stance, gives a greater body to the leather, fills up more effectually and durably the minute interstices of the skin, and renders it water-tight from the first.

Two months is, perhaps, an early period, to give any decided opinion as as to the preservative effects of this composition when applied to leather; but the present appearances, together with my previous knowledge of the subject, leave no doubt in my mind of the correctness of Col. Maceroni's statements respecting it.

I remain, Sir,
Your's respectfully,

London, April 4, 1838.

WM. BADDELEY.

INVENTION OF THE CAMERA OBSCURA.

Sir, The camera obscura is now so well known, but still considered so curious and amusing, that some account of its early introduction may perhaps not be uninteresting. The invention of it has been claimed by two or three persons; it was ascribed by Vasari, to Leon Baptista Alberti, a celebrated architect of the 15th century, but there are the strongest grounds for attributing it to John Baptista Porta, who was born A.D. 1445, and was a Neapolitan; since he gives a very minute detail of it in his book entitled, "Magia naturalis, sive de miraculis rerum naturalium, libri 20. Jo. Baptista Porta auctore"-Speaking of the various effects of concave glasses he says " Before I have done speaking about the operations of this glass, I will tell you a use of it, which is very amusing and ingenious, and by which we may observe many curious natural phenomena; it is to see all things in the dark that are done outside in the sun, in their true colours. First, the room must be thoroughly darkened by closing up all the windows and crevices for if there be any light in the apartment, the effect of the experiment will be entirely spoiled. Make a hole, the breath and length of a person's hand; above this, place a little leaden or brass table and cover it with a thin coating of glue; make a round hole in the middle of it the size of a finger; opposite this let there be a wall of white paper or white linen, and by this means you will see all that is done

outside in the sun, those that walk in the streets like your antipodes; whatever is on the right will be seen on the left; all things will have changed their direction, and the further they are from the hole, the larger will they appear. If you bring your paper or white table nearer, they will look smaller, and more distinct, but you must wait a little, for strong similitudes sometimes cause affections in the. sense, which are often so great that they trouble the organs not only while the senses are acting but after they have ceased acting. For instance, if we walk in the sun, and then suddenly go into the shade, the sensation continues, so that we can hardly see, because the affection made by the light is still in our eyes, and when that is gone, we are then able to see clearly in shadowy places. I now mention what I have hitherto concealed, and what I had some thoughts of never revealing-if you put a small centicular crystal glass to the hole, you will perceive all the figures much clearer, the countenances of people walking, &c.'

The description here quoted is certainly sufficient to establish Porta's claim to the invention, since it appears that he speaks as if he had not the least idea of its being ever attempted before, and reveals it as something not previously known. It is very probable that the part of Leon Alberti's book on architecture referred to by Vasari, alludes only to an instrument for reducing drawings or views to smaller sizes, in their proper colours. Porta's description, however, is so exceedingly exact, that it is impossible to mistake the instrument he ineans, which answers precisely to the camera obscura now in use, having been materially improved by Gravesande, the Dutch philosopher. Your's, &c.

J. C. W.

MESSRS. SEAWARD'S PATENT SLIDE

VALVES.

Sir,-In your last number, for April 7th, I observe that Mr. Francis Humphrey lays claim to a priority of invention for the slide valves, which I have patented and used successfully these four years, without ever having had my claim for a moment disputed: he says that in the year 1832 he made a drawing which was

shewn to one or two individuals, and then put into a tin case (which he believes was unsealed); and that a moving diagram was exhibited which might have been inspected by fifty persons for aught he knows.

1 should not have deemed it necessary to take notice of the claim put forth by this gentleman as being also the inventor of these valves, had it not been for the inference intended to be drawn, from the tenour of his observations, regarding the originality of my idea; as well as for some misstatements which require refutation.

I believe, Mr. Editor, that nothing is now more common, directly an inventor has brought his views into successful practice, than for other persons to spring up and claim to have imagined and invented the whole before; although they have never either published or applied the invention,

As regards the observations made in reference to the 'Emerald's" engines, my foreman states that he never accompanied or asked Mr. Humphrey to go on board the "Emerald" at all: and what he states about the Lords of the Admiralty, Mr. Ewart, and Mr. Brunton, must exist in his own imagination only, as at the time he speaks of, none of those gentlemen had ever been on board that vessel.

As to the conversation he alleges to have had with me, I unequivocally deny that I ever had any conversation with him upon the subject of these valves; and I further declare that nine months previous to the time he mentions having viewed the "Emerald's" engines, I had applied for the patent, and it had been then sealed for some time and I must also observe, that the drawings of a set of slides from which those of the "Emerald's" engines were afterwards made, were prepared by me in the year 1831.

I have also by me a full description of this plan of arranging the working valves of steam-engines in my journal dated May, 1830: so much for Mr. H's originality.

I think it a little extraordinary that this gentleman should have waited till the present day to put forth his claim, when, according to his own showing, he has known of my successful application of the valves for so long a period; particularly after his "enthusiasm for the

invention had somewhat subsided, and he had been lead to devise the other valves of great simplicity, used on board the 'Wilberforce'."

In conclusion, Mr. Editor, I have only to remark that at the time I first entertained the idea of using slides in the way I have patented, I had never even heard of Mr. Humphrey, nor until this claim made by him, had I ever seen or heard of any arrangment of valves like them; either of his invention, or that of any other person whatever,;-the idea first originated with me when in Cornwall, twelve years ago, while fixing a 90 inch pumping engine for the late Mr. Woolf at the Weelalfred mine: where I had abundant opportunity of observing the great expence of manufacturing, as well as the difficulty of keeping tight the double beat lifting valves, used with that description of engine: had I remained in the county I should certainly have had these slide valves then tried upon that sort of engine, if I could have got an opportunity :-however, I am happy to say they are now likely to be applied there, as I have recently sent drawings and descriptions of them to an engine manufacturer in the county, who contemplates using them.

I am, Sir,

Your obedient servant, SAMUEL SEAWARD.

Canal Iron Works, Limehouse. April 11th, 1838.

PREMIUM OF TEN POUNDS FOR A MACHINE FOR WASHING AND MEASURING WINE BOTTLES.

We are authorised to offer a premium of Ten Pounds for the best design for a machine for the washing and measuring of wine bottles; the power to be used, either steam on a small scale, or horse. The great objection to the machines in common use are, that the bristles which are always employed do not clean port wine crusted bottles. It is required that the machine entitled to the premium should perfectly clean old crusted port wine bottles. The parties who offer the premium, have heard that there was a machine invented by a person in London, by which this desideratum was completely effected, and the bottles filled by their own weight. Designs to be sent

free of expense to the Editor of the Mechanics' Magazine, on or before the 14th of May, shortly after which the premium will be awarded.

EXPERIMENTS ON THE STRENGH OF CASTIRON BEAMS. BY JOHN U. RASTRICK, ESQ., C. E.

[Extracted from Report by Mr. Rastrick, to the Architects employed to examine the construction of Buckingham Palace.]

On my arrival in town, I found that the method of putting the cast-iron beams in the floors to a satisfactory proof, had particularly engaged your attention, and that it had been proposed to effect such proof by loading a carriage with a sufficient weight, and running it leisurely over the centre of each floor at right angles, to the beams; and if the floors stood this proof without failure, or any indication of weakness, it might be concluded they were perfectly secure.

On discussing this point, we were soon aware this could not be so easily effected; in the first place, it would have been necessary to have had a carriage constructed on purpose, capable of carrying twenty tons; and this weight, to produce the desired proof, must all have been concentrated in a small compass; and as the wheels of the carriage would have broken in the boarding of the floors, unless strong beams had been laid to have run the wheels upon, which beams would have taken a bearing upon not less than three cast-iron beams, it would have become necessary to have increased the weight in proportion, which would have been quite impracticable; add to which, as the weight would have been brought nearly all at once upon the beam, it would, if defective, have given way instantly, and, even with strong framing constructed under the floor, any sudden fracture might have been attended with the most serious consequences. Several other methods had also been proposed, as loading the floors with an accumulating weight of materials, such as sand, bricks, &c.; but by these methods we could not have so well decided upon the nature and quality of the castings, as it would have been difficult to have discovered what the deflection was, and it was at all events desirable that we should prove some of the beams to determine this point.

After further consultation, it was ultimately agreed, that the most effectual proof would be, to place the weight necessary for the proof in a scale suspended from the centre of the beam to be proved, guarding

the agents and workmen employed below during the proof from any risk, by having substantial wood frames covered by strong planks set up on the floor below; this method of proof had this further advantage, that as the weight would be put on gradually, it would give us the opportunity of ascertaining the deflection of the beam at any stage of the proof; should there be any appearance of failure, we could stop short, and even should the beam give way suddenly, the scale being hung within an inch of the wood wedges resting on the floor, no damage would

ensue.

Having given orders for the iron-work for the scale (which was executed by Mr. Bramah, in the most satisfactory manner), while the carpenters were preparing the timber-work, I took the opportunity of examining some of the cast-iron beams, and, having discovered some defective ones in the ground-floor, of the bow room, I selected them for trial; but as the floor was laid over these beams, and they were arched between with brick coombs, it would have been impossible to have made any satisfactory proof of any one beam under these circumstances; having, therefore, applied and obtained your authority, I ordered so much of the floor to be taken up, and three of the coombings to be removed, as to set the beams completely at liberty.

The weights that were put on the scale were pigs of lead; I had them numbered from No. 1, upwards; they were accurately weighed, and the number and weight marked on each; the weight of the scale and ironwork complete, was found to be exactly one ton; a table was formed by adding the weight of the pig, No. 1, to the weight of the scale, and the weight of every other pig was added successively thereto; and as the weights were placed in the scale according to their numbers, this table gave us the total weight on the beam without further calculation, as every pig was put in the scale.

At a distance of ten or twelve feet on each side from the centre of the beam, was fixed an upright or standard, between which was stretched a fine silken line, at right angles to and over the centre of the beam; a diagonal scale of inches, divided into hundredths, was fixed to the beam itself, and adjusted to correspond with an even inch with the silken line.

I shall here state the data from which I determined the weight that this beam in question, or any other, should be proved to.

In trying experiments on cast-iron, I have found that bars one inch square and twelve inches long, supported at each end, and weighted in the middle, have taken from 2,180 to 2,600 pounds to break them, ac

cording to the quality of the iron, and some bars much more. In making such small castings as these experimented upon, you can always ensure a pretty sound casting; whereas, in making large castings, you cannot depend on its being perfectly sound throughout; neither is the iron itself so strong, the grain being generally larger and

coarser.

It has therefore always been my practice, as a general average of cast-iron, to take one ton as my datum for the weight that will break a bar one inches square and one foot long, supported and weighted as above, exclusive of the effect of the weight of the bar itself in producing the fracture.

Now, although it would have been more scientific to have taken a higher number or greater weight as a datum, and considered the weight of the beam itself as a part of the weight with which it was loaded, yet this condition would have involved two considerations; for it would always have been necessary to have found the weight of the beam itself, and subtracted that portion of its weight which contributed to its load, from the calculated weight which would produce fracture before the precise weight to be applied could be determined. I therefore chose the lower number, or less weight, as we practical men wish to arrive at our conclusions with the least trouble, and by the shortest rules.

I have, however, had such extensive practice in the use of cast-iron, and had so many opportunities of breaking cast-iron beams of all sizes, from a load of one ton to twenty, that I now place the most implicit confidence in my data, having found them, on large beams in general, true to less than the twentieth of the calculated weight; and more frequently the calculation has overrated the weight that produced fracture, than come under it.

It has also been my practice, as it is of every prudent and experienced engineer, never to expose a cast-iron beam, under any circumstances, to more than one-sixth of the weight that would break it; and, from calculations I have occasionally made, on castings made by other engineers, I find it accords very nearly with their general practice.

With this object in view, and to ensure security, I always prove every casting to double the weight that I intend there should ever come upon it; for I wish it to be distinctly understood, that I consider no casting safe till it has undergone a satisfactory proof; for a casting shall appear perfect and sound on the surface, yet air-holes and such like imperfections may be within,

and nothing but a severe proof can detect theni.

Having calculated the weight applied at the centre that will break the beam or casting to be proved, I take one-third, of that weight, and apply it to the centre of the beam, and while this weight is upon it, I have the beam forcibly struck all over its upper edge with a sledge-hammer, and afterwards all over its flat side with moderate blows by the same hammer. Previous to applying the weight, we strike a chalk line upon the beam, and after the weight has been applied, and the beam well hammered, we apply the chalk like again, and find what deflection has been produced by the weight. The weight is then suffered to remain upon the beam for a short time, and if the deflection does not increase, the weight is removed, and the chalk line applied again, and then, if the beam has recovered its deflection, we conclude it is perfect enough for the intended purpose, and it is accordingly sent off to its destination.

Should the beam, however, have not recovered its deflection, we conclude it has some imperfections; and it is again put to a more severe trial by hammering with the same weight upon it; and if the deflection does not increase, the beam is passed, but if the deflection does increase, the proof is continued, by adding more weight, and well hammering, till it gives way; and I seldom remember an instance of a beam giving way under such circumstances that did not turn out to have a material defect in it.

With regard to the weight of the hammer with which the beam is struck, it has been in general discretionary; but the workmen who have made the proof for me have never been very nice about it, always taking the first hammer that came to hand, and not being very ceremonious about the use of it, concluding, according to my observations to them, that it is much better that the beam should be broken in the proof, if imperfect, than be sent off, and found defective afterwards

A good general rule may be, to have the weight of the hammer one pound for every ton, applied as a proof.

The deflection which one-third of the breaking weight occasions to a beam, varies from one six-hundredth to one five-hundredth of its length, according to the quality of the iron; but as a ready means of calculation for the proof-weight, I take the one five-hundredth of the length, which I find near enough for all practical purposes, observing that the deflection is directly as the weight. When a beam is loaded with about one-half its breaking weight, permanent deflection begins to take place, consequently

a beam should never be loaded to such an

excess.

I need hardly observe to you, gentlemen, that the strength of any beam is directly as the square of the depth and thickness, and inversely as the length, a truth you are no doubt fully acquainted with; but I thought it best to insert it, that the report on this subject might be complete.

The proving of beams of large dimensions is a very tedious and laborious operation, and was attended with great expense, till, having so much proving to do, I contrived a method by which the business was performed with certainty and despatch, and at a comparatively trifling expense One of your association (Mr. Smirke) has seen the apparatus, and witnessed a proof, and can speak as to the efficacy of it..

I have stated, that I never exposed a beam when applied to the purpose for which it is intended to a greater load than one-sixth of its breaking weight, but that I always prove it to twice that weight; take an example: suppose I find by calculation that the strength of the given beam, or the weight that would break it, was fifteen tons; I then apply five tons on its centre as a proof: now five tons on the centre, is equal to ten tons uniformly distributed over the whole beam; but the load is to be equal to 5 or 2 tons applied at the centre, which would be equal to five tons uniformly distributed over the beam; consequently, the beam having been proved with five tons on its centre, will be equal to double the weight that should ever come upon it when the load is not more than onesixth of its breaking strength.

Having now given you a detailed account of my system of proof, and the several reasons thereof, which I considered as essentially necessary, that you might duly appreciate my proceedings, and also, that you might the better be enabled to draw your own conclusion on the following results, and further, that you might be assured I did not proceed at random, but that there was reason and method in what I did ;

Everything having been prepared as before described, I measured the beam we were about to prove, being the first from the centre beam on the east side of it, and I found the distance between the walls on which it rested, 35 feet, from which I calculated that it would require 47 tons 17 cwt. 1 qr. 17lbs. to break it; and consequently, the proof having to be one-third of this weight, there would have to be applied 15 tons 19 cwt. 15lbs. at the centre of the beam.

All parties present being now prepared, we commenced at twelve o'clock at noon to

put on the weights, taking the deflections at certain intervals (the particulars of which will be given in a table after the trial of the second beam, as one table will answer for both, and shew the effects on each at one view.) The operation was performed with as much dispatch as was possible, only just allowing time to take the various deflections; nevertheless it was a quarter past three o'clock by the time we had got on 15 tons 13 cwt. 1 qr. 16lbs. (the time employed being three hours and a quarter); when a question being raised whether there was not already more than double the weight upon it that could ever be brought upon the beam, I agreed to let the weight remain upon it all night, and decide this question in the morning; the deflection was now 9 of an inch; the doors were locked, and every thing made secure, and the weight left to hang upon it all night.

The same parties that met yesterday having assembled again this morning, the beam being examined the deflection was found to be exactly the same as when we left it last night; the weight was removed, and the deflections noted with the same weights at those they were taken at in putting the weights on, when the whole of the weight was removed, and the scale taken of; the beam regained its original position within one-tenth of an inch. It was however found, that on applying the chalk-line to a line that had been struck on the beam, before beginning the proof yesterday, that it exactly corresponded, so that it was evident no permanent deflection had taken place; and the apparent deflection, as indicated by the diagonal scale, was accounted for, by supposing the weight to have settled the beam one-tenth of an inch closer, and more firmly down upon the wall.

The question now came to be decided, whether or not I had proved the beam to double the weight that could ever be brought upon it (for this being a very defective casting, I was desirous of proceeding to prove it up to twenty tons); and if so, whether it was advisable to proceed to such an extremity.

The principal difference amongst us was as to the weight that a dense mass of people would bring upon the floor: to decide this point, I had a square of ten feet struck out upon the floor, and by getting as many of the workmen together, upon a portion of it, as could stand close wedged together, it appeared that seventy people might stand upon a square of an hundred superficial feet; this gave 143, or about 1 superficial feet for each person; ten of the men taken promiscuously were weighed, and found to be 13