1850.J 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



293 



representing the usual form and arrangement of "a pile" of 

 "slabs" such as are employed in forming, when welded togetlier, 

 a mass of iron from which boiler plates or bars of iron are rolled. 

 Fig. 4. rejiresents such "a pile" of " slabs," which having been, as 

 is generally the case, produced under the action of a forge hammer 

 and anvil, having flat, or as is generally the case, slightly conve.v 

 surfaces, causes the slabs so produced to have certain hollow parts, 

 or slightly concave portions of their surfaces, so that when piled 

 one upon the other, as in fig. 4, the risk of having hollow spaces be- 

 tween is almost certain. The hollow spaces are represented in the 

 figure by the dark irregular lines between the slabs. 



Referring to fig. 4, a, b, c, d, represent a pile of four slabs laid on 

 the anvil welding hot ; owing to 

 the concave irregularities of the 

 surfaces, the parts most cei^tain 

 to come into contact first are ge- 

 nerally the exterior edges of the 

 slabs. The efi'ect of the blows 

 of the hammer is first to weld 

 the parts in natural contact; and 

 by continuance of the blows, the 

 interpiwiiig scoria or "cinder" is 

 expressi.?!, in a degree more or 

 less pcvt.' -tly, according to the 

 energy oi ihe blows and the deep- 

 ness of the convex or hollow 

 patches betwixt the slabs. So 

 long as there exists an exit or 

 passage for this scoria, all is 

 well; but, as generally happens, 



some portion of this scoria lurks behind after all chance of escape 

 is removed by the welding of the exterior portion of the surfaces 

 of the slabs. The result of this is, that we have to a certainty a 

 defect greater or less in amount, according to the quantity or sur- 

 face over which the inclosed scoria extends: once such scoria is 

 shut up between the surface of the slabs, no amount of after ham- 

 mering will ever expel it, but on the contrary, will only tend to its 

 extension over a larger surface; and, as before said, so long as 

 a particle of this scoria is left interposing, so have we a degree of 

 unsoundness in proportion. 



Great as this evil is, and common as it is as a fertile cause of 

 defective iron work, and the more especially so in the case of 

 boiler plates, the means of avoiding such source and cause of 

 defect is as simple as the results are important; and it is to be 

 hoped that the free and open communication which Mr. Nasmyth 

 has made of his views on this subject will be answered in the most 

 acceptable way by the general adoption of his improvement or 

 certain means of avoiding the occurrence and existence of all such 

 causes of defective boiler plates, and forged work generally ; w hicli 

 improvements consist simply in so fonniny the surfaces which we 

 desire to weld together that a free exit may be preserved to the last for 

 the escape of the molten oxyd or scoria until the entire surface of the 

 parts we desire to weld are thorouyhly incorporated by the welding pro- 

 perty, aided liy the action of the hammer or rolls, as the ca.^-e may be. 



In order to accomplish this most important and desirable object, 

 Mr. Nasmyth forms the surfaces of his slabs convex (see fig. 5); 

 by which most simple common- 

 sense-means a perfect free exit 

 to the scoria or interposing 

 impurity is maintained to tlie 

 last moment, the welding com- 

 mencing at the centre part of 

 contact w, and extending out- 

 wards towards the edges under 

 the action of the successive 

 blows of the hammer, or 

 squeeze of the rolls; but, as 

 before said, an open door is 

 kept for the escape of the 

 scoria until the surfaces unite 

 from the centre w to the outer 

 edge z, z, z, z. Here, then, by 

 an arrangement or formation 

 of the surfaces we desire to 

 weld, we have the most cer- 

 tain and simple means of pro- 

 curing a perfectly solid sound 

 mass of iron which, when '^' 



beaten, hammered, or rolled down to whatsoever thickness we 

 desire, vvill retain, to the last, all the qualities of the one sound 



solid mass we had converted it into hy this most simple improve- 

 ment — namely, giving to the surfaces we desire to weld a convex 

 form and relation to each other.— Mr. Nasmyth concluded his 

 observations on these important subjects by an earnest appeal to the 

 members of the Mechanical Section to diffuse, by all means in their 

 power, the information which, on this as on all such subjects, he 

 shall ever feel the highest pleasure in communicating to the prac- 

 tical men of his profession, and the world in general, who may 

 think fit to accept these results of an active life, which he finds so 

 much real pleasure in freely sharing with them. 



POWERS OF MINUTE VISION. 



On the Powers of Minute Vision. By Mr. W. Petrie. 

 Resulis from experiments for determining the best sort of 

 station-marks, and the errors liable, in observing with optical 

 instruments that measure on the principle of bringing tivo reflec- 

 tions together. The experiments were performed in bright day- 

 light (but not sunshine), being light of the maximum of advan- 

 tage for perceiving black against a white ground. The general 

 circumstances of the experiments were arranged rather to deter- 

 mine the facts of common practice, than the theoretic powers of 

 vision. 



Mr. Petrie then detailed the various distances at which circular 

 spots, lines, &c., white on black as well as black on white, could 

 be seen, the distances being given in terms of the breadth of the 

 object seen. An arrangement of lines was described, by which an 

 alteration of their position to the extent of only one millionth part 

 of the distance of the observer was made visible. One result of 

 the experiments would be to show what would be the proper pro- 

 portions of parts to be observed in forming letters to be read with 

 the greatest distinctness at a distance,— a subject of much practical 

 use in the present day, and admitting of a strictly scientific system, 

 although generally left to the fancy of incompetent persons. White 

 letters on a black ground should have their component lines of only 

 half the breadth that black letters should have on a white ground. 



The direction of the eye, while appearing to gaze steadily at any 

 object, does in reality keep wanderiny to an imperceptible distance 

 on every side of tlie object looked at, but very rapidly. This ican- 

 dering is not accidental or an imperfection of sight, but an essentinl 

 feature of vision; because it is not the continuance of an impres- 

 sion that is perceived (by any of the animal nerves), but its com- 

 mencement and termination, or, more strictly speaking, its in- 

 crease and decrease. This principle is probably analagous to that 

 by which a magnet creates an electric current in a neighbouring 

 wire, not by its constant presence, but by the increase or diminution 

 of its influence, either by a variation of its power, or of its position. 



This wandering propensity of the eye was shown to account for 

 the relative facility with which different sorts of marks were seen 

 at great distances : it takes place, apparently, in a minimum case, 

 to the extent of an angle of 1 in 2500. A dislocated line (as in a 

 vernier), its fault being half its breadth, can be perceived to be so 

 at a distance" of 10,000 times its fault, if black on a white ground ; 

 and at 12,000 times, if white on a black ground. It shows itself, 

 however, by giving the line a less steady appearance, than a per- 

 fectly even line would have, when narrowly watched, by running 

 the eye along the line, at about half as far again. 



Experiments were tlien described, on the visibility of the posi- 

 tions of the ends of lines, and of hiatuses in lines, and of square 

 dots as compared with round. But the last conclusion of practical 

 importance was in respect of observing the angular position of 

 station-marks, or of stars, by reflection, as in a sextant. From 

 these experiments it appeared that the position of two closely 

 adjacent dots or images, in sensible parallelism to a given direc- 

 tion, while it affords one of the simplest kinds of observation, is 

 more accurately observable than their actual coincidence, or even 

 tlian the junction of two lines, as if in a vernier. 



On the Gradual Subsidence of a Portion of the Surface of Chat 

 Moss, in Lancashire, by Drainage. By Mr. G. \V. Oumerod.— Tliis 

 was the continuation of a paper read at the Swansea Meeting. It 

 was shown by a series of levellings made in the last four years, 

 over an extent of about 200 acres, where drainage was carried on, 

 that a subsidence had taken place to the amount of one inch per 

 annum. 



