316 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[September, 



of the saiulv subsoil ; in this state it has the a]i])carancc of a slimy grit, con. 

 sistiug of [laiticlcs of the saiul-stoiie luliricateil with clay; it is f.ciuiliaily 

 called "the blue slipper." A continuation of this infiltration for any length 

 of time must end by undermining certain portions of the face of the Clitt'. 

 which, being unsupported Ijcneath, detach themselves from the main rock 

 and settle ; the first settlement may not exceed a few inches, but a fissure 

 having been formed the whole length behind the subsidence, the surface 

 water ))0urs into it, and continuing to moisten and undermine it, at length 

 causes the slip to assume its present aspect. This soakage of water at the 

 back of the mass may be supposed to sap its foundation at the rear and to 

 give it the dii) inwards, which is observed in all cases, and most evidently in 

 such as are farthest advanced in their descent. A number of natural terraces 

 are thus formed, and the process may be traced in every stage of its progress 

 at ditferent parts of tlie Clitf, as at Mirables, in the Pelham Walks, at Vent- 

 nor, and at the Luccondie landslip. These subsidences appear to have suc- 

 ceeded each other at long intervals of time, but there is no record of any so 

 extensive as that which occurred in 1799, at which time upwards of 100 

 acres were set in motion. That the principal landslips took place prior to 

 the Norman Conquest is proved by the existence of Bonchurch and St. Law- 

 rence Chapel, which are supposed to have been Inult soon after the manor 

 was surveyed for entry in Doomsday Book. 



The President oljserved, that altliough papers of this kind did not appear 

 to be exactly adapted for the meetings of the Institution of Civil Engineers, 

 yet, as geology was so intimately connected with engineering, and it was 

 always essential to ascertain accurately the nature of the ground where works 

 were to be executed, such commnnications became not only acceptable, but 

 very valuable, to the profession. 



jlr. Lowe liad paid ranch attention to a similar formation at Hastings, and 

 while he agreed to the general correctness of the observations, he diil not 

 think a sufficient reason had been assigned for such a mass of iron sand with 

 its incumbent chalk being driven seaward. He would attribute the subsi- 

 dences at the Undercliff to the action of water percolating through the 

 fissures into the thin beds of clay interspersed with lignites, with which the 

 iron sand abounded. This, when moistened, would ooze out and permit the 

 chalk to crush it outwards, causing the sidjsidences so ably described by Jlr. 

 Rickman. 



March 24, 1810.— The President in the Chair. 

 The following were balloted for and elected: — Charles Lanyon, as a Mem- 

 ber ; Henry Addams, Thomas Macdougal Smith, and Robert Richardson, as 

 Graduates ; Henry Heathorn and Ardaseer Cursetjee, as Associates. 



" On the manufacture of Flint Glass." By ApsleyPellatt, Assoc. Inst. C.E. 

 Flint glass, called by the French " cristal," from its resemblance to real 

 crystal, is composed of silcx (whence the English name), to which is added 

 carbonate of potash and htharge, or red lead ; to whicli latter material is 

 owing, not only its great specific gravity, but its superior lustre, its ductility, 

 and ]iower of refraction. 



It is necessary for optical purposes that flint glass should be perfectly free 

 from strise, otherwise the rays of light passing through it diverge and become 

 distorted, and this defect is caused by the want of homogeneity in the melted 

 mass, occasioned by the difficulty of perfectly fusing substances of such dif- 

 ferent density as the materials employed. The materials, being properly 

 prepared, are thrown at intervals into a crucible of Stourbridge clay, which 

 will hold about 1600 lbs. weight of glass when fused. The mouth of the 

 crucible is then covered with a double stopper, but not luted, to permit the 

 escape of th.c moisture remaining in the materials, as well as the carbonic 

 acid gas and excess of oxygen. It reiiuires from 50 to 60 hours application 

 of a rapid, intense, and equal heat to etfect the perfect fusion of the materials 

 and to drive ott' the gas ; during whicli time the unfused particles and excess 

 of salts are skimmed off as they rise to the surface. The progress of fusion 

 cannot be watched, nor can any mechanical means for blending the material 

 during fusion be resorted to, lest the intensity of heat requisite for the pro- 

 duction of a perfectly homogeneous glass should be diminished, the quality 

 of the product being influenced by any inattention on the part of the fireman, 

 as well as by the state of the atmospliere or of the wind. It has been ascer- 

 tained, that there is a certain point or crisis of fusion at which the melted 

 metal must be kept to insure a glass fit for optical purposes, and even when 

 that point be attained, and the crucible shall furnish proper glass during 

 several hours, should there be such diminution of heat as to require the fur- 

 nace to be closed, the rciuainder of the metal in the crucible becomes curdy 

 and full of stri;e, and thus unfit for use. It is the same with tlie glass made 

 for the flat bore tubes for thermometers, which are never annealed, because 

 the smoke of the annealing furnace would render the interior of the bore 

 unfit for the reception of the mercury. These tubes will only bear the heat 

 of the blow-]iipe when they are made from a metal which has been produced 

 under all the favourable circumstances before described. It is, therefore, to 

 be inferred, that the most homogeneous and perfect flint glass can only be 

 produced by exposure to an intense anil equable degree of heat, and that any 

 excess or diminution of that heat is injurious to its quality. 



The English method of manufacturing the flint jilate for optical purposes 

 is thus described. About 7 lb. weight of the metal is taken in a ladle of a 

 conical shape from the pot at the proper point of fusion, and then blown 

 into a hollow cylinder, cut open, and flattened into a sheet of glass of about 

 14 inches by 20, and varying in thickness from J to , of an inch. This plate 

 is afterwards annealed, aud iu this state goes into the hands of the optician, 



who cuts and grinds it into the requisite form. When a glass furnace is 

 about to be put out, whole pots of metal are sometimes suffered to remain in 

 it, and cool gradually. The crucibles' being destroyed, pieces of glass may 

 be cloven from the mass of metal, softened by heat, and made to assume the 

 requisite form, and then ground. It is believed that the excellent glasses 

 made by Frauenhoft'cr, and other manufacturers on the continent, are jiro- 

 duced by some such means. On aMempting to cut glass ware, it is easilv 

 ]ierceived if it be sufficiently annealed ; if not, the ware is put into tcpiil 

 water, which is heated, and kcjit at the boiling point during several hours ; 

 it is then suflfered to become gradually cold. Tliis method is more efficacious 

 than re.anncaling by the ordinary means. A |)iece of imannealed barometer 

 tube of 40 inches in length being heated and quickly cooled, contracted only 

 -^ of an inch, whereas a similar piece, annealed by the usual means, con- 

 tracted nearly J of an inch. Unannealed flint glass, being heated and sud- 

 denly cooled in water, exhibits the appearance of a mass of crystals ; it is 

 thence inferred that the process of annealing renders the glass more compact 

 and solid ; it thus becomes incapable of polarization. 



Flint glass being remarkably elastic, has caused it to be used for chro- 

 nometers. To prove its elasticity, a hollow ball of unannealed glass of 3 

 inches diameter, weighing about 10 ounces, was dropped, when cold, from a 

 height of 7 feet upon a stone floor; it rebounded uninjured about 31 feet, 

 but broke on falling to the ground after the rebound. Similar balls, both at 

 a bright and a low red heat, were dropped from the same height, aud both 

 broke immediately without any rebound ; thus demonstrating that its elasti- 

 city only exists while cold. Glass being sometimes deteriorated in the pro- 

 cess of reheating, not only in colour, but in its faculty of welding, by the sul- 

 phur existing in the coal or coke used in the furnace, this is prevented by 

 occasionally throwing about a quart of cold water on the fire ; the explosive 

 vapour thus raised carries off the sulphureous gas. 



The process of annealing has the remarkable property of carrying off from 

 the glass tlie reddish tint imparted to it by manganese ; and in large masses, 

 not only the reddish tint disappears, but the glass sometimes becomes green 

 or blue, probably by the action of the sulphureous acid gas from the coke. 

 The reddish tint will however return, and the greenish one disappear, should 

 the annealed glass be afterwards heated or renielted. Should the pot crack 

 during fusion, and the flame or smoke come in contact with the melted metal, 

 a green tint and abundance of dense stria: will be the consequence. Such an 

 accident can only be repaired, if the crack be accessible, by throwing cold 

 water on the exuding metal, which thus becomes gradually cooled, and itself 

 forms a lute, so as to enable the process of melting to be continued. Long 

 experience has shown that the best fuel for melting glass in the furnaces is 

 oven-burnt coke mixed with a small quantity of screened coal. 



Mr. Pellatt illustrated the preceding paper by specimens of glass exhibiting 

 peculiar eftects of crystallization ; among them were cylindrical solid pieces 

 of flint glass, which, from being suddenly cooled by plunging them into water, 

 bad the interior entirely dislocated, and were merely held together by the 

 exterior coating; portions of tubes showing the same effect; a portion of a 

 vase of white glass dipped into blue glass of a greater density — in cooling, 

 the interior white glass appeared to be crushed by the contraction of the ex- 

 terior coating ; a similar vase of white and blue glass of more equal density 

 had cooled, and bore cutting without cracking ; a mass of optical glass, ex- 

 hibiting stria:, specks, and imperfections ; which, together with the modes of 

 manufacture, he explained. 



In answer to several questions, Mr. Pellatt was not aware of any attempt 

 having been made to cut the bulb of Prince Rupert's drops : he believed the 

 perculiar property of the bursting of these drops or tears, on the end being 

 broken, arose from a crack suddenly commencing and extending itself rapidly 

 throughout the mass, causing the dislocation of the particles. Flint glass is 

 seldom sufficiently fluid to make these drops ; they are generally made from 

 glass which does not contain lead. 



Alluding to the use of plate glass in Nasniyth's Pneumatic Mirror, he ob- 

 served that, owing to the absence of lead, plate glass was purer and more 

 homogeneous than flint glass, and the equality of thickness produced by 

 grinding and polishing enabled the curve caused by the pressure of the atmos- 

 phere to be very regular. 



The use of coke as a fuel, by the regularity of its combustion, assists ma- 

 terially in producing good results, and prevents the injury which frequently 

 arises from a difi'ereuce in the heating powers of various coals ; unfortunately, 

 the form of the furnaces causes the greatest heat to be in the centre, thus 

 acting most powerfully upon the backs of the pots, instead of being equally 

 distributed around them, which would be more desirable and would insure 

 better results. 



Mr. Pellatt still continued to use nine parts of coke mingled with one part 

 of small coal iu preference to any other fuel. He had abandoned the use of 

 gas coke, and now purchased small coal at a low price, which he converted 

 into a moderately-hard coke, rather less dense than that used for smelting 

 iron. In the north of England, a charge of coal generally remained in the 

 oven during 48 hours ; in London, only 30 hours; lie made lighter charges 

 and coked them in 24 hours. He still found the calorific efl'ect of 8 or 9 lb. 

 of coke to be equal to that of 12 lb. of coal ; in his ovens, 20 cwt. of coal 

 produced 14 cwt. of coke. 



Mr. Parkes inquired, which was the best method of annealing tubes for 

 water gauges on boilers ? He generally used those prepared by Mr. Adie, of 

 Liverpool, who annealed them by placing them in cold water and gradually 

 raising the temperatiux to the'boiUng point, at which it was kept for 24 



