to think, attach some importance to the circumstance that an appreciable time should be allowed between each stroke of the engine for the purpose of effecting a more complete condensation in the cylinder.

I hastened to point out more clearly than in my former letter, that no argument against "Scalpel's" views could be founded on the slow stroke of the Cornish stamping engine-fearing, as the result proved, that the limitation, that the "subject was only brought forward for the purpose of pointing out the propriety of strict definition of the class of rotative engines," might be insufficient.

Some doubt did exist in my mind, whether the Cornish engineers had not derived their method of condensation from Watt, as they succeeded to the care of his engines after 1800. Holding opinions similar to those of "Scalpel" respecting Watt's knowledge of condensation, as derived from his original and most beautiful experiments on a small scale, and subsequently proved on the largest scale, I fancied the advantage of such a plan could not have escaped his notice; especially as slow speed, so often required in mining engines, must from the earliest period have afforded an opportunity for the trial of this plan, by means of the cataract employed for the regulation of the number of strokes per minute. If it should be proved, (and the fact seems admitted,) that Watt was unacquainted with this expedient for the improvement of cylinder condensation, perhaps an interest may be felt in ascertaining to whom among the mining engineers the improvement may be attributed, and when and where it was effected.

In regard to "Scalpel's" first deduction, though the meaning is obvious, yet the propriety of the term "duty," as derived from indicators attached to rotative engines, when contrasted with the duty of lifting engines, derived from a different cause, the weight of water raised exclusive of pump friction, seems doubtful, (see page 51, vol. iii., Trans. of Inst. C.E., Mr. Parkes's observations on Steam Boilers.)

The indicator shows the steam pressure exerted on the piston, and by making allowances for friction, &c., the effective steam pressure may be calculated by the comparison of rotative

engines with each other; but confusion will inevitably arise, unless the distinction between duty and effective power is kept up. The work performed, or duty of stamping engines, is obtained. directly from the weights lifted, and perhaps in some of the recent engines it may have been increased by the omission of a portion of the gearing for the communication of the power.

In the second deduction, it is stated, "that Mr. Watt's rotative engines consumed in 1787 an average of 8 lbs. of best coals per nominal horse power per hour. That the actual power was from one-half to two-thirds greater than they were rated at. The steam pressure was nearly always 2lbs. less than the atmosphere, and never used expansively."

Now as the atmosphere at 30 inches of mercury, is nearly 14 lbs., a deduction of nearly always 2lbs. will give a mean steam pressure on the piston of 12.75 lbs. The cylinder vacuum is subsequently shown to vary from 9 to 11 lbs. at this period; hence, the resistances ofthe uncondensed steam would be from 5.75 lbs. to 3.75 lbs. per square inch; and further, deducting the friction of the engine itself, it appears to me that the mean effective steam pressure must have nearly always coincided with 7 lbs. steam pressure, assumed by Watt for the basis of nominal horse power, a view that might be supported by a reference to Watt's well known accuracy.

At present, steam of less than atmospheric mean pressure is seldom used in rotative, especially marine engines; hence, the 8lbs. of best coal per nominal horse power per hour, in 1787, seems to have represented a smaller amount of actual power than the 8lbs. now consumed, (as I have always understood in non-expansive engines ;) a few of the larger marine engines use expansion, when their power requires to be reduced, and steam tugs with higher steam, and the stamping engines in Cornwall are obviously exceptions. When 3lbs. steam above atmosphere was used, then from onehalf to two-thirds more than the nominal power was obtained; this however is clearly stated to have been the exception, as above shown.

The amount of expansion should be stated, since the theoretical difference between cutting off the steam at one-sixth and one-half is as great as

between full pressure, and steam cut off at stroke; on the whole, I think the proofs extremely defective, that rotative engines of the present day are inferior to those erected by Watt.

Watt's views I have no doubt will be confirmed whenever accurate experiments are again made relative to the amount of loss due to a less perfect vacuum, in comparison with the greater expenditure of power to raise the larger quantity of injection water, and the coal expended in raising the temperature of the water from the hot well, when thrown into the boiler. As a rough approximation, 10 degrees of heat require aboutth of the coal, for the conversion of water into steam. I would rather urge reference to such experiments than decide on modern performances, by Watt's opinions.

I am not aware of any difference of opinion on principles between myself and "Scalpel"-difference of values to be assigned to averages can sometimes be arranged by explanations, when parties are desirous of the truth, and are not disposed to urge arguments too far. At present there seems a disposition to account for the high duty of the Cornish engines by bringing forward some favourite cause, instead of attributing it to the minute attention which the Cornish engineers pay to every available source of improvement, occasioned by a continued rivalry with each other, and a degree of publicity unknown elsewhere. Mr. Baddeley refers their superiority to the variation of the power required to move and continue motion in heavy bodies (probably Columb's experiments) -Mr. Parkes to percussion-and "Scalpel" to condensation.

Two advocates have recently joined Scalpel's cause-Mr. James Pilbrow, who has proposed an improved engine to realise similar advantages, and Mr. Boyman, who has written a pamphlet in its support.

As many persons may be unacquainted with the method of converting the 8 lbs. of coal per horse power per hour into the form commonly used in Cornwall, and as clearer ideas are obtained from figures so arranged as to be capable of direct comparison, I have submitted the following, more with a view of calling attention to this part of the subject, than of founding any decisive argument thereon.

The first difficulty is the present Cornish bushel of 94 lbs., arising, I believe, from the use of the imperial bushel of damp Swansea coal. At Herland, doubtless the Winchester bushel was used. Newcastle coal was taken at 81 lbs. the Winchester bushel; the Welsh coal always employed in Cornwall is heavier. I believe it will be in Watt's favour if the best performance in pumping was taken at 30 millions per 94lbs. of Welsh coal in 1798. Now as one horse power per hour is 1,980,000lbs. 1 foot high, 94 lbs. would give 23,265,000 pounds one foot high, provided the actual and nominal power coincided; but at one-half greater it would become 35,897,000lbs., and at two-thirds greater

38,775,000 lbs., subject to some variable deductions for pump friction, &c., to reduce the power derived from the effective steam pressure to the duty derived from the weight of water raised.

I am not as yet prepared to admit, that the performance of Watt's rotative engines equalled that of the lifting engine at Herland, (especially as expansion was in all probability employed to some extent,) yet I do not perceive how this conclusion can be avoided, if the positions laid down by "Scalpel" are proved to be correct, and no error has crept into the estimate of actual horse power exerted, in particular instances assumed as an average. Sir, I remain,

Your obedient servant,

July 1, 1841.

S.

PILBROW'S CONDENSING CYLINDER
STEAM-ENGINE.

Sir,-Permit me through the medium of your pages to put a question to Mr. Pilbrow, the inventor of the Patent Condensing Cylinder Steam Engine. I wish to know upon what grounds Mr. Pilbrow considers he shall gain an additional pressure per square inch, in consequence of the working piston in his condensing cylinder preventing that. accumulation of air and gas which is incident to the common condensers? He assumes that such will be the case, but I think he is in error; for if 1lb. (2 inches of mercury) of the resistance in the condensers of steam engines, arising from uncondensed vapour, consists of air or other fixed

gases, then it appears to me it would be impossible ever to obtain a more complete vacuum by injection than is indicated by a column of mercury of 274 inches, taking the average standard of the barometer in this country at 29 inch.

I am, Sir, your obedient servant,
MACHINATOR.

London, June 7, 1841.

THE THAMES STEAMERS.

Sir, I have read in your very useful and highly-esteemed Work of the 26th instant, an interesting account of some of the fast-going steamers on the Thames, by "Nautilus;" but to do full justice to the distinguished engineers who have constructed the engines of these remarkable boats, I must dissent from the assumption of "Nautilus"that the Railway has greater speed than her sister vessel, the Blackwall; which question at present cannot satisfactorily be decided, as they have not yet made a voyage at the same time; one making the up voyage, whilst the other is going the down one, and, if we may infer from the time the several voyages have been made by each of these boats, we may fairly assume that the speed of the two is equal. Or, if we may judge by a voyage which I lately made from Blackwall to Gravesend, on the 11th instant, the preference in speed is due to the Blackwall. This vessel left the pier at the Railway terminus at 20′ 30′′ past eight in the evening, and arrived opposite the church at Gravesend, 19' past nine, having made the voyage in 584 minutes, which is, I believe, the shortest time the passage was ever made by any steamer on the River Thames. "Nautilus" also is in error, in stating that each of these boats is fitted with a pair of 90-horse engines: the Railway has a pair of 50-horse engines by Messrs. Penn and son, and the Blackwall has a single 90, by Messrs. Miller, Ravenhill and Co. I fully agree with him in giving due praise to the performances of the Ruby, which boat has been lengthened since last season, and received new boilers, and I believe some changes and additions to her machinery, and certainly does great credit to those able engineers, Messrs. Seaward and Capel, who constructed the engines. It may also be presumed, that when the engines of

the Brunswick, by the same makers, have attained their full speed (which they will soon do,) she will equal her rivals in the quickness of her passages.

By inserting this in your widely-circulated Journal, you will oblige,

Sir, a Constant Reader,

JUSTICE.

P. S. Since writing the preceding, I think it but right to inform you that I have received some particulars with respect to the Ruby steamer, of which I was quite ignorant before, and which also account for her great increase of speed this summer. The Ruby, I find, before commencing this present season, was lengthened 15 feet, and made considerably lighter by having her boilers removed, and those of a much lighter and peculiar kind (being tubular) put in their place by Mr. Seaward. These alterations have had the desired effect, and the Ruby is now almost equal to any steamer on the Thames in speed, and only the other day completed the passage from London to Gravesend in one hour and twenty-five minutes. The Diamond Company, I understand, are likely to make the same alterations in one or two more of their boats.

I should think Mr. Napier, when he hears this, will not be so ready to match the Fire King against the Ruby as he was last year.

July 2, 1841.

IMPROVEMENTS IN PIANO-FORTES.

Sir,-Among the many attempts to improve that most universal of musical instruments, the piano-forte, which have been offered to public notice, none have excited the attention of musical and scientific persons more than the application of equal tension to strings of equal size, for which improvement we are indebted to John Isaac Hawkins, Esq., who applied it to the upper half of a very ingeniously constructed instrument, manufactured by him many years ago. It has since been carried throughout the entire compass, by Mr. Wornum, whose name is honourably associated with many other improvements in this instrument; and this construction has the very great advantage of standing in tune better than any other; but this advantage is obtained at the expense of the under-mentioned

defect, which in the humble opinion of the writer, more than counterbalances its advantages.

In selecting the strings for a musical instrument, especially one having a free vibration, as the harp or piano, it will readily appear some proportion between their length and thickness should be observed, for if we employ wire of the same diameter for a string an inch and a half long, as we use for one of four feet in length, it is obvious the stiffness of the short string must immensely exceed that of the long one, the effect of which, as might be expected, is either want of vibration in the short string, or if that be thin enough to vibrate well, want of firmness and fulness of tone in the long one.

In instruments which have equal tension applied to strings of a considerable difference of size, a yet greater defect results from deficiency of the tension of the bass strings which are not covered; nor do the different parts of the instrument remain in tune with each other, for although the unisons may stand in tune, the octaves soon become false.

The above observations do not apply with equal force to stringed instruments such as the bass-viol, whose sounds are sustained by friction, applied directly or indirectly to their strings; but even in these, difference in their size is indispensable to an equal quality of tone, as is evidenced by putting a gut string of the same size as the first string of a violin on to a double bass, subjecting it to the same tension, when a very thin feeble note results, far inferior to that yielded by it on the violin. The conditions of sound in a musical pipe vary so much from those accompanying the vibrations of strings, that any analogy derived from them might be considered irrelevant; but from the necessary increase of the diameter of organ pipes towards the base to preserve an equal quality of tone throughout the compass of that instrument, and the fact of the absence of that quality of tone in tube instruments of nearly equal bore, we might be almost justified in concluding that there is in all sounding bodies some proportion between the length and bulk, which is most practically advantageous. Sir, I remain, respectfully, 16, Garlick Hill. ALFRED SAVAGE.

P.S. The writer of the foregoing observations can suggest some improvements in the construction of piano-fortes and other stringed instruments, which if it is consistent with the plan and objects of your Journal, Mr. Editor, to receive, he may at a future time send for your acceptance. [We shall be happy to receive an account of them.-ED. M. M.]

WIRE GAUGES.

Sir, From the nature of the business in which I am engaged, I have frequently occasion to use the wire gauge, to ascertain the thickness of sheets of metal, the same number being used to express the thickness of sheets of iron and the diameter of wire. Great difficulty is experienced very frequently in correctly ascertaining the number of thin sheets, such as are thinner than No. 24, which is, I think, about th of an inch. The wire gauge, as I dare say is well known to your readers, consists of a plate of steel having notches cut in the edge numbered from 1 to 40, beginning with the largest size. My object in troubling you, is to ascertain, if possible, by what rule these notches are made, as it seems to me that there is no regularity in the sizes of them, and the gauge makers with whom I am acquainted only copy from each other.

By inserting this in your valuable Journal, you will confer a favour on, Yours very respectfully, Wolverhampton, June 15, 1841.

MENSURATOR.

PROPOSITION IN PROJECTION. Sir, I am not aware whether the following proposition, or the analytical demonstration which accompanies it, is given in any work upon projection: it has reference to a case of every-day occurrence in mechanical drawing, and is at your service if you think it worthy a place in your Magazine.

I am, Sir, yours very obediently, WILLIAM POLE. A. S. C. E., Mem. Math. Soc. 85, Great Russell-street, Bloomsbury, March 2, 1841.

Proposition.

In the orthographic projection of a cylinder having another cylinder of