)S40.J 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



315 



differs from Mr. llenwood as to tliat portion of the curve wliich the latter 

 selects as represenliiis the true value of expansive working. 



" Description of a Riimiing Gauge for ascertaining the Parallelism of a 

 Raihcay." By Edward Cowper. (Described in the Journal, vol. ii, p. 245.) 



" All Azimuth Cap as an addition to the common Level." By Edward 

 Cowper. 



It is sometimes desirable in levelling operations to ascertain the bearing 

 of objects which are cither above or below the tield of view of the telescope. 

 The comiuon level alone cannot take the bearing of such objects ; for, by 

 elevating or depressing the telescope, the action of the compass is destroyed ; 

 but, by slipping the azimuth cap on to the end of the telescope of the level, 

 object's 50° above or below the field of view may be observed without dis- 

 turl>ing the compass or altering the level of the telescope. 



This instrument consists of a brass cap containing two slips of looking- 

 glass placed at an angle to each other, precisely as in Iladley's quadrant ; 

 one glass being fixed at an angle to the axis of the telescope, and the other 

 being moveable about a centre. When any object is required to be brought 

 within the field of view, the cap is placed on the end of the telescope, and 

 the angle of the moveable glass is varied until the object is reflected on the 

 fixed glass, and thence to the eye. 



March 10. — The President in the Chair. 

 The following were balloted for and elected : — John Manby, as a Graduate; 

 Frederick John Evans, Richard Ravenhill, and John Glutton, as Associates. 



" A mode of li ending Discs of Silvered Plate Glass into Concare or Conre.v 

 Mirrors hg means of the pressure of the Atmosphere." By James Nasmyth. 



The dilficulty of obtaining large specula for telescopes, together with the 

 disadvantages attending the weight, the brittleness, and liability to oxidation, 

 of the speculum metal generally used, induced llr. Nasmyth to turn his at- 

 tention to the employment of silvered plate glass for telescopic purposes, as 

 it possesses perfect truth of surface, is lighter than metal, is not liable to 

 oxidation, and a greater quantity of Ught is reflected from it than from any 

 metallic surface. 



To give a concave or convex form to a disc of plate glass, a certain pres- 

 sure must be made to act equally over the surface. This equal pressure is 

 obtained on Mr. Nasmyth's plan, by taking advantage of tlie weight of the 

 atmosphere. 



A- disc of silvered plate glass, 39 inches in diameter and f^ of an inch in 

 thickness, is fitted and cemented into a shallow cast-iron dish, turned true 

 on its face so as to render the chamber behind the glass perfectly au:-tight; 

 by means of a tube communicating with this chamber, any portion of air can 

 be withdrawn or injected. 



To produce a concave mirror so slight a power is required, that on apply- 

 ing the mouth to the tube and exhausting the chamber, the weight of the 

 atmosphere, which amounts in this case to 3558 lb., acting with equal pres- 

 sure over a surface of 1186 square inches, causes the glass to assume a con- 

 cavity of nearly three-quarters of an inch, wliich, in a diameter of 39 inches, 

 is far beyond what would ever be required for telescopic purposes. On re- 

 admitting the air, the glass hnmediately recovers its plane surface, and on 

 forcing in air with the power of the lungs, it assumes a degree of convexity 

 nearly equal to its former concavity. The degree of concavity or convexity 

 may be regulated to the greatest nicety, and it is proposed to render the 

 degree of concavity constant, by placing in the air tight chamber a disc of 

 iion turned to the required form, and allowing the pressure of the atmosphere 

 to retain the glass in the form given to it by its close contact with the iron 

 disc. The curve naturally taken by the glass when under the pressure of the 

 atmosphere is believed by Mr. Nasmyth to be the catenary, inasmuch as its 

 section would be the same as that of a line suspended from each end, and 

 loaded equally throughout its length. 



Mr. Lowe did not feel weU assured that the curve naturally taken by the 

 " Pneumatic Mirror" was a catenarian, as the plate being set iu a frame was 

 supported all round its periphery, and resembled an arch resting on its abut- 

 ments. He suggested the ]>ropriety of attempting to attain given curves by 

 grinding the plate of dilfereut thicknesses ia paits, so that the pressure of 

 the atmosphere should affect it unequally. 



Mr. Macneill was inclined to believe the curve assumed was the " Elastic 

 Curve," the properties of which were exaudued by James Beruouilli, iu the 

 Memoirs of the Academy of Science, 17U3. 



March 17. — Henry R. Palmer, V. P., in the Chair. 



The following were balloted for and elected : — Theodore Budd and Thomas 

 Steel, as Graduates ; Geddcs Pearce, William Lane, Thomas Jevons, and 

 George ilills, as .\ssociates. 



" An Account of the Performances of the Locomotive Engines on the Lon- 

 don and liirmiwjham Railtray during the gear 1839." By Edward Bury, 

 M. last. C. E. 



The engines used on the London and Birmingham Railway are all con- 

 structed on the same principle as to the main parts, the whole being upon 

 four wheels, and oidy differing from each other in some of the minor details. 

 The engines used for the conveyance of passengers have cyUnders 12 inches 

 diameter, with an 18 inch stroke ; the driving wheels are 5 feet diameter, 

 and the carrying wheels 4 feet diameter. The merchandize engines have 

 cyhnders 13 inches diameter, with an 18 inch stroke, and differ from the 



others in having all the wheels of 5 feet diameter and coupled togetl'.er. The 

 framing is of wrought iron, fixed inside the wheels for the greater conve- 

 nience of connecting it with the boiler. The cylinders are attached to the 

 frame by two strong wrought-iron bars passing beneath the lower semi- 

 diameter, and secured by bolts to the ears cast on them. The cranks and 

 fore axles are also fixed to the frame. By this arrangement, any concussioa 

 is received directly by that part of the machine best calculatcil to bear it, 

 and when the force of the engine is exerted in either pushing or drawing, it 

 is done directly tlirough the line of the framing, and thus any strain is di- 

 verted from the boiler or from those paits of the machine liable to be injured. 

 There are only two bearings on the axles, and they are inside the wheels. 

 .-Vny tendency towards depression iu the centre from the weight would be 

 counteracted Ijy the continual upward pressure, arising from blows received 

 by the flauneh of the wheels striking against the rails on curves, passing 

 crossings, lic. The bushes which the axles run in are fitted ijito the frame 

 in such a manner as to allow the springs to play vertically, but have flauches 

 which prevent any tendency to lateral action beyond that necessary for the 

 irregularities of the road, and they are of such a length as to enable them to 

 hold up the engine in case of the breakage of one of the axles. It would 

 appear that the breakage of the axles is a ver)- rare occurrence, and that even 

 when it has happened, the engines have performed the remainder of the jour- 

 ney and brought home the train with only a slight diminution of speed. The 

 engines differ in weight according to the class they belong to. A passenger 

 engine, with its coke and water in the fire-box and boiler, weighs 9 tons, 

 13 cwts. 1 qr. 



Tons. cwts. qrs. 

 The fore end .... 3 19 1 



The after end .... 5 17 2 

 A merchandize engine, with coke and water, weighs 11 tons, 13 cwts. 1 qr. 



Tons. cwts. qrs. 

 The fore end ...-541 



The after end .... 6 12 3 



This form of engine was adopted by the atithor as early as the year 1R2!>, 

 when he constructed the " Liverpool," which was the original model engine 

 with horizontal cylinders and cranked axles. It was set to work on the 

 Liverpool and Manchester Railway in July, 1830. This form of engine has 

 been invariably used on the London and Birmingham Railway since its 

 opeiuug. 



The paper is accompanied by complete drawings of the engines, and tabu- 

 lar statements of their performances during the year 1839, showing the 

 number of miles traversed by each engine, the weight conveyed, with the 

 cost in detail of coke, oil, tools, wages, repairs, and general charges. 



The performances of the engines extend over a distance of 700,000 miles, 

 and a period of 12 months ; and it appears that with the passenger engines. 

 For the first 6 months the average total 



cost of conveyance was 

 For the second months the average to- 

 tal cost was ..... 

 While with the merchandize engines — 

 For the first G months the average total 



cost was i^Mr '^i**''- 



And for the second G months the average 



was -nnnx '^'t*0- 



" Earth Falls at the Undercliff in the Isle of Wight." By William Rick- 

 man. 



The remarkable tract of coast called the " Undercliff" extends from the 

 south point of the Isle of Wight, nine miles to the eastward. Its surface is 

 distorted in fonn, somewhat resembling in miniature tlie volcanic features of 

 Southern Italy ; for although the latter has been formed by the action of fire, 

 and the former by that of water, botli have been moulded when in a state of 

 partial fluidity. The soil is of a boggy nature, is intersected with numerous 

 springs, and in it are imbedded, in the utmost confusion, detached masses of 

 the weather-worn cliff-rock, forming in places natural terraces on the face of 

 the clitf, and inclining inwards at different angles towards the land. 



A sectional view taken through the south point, bearing north to the sum-, 

 mit of St. Catherine's Down, would jiresent tliese features. 



From the sea beach of iron sand, strewed with shingle and boulders, rises 

 a cliff of GO feet, and from it a rugged and irregular ascent of 320 feet iu 

 height, half a mile in extent, composed of vegetable soil, chalk, green sand- 

 stone in masses an<l fragments, and of blue marie, the whole mingleil indis- 

 criminately and irri::rited by numerous springs. Thus much constitutes the 

 Underclift"! Above it appears the perpendicular, serrated profile of the Upper 

 Cliff, 260 feet in height, from which the surface of the Down jirocceds with 

 a slight descent for :i. quarter of a mile, and then gradually rises in the extent 

 of half a mile to a v.- tical height of 200 feet, being the highest land in the 

 island — 780 feet above the level of the sea. The strata are nearly horizontal, 

 with a slight dip to the north-east. They are the upper part of the secon- 

 dary or supcrmedial order, and consist of cliallc, chalk-stone, gi'een sand., 

 stone, blue marl, and iron or red sand. 



This stratification would account for the subsidences of the Cliff which 

 have occurred so repeatedly. The water collected by the extensive surface 

 of the Down would percolate througli the chalk and sand-stone beds until it 

 reached the impervious blue marlc, where it would accumulate until it finally 

 escaped by oozing out over the edge of the stratum, carrying with it portions 



of a penny per ton per mile. 



ditto. 



