1841.1 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



373 



MR. STEVENSON'S IMPROVEMENTS ON LEVELLING 

 INSTRUMENTS. 



S[R_Intlie minutes of the Institution of Civil Engineers wliich 

 were publislied in last montli's Journal, there wns a notice of my im- 

 provements on levelling instruments; but that notice was not correctly 

 given, and strangely enough omitted all mention of two of these im- 

 provements. I have therefore as a favour to request that you will 

 give the following correct account of these improvements a place in 

 your Journal, which may be considered to be the present acknowledged 

 organ of the profession. 



Lcrelling S/aJ'. — The improvements on the levelling staff is as fol- 

 lows : the first is the introduction into the staff with sliding vane of 

 an adjusting screw with clamp, by which, af'er the vane has been 

 brought within i or i inch of being correct (which is readily done 

 wil'Ahe hand), the vane is firmly clamped, and the final adjustment 

 up "V down is at once made with the screw. A similar plan, I have 

 since been informed by Mr. Manby, Secretary to the Institution of 

 Civil Engineers, was used by Capt. Lloyd (Roy. Soc. Trans.), and idso 

 by Mr. Bunt, while prosecuting their scientific researches; but Capt. 

 Lloyd's staff' was only 6 ft. long, and Mr. Bunt's 9 ft., being too short 

 for engineering use. The application of this arrangement to engi- 

 neering purposes I have found of great utility. The above adjusting 

 apparatus is fur the vane when on the lower half of the rod, wiiich is 

 32I ft. long. But when the level line (i. c. the optical axis of the 

 telescope produced) is above the lower half of the statf, the vane is 

 as in all rods with sliding vanes, first clamped at the top of the upper 

 half of the staft", wdiich is then pushed or slid along the face of the 

 lower half, until the vane is within ] or s inch of being correct, when 

 the upper half 0/ /he rod is clamped and finally adjusted up or down 

 by a clamp and screw apparatus fixed at the back of the lower half. 

 By this plan, although the vane may be far out of reach, the adjusting 

 apparatus never moves, being a fixture and within reach of the hand, 

 thus obviating the necessity of having a long adjusting wire fork for 

 working a capstan-headed vane screw, which, although perfectly 

 suitable fur Mr. Bunt's cp/imum levelli-ig would of course not answer 

 the long rods used by engineers. The next improvement is //le com- 

 hinations in one rod of all the advtintages of the sliding vane staff, with 

 vernier readings for accurate rcork, and the scf-readtng staff fur rough 

 norl;. This is effected by having the back of the rod graduated to 

 feet and decimals by means of inlaid bone figures which are read 

 through the telescope level. In tliis way I have not lost sight of Mr. 

 Gravatt's ingenious contrivance of self reading. In making a section 

 where it is necessary, in order to carry forward a correct result, to be 

 careful only with the first and last sights after and before moving the 

 instrument, the intermediate sights are read off the back of the staff 

 by the telescope. The above instrument, vvhicli is 122 feet long, and 

 may be said to consist of a sliding vane staff and a seif-reading staff, 

 is, by the contrivance of a late friend of mine, packed up in a box 

 3A feet long and 4 inches sc]uare ' 



Level. — 'I'lie first impruvement in the level is the fixture to the 

 telescope tube of a circular level sluggish in its motions, instead of the 

 comparatively delicate cross bubble of Mr. Gravatt's level. 



The Ui'^t improvement is the introduction of a ball and socket Joint 

 between tlie head and legs of the iiistniment, so as to have a motion 

 intermediate in fineness between that of the parallel |)l,ite screws and 

 that of the legs. Befure the introduction of the parallel plate screws, 

 1 understand that levels had a ball and socket motion instead of the 

 screws. The present improvement consists in restoring the ball and 

 socket for the rough setting, and at the same time retaining the parallel 

 plate screws for the Jinal adjustment. The clumsiness of the present 

 arrangement consists in the instrument being at all dependent on the 

 setting of the legs. With the instrument thus improved, the surveyor 

 is made quite independent of the level of the gn.und where he sets 

 the legs of his instrument, and may place them without regard to the 

 inclination of the telescope to the horizmi. Looking first to the cir- 

 cular level, and releasing the clamp of the ball and socket, he with 

 one hand moves the head of the instrument till the bubble is in the 

 centre of the circle, an operation which is done almost instantaneously. 

 The socket screw is then clamped, and the telescope bubble is brought 

 to the absolute level by a slight touch of the parallel plate screws. 

 In this way the legs of the instrument need never be moved after the 

 instrument has been pi, iced on the ground, and the parallel plate 

 screws have almost nothing to do, advantages which all who are 

 accustomed to levelling wdl fully appreciate. 1 may mention that the 

 price of altering my old level tu the new form was 1/., but 1 have no 

 doubt that in future the charge may be less. That the above im- 

 provements are well worthy the attentiun of the profession I am fully 

 assured, else i should nut have brought tbein forward. My own ex'- 



perience on liundreds of occasiuns, as well as that of others, has 

 placed any tiling like doubt completely aside, and with the ball and 

 socket joint I have been enabled on very many occasions lately to take 

 in sights with one setting of the instrument, that would otherwise 

 have required two, owing to very irregular and precipitous rocks, 

 vvhich did not afford snlficiently level rests for the legs to allow the 

 bubble to be brought right. 



I remain. Sir, 

 Little Ross Island Light House Your obedient servant, 



IForis, Kirkcudlirighl. Tho.mas St£V£nso.\. 



Oct. 11, ISU. 



ON THE ECONOMY OF FUEL IN LOCOMOTIVES CONSE- 

 QUENT TO EXPANSION AS PRODUCED BY THE 

 COVER OF THE SLIDE VALVE. 



Sir — ^The confidence of anonymous writers is sometimes exceed- 

 ingly amusing. For reasons best known to themselves they seem to 

 put but little value on the idea that they themselves may perhaps be 

 labouring under misapprehension; and such is singularly the case of 

 a gentleman signing himself M., the author of a letter in your October 

 number, containing some strictures on a paper of mure in your August 

 number, on the subject which is the title of this letter. This gentle- 

 man, Mr. M., having discovered, he says, some errors in my calcula- 

 tions, hopes you will allow him to point them out for the beriefit of 

 your readers; and he begins by premising that although some.formulac 

 of mine, which he points out, are correct, yet in two of them there are 

 expressions which are identical, and which, he says, might perhaps be 

 better expressed, now whether or not these expressions are identical, 

 any schoolboy can find out, and whether or not they are identical, 1 

 hope Mr. M. will have the generosity to allow me to suppose them 

 better stated in the form 1 have adopted. Mr. M. now proceeds to 

 state that the value of (s) (s being the area of the piston) is incorrect, 

 and that it is impossible to deduce the area of the piston from the 

 length of the stroke cover and lead of the slide, and ratio of the 

 greatest to the least pressure of the cylinder, without knowing how 

 much steam is generated in the boiler, if Mr. M. would have the 

 goodness to read over again a part of my paper, he would find it there 

 stated that in the mode of expansion virtually adopted in locomotives, 

 the cylinders are enlarged so as to consume the same quantity of steam, 

 (which is just the wdiole steam that the boiler can produce) ; and this 

 is the only way in which the question of advantage from expansion 

 can be treated ; now if Mr. M. would consider for a little he would see 

 that what I am about with the equation, 



a' |=2rfXl is finding wdiat area the piston 



must have so that the steam may be cut off at (a'), and yet the same 

 constant quantity of it consumed, and he would see that so far from 

 being able to give (s) any value we please, it can on the contrary have 

 only one definite and particular value, which is assigned by this equa- 

 tion, and so far from 2d X i being an arbitrary quantity, it is the very 

 quantity that properly represents the quantity of steam produced by 

 the boiler, or which is the same thing in the question under consider- 

 ation, the volume that the cylinder would have if no expansion took 

 place. 



Ml-. M. in the next paragraph leads me to doubt if he understands 

 the mode of analysis it is necessary to follow in estimating the work 



performed by an engine w orking expansively, for the expression — — I 



does not express the effective working pressure during the expansion, 

 and only at that part of expansion wliere the piston is at a distance 

 (iv) from the beginning of the stroke ; and how he has discovered that 

 this gives the effective working pressure 3 or 4 t6. per square inch too 

 much lie does not mention, and 1 am unable to find out. Mr. M. men- 

 tions that 1 make no alluwanee either for the diminution of tempera- 

 ture of the steam during the expansion, or the waste sjiace which has 

 tu be filled with steam. Perhaps Mr. M. for the benefit of your readers 

 will be kind enough to tell us liuwmuch the temperature of the steam 

 will be reduced in the hot climate of a locomotive's smoke box? 

 Would Mr. M. seriously propose to introduce into this analysis the 

 slight modification necessary for the waste space filled by the steam; 

 which waste space varies in all locomotives, and will yet be reduced 

 to almost nothing? Again, Mr. M. states that the effect during the 

 part of the stroke from llie opening of the eduction port the termina- 

 tion ot the stroke, must not be neglected on account of the small open- 

 ing of the port during that period, and that the pressure of the steaiu 



111 'ill 



