000 to 1200 feet ; but the surface is broken by numerous masses of 

 rock, which run up to three, four, and five hundred feet above the ordinary 

 level of the moor ; these are ranged in short chains, or they rise in rifted 

 blocks, or in insulated hillocks, which ere distinguished as Tors, or provin- 

 cially Tars. 



The surface granite of Dartmoor, which is in detached blocks, in infinite 

 quantity, had been employed in the neighbourhood for the ordinary purposes 

 of building from time immemorial. Quarried granite from Dartmoor was 

 first brought into the market about 1820 by the Haytor Granite Company. 

 A stone tramway was constructed from the llaytor quarries on the south- 

 eastern face of the moor to the Stover Canal, near Newton Bushel, a few 

 miles above Teignmoutli, the nearest available port at which stone couhl be 

 shipped, and by tliis means Dartmoor granite caiue into the market at con- 

 siderable advantage, especially as the quality of the stone is such as to enable 

 it to compete with the best .\berdGenshirc, and, for many purposes, the 

 lightness of its tint aided by the fineness of its texture, and the almost un- 

 limited size of the blocks in which it could he procmed without defect se- 

 cured it a jircfcrence. The western face of London Bridge is mainly com- 



posed of granite from Dartmoor. Dartmoor granite has also been intro- 

 duced into many of the |)ublic buddings in the metropoUs; amongst others 

 the New Post Office, the Goldsmith'* Hall, Fishmougcr's Hall and Bucking- 

 ham Palace. 



The establishment of the Plymouth and Dartmoor Railway, which was 

 completed in the year 1825, directed attention to the fine granite on the 

 western face of the moor, by affording ready means of transport to the port 

 of Plymouth for stone from Foggintor and other points adjacent. 



The length of the Plymouth and Darmoor Railway, from Prince-town to 

 the Tide Docks, known as Sutton Pool, at Plymoutii, is about 25 miles; 

 though the distance from one terminus to the other, by the carriage road, is 

 not more than 16 miles ; whilst a right line, from point to point, does not 

 exceed 13 miles in length, the greater length of the railway being occasioned 

 by windings to save expensive works, and to obtain tolerably equable incli- 

 nations upon which the trains might nui down freely and allow the empty 

 or slightly laden waggons to be drawn back without much waste of power. 



The whole rise of the railway from its toll-house in Plymouth to the 

 Prince-town terminus, by the Prisons of War, is 1350 feet; which, upon the 

 net distance of 24 miles between those points, gives a rise of 56'25 feet per 

 mile, or a ratio of nearly 1 in 94. The road is almost entirely upon the sur- 

 face, with occasional slight cutting and filling, and a short tunnel (30 chains 

 in length) occurs within the fourth mile from Plymouth. The wagons or 

 trucks used upon the line consist of a platform or bed, set upon two centres, 

 on two under carriages, to a'lapt them to their work and to the excessively 

 sharp curves and irregidar surfaces that frequently occur upon the line. The 

 power used in working the line is the gravitation of the load, assisted by 

 horses, the return being by horses entirely. Three horses draw eight single 

 wagons, or four double ones, with from 30 to 40 tons of granite down, and 

 take the wagons back when empty. There are at present no planes upoa 

 the line having gear worked by fixed power, or as self-acting planes. 



The quarry which affords this railway its principal occu^tion, is worked 

 by the Haytor Granite Company. It is situated witliin a quarter of a mile 

 of the main line of the railway, from which a branch is laid into the quarry, 

 at two miles from Prince-town, the floor of the quarry being at a level of 

 48 feet above the turn-out, and 12C0 feet above the level of the quay at 

 Plymouth, from which the stone is shipped. 



The quarry is on the side of the Mount known as Foggintor, at from 350 

 to 400 feet below the summit. A gullet was first driven in horizontally, 

 until a face of rock 50 feet high was obtained, presenting a most beautiful 

 section of stone, in beds or layers of from eight to ten feet thick. The gullet 

 has since been carried forward from 110 to 120 yards, and extended laterally 

 until the bed of the quarry presents a cleared horizontal surface of nearly 

 4000 square yards. The benching onw ards and outwards of the upper layers 

 exceeds 2G00 square yards, and the highest bench is 80 feet above the rails 

 on the floor of the quarn-. A considerable further extent of surface, beyond 

 and above this, is uncovered of earth, and, the crust being removed, a ver- 

 tical section of the granite tor is exposed, of nearly 100 feet in height. 



A single blast in this quarry has been known to separate and remove 3000 

 tons of stone, and single blocks have been sent out weighing 20 tons, com- 

 puted to contain about 250 cubic feet. 



From the lower benches, in the face of the quarry, the stone has beeu 

 taken up by derricks, or moveable cranes, and placed by them in the usual 

 manner upon trucks on the railways, laid on the floor of the quarry ; whilst, 

 from the topmost benches, the stone is delivered over the sitle of the hill, 

 and skidded down an inclined plane to the masons' sheds, where the ope- 

 rations of converting and dressing are performed, and from whence the 

 blocks are conveyed upon railway trucks to the place of shipment. 



The existing facilities for working the quarries, are now in course of fur- 

 ther extension, by the construction of stroi)g timber stages or scaffolds, with 

 travelling frames, and upon the frames iiowerful traversing crabs, avoiding 

 thereby the excessive labour and delay of lifting by the ordinary means of 

 derricks and cranes. These stages rest upon the floor of the quarry in front, 

 and run in parallel lines of 30 feet in width onwards upon the benches ac- 

 cording to their heights, and give the means of taking up the stone wherever 

 the blocks may be thrown out in blasting upon the different benches, and 

 placing them at once upon the tnicUs on the floor of the quarry, by which 

 they are taken to the mason's sheds, so that the quarry is kept constantly 

 clear, and the largest blocks of stone are moved out with the greatest pos- 

 sible ease. An arrangement is in progress for transferring the travelling 

 frames, with the crabs upon them, from one line of scaffold to another, by 

 which means power may be accumulated to almost any extent upon any one 

 stage, to operate upon blocks of extraordinary size. 



Foggintor granite is at present extensively used for all the purposes to 

 which granite has been hitherto ajiplied. It is superior to any other in this 

 country for steps, plinths, string and blocking courses, ashlars, pedestals, 

 obehsks, columns, cornices, and indeed for all the purposes of architecture, 

 because of the freedom and comparative ease w ith which it can be worked, 

 being alike capable of the finest arris and of the fairest face, whether moulded 

 or plain ; whilst the purity and evenness of its colour, and the fineness of its 

 texture in the deep beds, give it advantages not possessed in an equal degree 

 by the |)roduce of any other quarry in Great Britain. Foggintor granite is 

 moreover iiecidiarly fitted for the more massive works of the hydraulic arcLf« 

 tcet or engineer, on account of the magnitude of the blocks in which it 

 procurable. It is in great demand for the quoin, hinge, and heel stones 

 dock :!nd other lock gates, for altars iti graving docks, as well as for 



