of Procyon is less than 90° since it has already risen. It is not till after b that it becomes necessary to produce D Z'".

No one can be so dull, as not to see at once that the latter part of the above quoted observations, as well as the first, was intended for the direct purpose of proving that O. N. was in error in the answer he gave to my question, and I must say, that a more disengenuous mode of going to work I have never seen (and I hope I shall never see again,) used by any of the scientific writers in the Mechanics' Magazine. Now what is the truth? Why simply this, that all these observations of Nautilus only hold

true within the limits of the two latitudes 57..59.10 N, and 57 .. 22..32 N. Now in O. N.'s solution of his own question, the lat. of the place was not within these limits, as it happened to be 55.. 58 N. So that Nautilus in his vain attempts to prove that O. N. was in error has been under the sad necessity of contradicting himself. This is really too bad.

Had Nautilus attended to the general principles demonstrated by O. N. (732), he would have discovered that all the information which he fancies he has given us (751), are only simple deductions, which necessarily follow from O. N.'s premises; the only thing new that I can discover, is in giving the lesser latitude 57.. 22.. 32, and this is found at once by calculating PZ on the supposition that D Z' is 90°.

But in the teeth of all this, Nautilus, in his last letter, now pretends to say, "that in the latitude of Edinburgh he had never denied that the perpendicular PN must fall upon DZ' produced," and this information he gave a week previously to the date of my last letter. Well, suppose I should be inclined to admit this (which I do not) to be true, then I would insist upon Nautilus telling us what slips ON in reference to my question had really made, and this Nautilus might have found would have been only leaping out of the frying pan into the fire.

In making the above remarks, Mr. Editor, I have been obliged to associate O. N. with myself, for this plain reason, that had he been wrong I must have been so too. O. N., I see from his last letter (761), intends to reply to that of Nautilus (758); he may do so, although I should imagine that both gentlemen

have already nearly exhausted all the information they can give us on that subject.

I am, Mr. Editor, yours, &c.
IVER MCIVER.

PATENT WARMING-PAN.

Sir,-Some time ago, Col. Maceroni in one of his ordinary de omnibus rebus communications, mentioned that in 1814 he had caused to be made by an ironmonger in Bishopgate-street, a warming-pan, which was to be heated by containing hot water. One would have thought that this would have been the natural gradation from the more primitive plan of warming a bed with a bottle of boiling water: but not so, the simplest plans, sometimes leading to the grandest results, are often overlooked. This was evidently the case with our forefathers in the instance now under consideration; they skipped from the hot-water bottle to the burning-coal pan, leaving the honour of the invention of the hot-water warming-pan to the present enlightened generation.

Col. M. does not appear to have been aware that the invention which he thought of so little importance as to throw out as a mere hint for the benefit of a thankless public, had been the subject of a patent. Such, however, is the fact; for, be it known, that on the 24th of May 1834, his late Majesty's royal letters patent were granted to Stephen Hawkins, of Milton House, in the county of Hampshire, gentleman, for" certain improvements in warming-pans or apparatus for warming beds and other purposes." It should be observed that this grand discovery was made by a gentleman, enjoying probably the otium cum dignitate, warm in the pursuit of science, ardent in his researches into the economy of nature. It is most likely the invention was the result of much study, and that it was not a mere discovery, fallen upon by chance. Mr. Hawkins, is therefore, deserving of the greater honour. From the specification enrolled in his Majesty's High Court of Chancery, pursuant to the proviso in the patent, I have taken the following particulars of this invention. The patentee proposes that a pan should be made of copper or other suitable metal, and which he represents in an accompanying drawing as

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.

Jo.

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. 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