ceding from one of the members of the Institution; but the individuals who manage its concerns are so apt to take up in a hostile spirit every exception taken to their proceedings, no matter how honest or well-intentioned, that, for the sake of avoiding controversy, we must pass over many things in his letter that would surely lead to it, and confine ourselves to an extract or two, which seems to us to admit of no question.

"A vote of thanks," says our Correspondent, "to the Lecturers, did not appear to be among the resolutions prepared for the adoption of the meeting; and on an officious member's proposing to supply the omission, a retiring member of the Committee replied, that the Lecturers now stood upon a different footing to what they formerly did'-meaning thereby, as we all understood, that they are now paid for their services. I was rejoiced to learn this, and thought, at the time, how much pleasure it would give you, Sir, to find, that a point for which you contended so strenuously, has been conceded at last. I must confess, however, that I was, and am, at a loss to perceive why the paying of the Lecturers should so entirely deprive them of a claim to our thanks."

"The Report spoke of the prosperous state of our finances, and of our highest hopes having been realized; while, at the same time, it was confessed that we are 37001. in debt to Dr. Birkbeck (beside a large debt of gratitude), and that the number of lectures is reduced from two or three to one a week. Precious prosperity this! Bright realities, indeed! But for the Elementary Schools, which are excellent, and the Library, the benefits we have to boast of would be but few."

"It is not to be denied, Sir, that the Institution, on the whole, is doing better than it did; but it would be more consistent with candour and fair dealing to say less of the past, and take credit for the present only."

373

OBJECTIONS TO MR. J. BUTTERS' TELEGRAPH.

1st. The expense and difficulty of putting down the pipe.

2ndly. The utter impossibility of finding the defective part in case of a leak, and the total interruption of a communication.

3rdly. The difficulty of adjusting the indices in the first instance.

4thly. I think that, perhaps, the great distance of one index from, and the proximity of the other to the pump, would admit of great difference in the density of the air at the two places, and thus one index would show one thing and the other something quite different, to say nothing of the difference of temperature at the two places, and thus wrong orders would be given and endless confusion arise. Mr. Butters sets out with a wrong supposition, viz. that Portsmouth and London are on the same level.

I remain, Sir, Your obedient servant,

POTATO-GRINDING MILLS.

SIR, A Correspondent in No. 65, vol. 111. of your valuable publication, requests a description of the mill used by bakers for the purpose of led into the belief of the reality of grinding potatoes. He was probably such a machine by the following barefaced assertion in 'Cobbett's Cottage Economy:'

"It is a notorious fact that the bakers, in London at least, have mills wherein to grind their potatoes, so large is the scale upon which they use that material."

Now, Sir, I beg to say that I have inquired of the principal machinists in London respecting these aforesaid mills, and have been assured by them that none ever were constructed for the use of bakers. So much for Cobbett's notorious fact!

I am, Sir,

Your obedient servant, ARGUS.

SIR,-I am one of a party whom your Magazine has induced into a habit of mathematical disquisition. We frequently have disputes; and although, as we have read veritus in puteo, yet, notwithstanding, some one among us has been able to fetch it out, without having, as yet, had recourse to what we consider the weekly dip of the master-bucket. At present we are troubled with two refractory members on a subject in dispute; and although they admit we (the majority) have dipped our buckets in the right well, yet the water appears so muddy to them, that they give us credit for stirring up the sediment without having obtained the object of our dip. But to the point, as simply as possible. Can a straight line touch the circumference of a circle in any POINT, so as to be at a less angle than 90 degrees, with the radius drawn from that point to the centre? We think it can; inasmuch as the deviation of the curve line from the perpendicular, AB, must be infinitely divisible, as the diagonal AC may be drawn so as to leave the perpendicular AB, in any imaginable distance, at a less angle than the curve line; and as A is a mathematical point from which it is palpably evident the curve line, the diagonal, and the perpendicular, may diverge.

BY A. B. QUINBY.

From Professor Silliman's American Journal of Science and Arts.

The following is the substance of a paper communicated by the author to the Committee appointed, by the Literary and Philosophical Society of the City of New York, to investigate the causes which gave rise to the explosion of the boiler on board the steam-boat Etna:

To determine the comparative eligibility of the high and the low pressure steam-engine, the two following things appear to me necessary to be cousidered. First, the liability of each engine to explode; and, secondly, the danger or injury which each engine is capable of producing in case an explosion takes place.

To determine the comparative liability of the two engines to explode, it will be necessary to consider the four following things:-The diameters of the boilers used in the two engines, the elastic force of the steam in each boiler, the tenacity of the metal of which the boilers are composed, and the thickness of each boiler. M

375

As, however, the cylinder which constitutes the boiler is not solid metal, but is composed of plates riveted together, it will be necessary to diminish the number which expresses the tenacity.

Let, therefore, the tenacity be put at 30,000lbs. in place of 60,000.

The thickness of the boiler in the Ætna was of an inch, and the thickness of a low pressure boiler for an engine of equal power would be about of an inch.

From these data it is easy to calculate the comparative liability of the two engines to explode; for, by Mechanics, the force of steam which a high pressure boiler 30 inches in diameter, and of an inch thick, is capable of resisting, is equal to the thickness multiplied by the tenacity of the metal, divided by half the diameter;

30,000

=

=

750lbs., which is 600lbs.

15 more than the usual working pressure, or 5 times the usual working pressure. And, next, the force of steam which a low pressure boiler 90 inches diameter, and of an inch thick, is capable 30,000 of resisting is=4x166 lbs. ; 45 which is 156 lbs. more than the usual working pressure, or 16 times the usual working pressure.

=

Hence, if the excesses merely be considered, laying aside the ratio of the elastic force of the steam in the two boilers, it appears that the high pressure engine is safer by 443 lbs. per square inch, than one of the low pressure kind. But, on the contrary, if the ratio of the elastic force of the steam in the two boilers be considered, and the excesses be laid aside, it will appear, from the above results, that the low pressure engine is more than three times as safe as one of the high pressure kind; or, that the safety of the low pressure boiler is to that of the high pressure boiler in the proportion

of 16 to 5.

But it can be shown that either

*The tenacity of any metal is usually expressed by the greatest weight in pounds which a bar one inch square, of that metal, is capable of sustaining when pulled endwise.

To prove this we have (by Mechanics) the thickness of a low pressure boiler 90 inches in diameter, capable of resisting 10lbs. per square inch, 45 x 10 = .015 inches; and that of 30,000

H

a high pressure boiler 30 inches in diameter, capable of resisting 150lbs.

15 x 150

30,000

= .075

376

REV. MR. CECIL'S GAS VACUUM
ENGINE.

SIR,-I beg to state, in justice to Mr. Cecil, whose Gas Machine is noticed in one of your late Numbers, that I have seen the machine work at the inventor's rooms at Cambridge, and that in the year 1822, so that there can be no doubt of the priority of Mr. Cecil's claim. The partial vacuum-for, though nearly perfect, it is not quite so-is produced by explosions of hydrogen gas and atmospheric air in a cylinder. It is always ready to work, without any previous preparation, at a moment's notice.

t

MUSLIN MANUFACTURE.

The improvements in the manufacturing of muslins appear daily on the increase. A very few years ago the bleaching of webs in the chain was introduced, and it is now become very general; and dyeing has been successfully performed in Mr. Forrester's starching work, Park-lane, Mile-end. This novel process, which is very simple, and all done by ma'chinery, reflects great credit on the ingenious inventor. It is a much superior and more level dye than can be done in the hank, and, what is of great importance, it is a saving to the manufacturer of from five to ten per cent. according to the fineness of the yarn. This department of Mr. Forrester's work has been partially going on for eighteen months, but, since its commencement, he has brought it to such perfection by the great improvements he has made in it, that it is now quite original, and the accession of business is liberally reAwarding the inventor. Glasgow Journal

A

MULTIPLICATION IN ONE Line.

SIR,-Your Correspondent whose own brother refused to tell him how to dye silk a particular colour, has reminded me of a friend who once asked his father to instruct him how he multiplied by a number of figures in one line, and who died without It is communicating the secret. not, however, lost, for I have lately learned there is a gentleman in the metropolis who can do the same thing; perhaps there may be many, and it has occurred to me if any one, through your means, would explain the way in which it is to be performed, it might be of service to many of your numerous readers. I am, Sir,

Your obedient servant,
WW

SHARPENING RAZORS.

-

SIR,-Having spent a considerable sum for hones, straps, and various articles usually advertised for sharpening razors, I have found them all nearly useless. The best cutler I ever met with was the late Mr. Offord, of Colchester, and in giving an edge to a razor he was unrivalled. His plan was this:-After whetting the razor on the hone and strap, he finished by drawing the razor backwards and forwards on a piece of calico or fine linen: this took off all the grease attached to the edge of the razor, and enabled it to cut. If any of your friends and readers will try this, they will find it answer, as I am in the habit of proving it every day.

I am, Sir, 1911tos Your obedient servant,

SIR,-An Alarum being a most useful piece of furniture, at one time or another, in every house, perhaps you will not think the above sketch of one, both cheap and simple as well as original, unworthy of your notice.

I think the Alarum of which you have given an engraving in a former Number of your work, and which is made to act by the running of sand, is liable to one great objection, viz. that, should it be required to run for ten or twelve hours together, the quantity of sand must be so great, or the passage so extremely small, with a proportionably fine sand, that either way it would be difficult, if not impossible, to make it answer the end proposed, unless it be made inconvéniently large and cumber

Description.

AB and CD are two uprights, of wood.

EF and FC are two small pieces of wood, with small hinges or pins, at E and C.

G is a vessel, made of glass or tin, and suspended from F, to contain water, under which is placed the lamp, H.

By the heat of the lamp the water gradually evaporates or flies off in steam, which, lightening the vessel, G, allows the piece of wood, EF, moving on the hinge at E, to escape from under the point, F, of the piece of wood, FC, which flies up, from the strength of the spring of the bell, to the wheel or pulley, I, and thus sets the bell at liberty and causes it to ring.

J is a bracket or triangular piece of wood, to prevent the vessel, G, from falling on the lamp after the bar, EF, has escaped from under it.

Observation.

The quantity of water for a certain number of hours, the size of the wick, &c. can be calculated, with ease, from a few hours observation. A slit should