304

MINOR CORRESPONDENCE..

Sir, Mr. Wynn's observations, quoted by his opponent, Mr. Harrison, vol. xii. p. 237, prove that his conception of the manner in which the teeth of wheels act upon each other when properly formed, is much more correct than Mr. II.'s: whose remarks on the subject are certainly not such as might have been expected from a man who pretends to treat the subject scientifically. The best mode of forming the teeth of wheels is by involutes of circles (by somc, perhaps, considered a sort of epicycloids). Teeth formed in this manner on a wheel ten feet diameter, and working three inches deep, will only slide on each other one-sixtieth of an inch. Whether Mr. Harrison would introduce rollers, in the expectation of lessening this friction, is for himself to decide. In support of what I have asserted, I beg to refer your readers to Gregory's Mechanics, vol. i. art. 147; Robison's Mechanical Philosophy, art. Machinery; Ency. Britt, vol. ii. pp. 103, 106, vol. xx. p. 104, and Rees' Cyclopadia, art. Clock-movement. Yours, respectfully,

Dec. 1, 1829.

S. Y. (a Young Engineer.)

Painting on Parchment.-Sir, Having obseryed in No. 329 of your Magazine the use of aluin recommended in laying water-colours on parchment, I beg to say that I have used that as well as other methods on prepared vellum without the desired effect. A succession of light tints on parts previously passed over with a brush dipped in clean water, I found the most successful; but the tediousness of this process is an objection. I have observed also, during these ope. rations, that some colours (especially gumboge) lie infinitely better than others (especially indigo and lake): so that to discover what one colour possesses different from another, might lead to the discovery of a remedy against the difficulty. I am told that prepared ox galls will answer, but have not yet had an opportunity of making the experiment. I am, &c. E. W. G.

Lathe Bands." The band I use to my lathe is a hempen one of three strands, cable laid, oneeighth of an inch in diameter. After I had preperly spliced it to make it traverse smoothly. over the pullies (my lathe being on the plan of 'Mr. Williamson's, described in the "Mechanics' Magazine," vol. i. p. 369), I soaked it well in unboiled linseed oil, and suffered it to dry. I have since wetted it several times with the oil, using it at the same time. It has been now in work more than twelve months, is perfectly smooth, has every appearance of catgut, and seems as strong as ever. One great advantage attending a lathe hand thus prepared, is that, besides wearing well, it is nearly impervious to the damp of the atmosphere." Номо.

tual. "Whatever may be the cause, the appr ratus offers a cheap and effectual remedy for smoky chimneys, when this fault in them is dependent upon the pressure exerted by winds upon the upper aperture of the flue."-Bull. Soc. Enc.

Potato Farina." I have been for some years in the habit of procuring potato farina by grinding potatos on a circular tin grater, fixed to a common lathe, with a tin shield over it to prevent the splashing; open in front, above three inches square, to admit the potato which is pressed forwards by hand on a small wooden trough placed opposite the hole. The operation is performed with very great facility and quickness; and the pulp is received in a trough, or pan, supported under the frame of the lathe."Extract from the Letter of An Amateur Mechanic.

Cure for the Bite of Rattlesnakes.—It is stated in the Colombian Telescope, on the authority of a gentleman than whom there is no man in the world upon whose veracity more reliance may be placed," that a decoction of the plant known among botanists by the name of the Asclepias Verticellaria, has been found in three different cases to cure effectually the bite of the rattlesnake. "A tea is made of the plant, root and top, and then given to the patient in broken doses.'

INTERIM NOTICES.

We must decline inserting any communication on the subject of the Mechanics' Institution which does not meet us fairly on the merits of the case. Is the number of Mechanics belong! ing to it as inconsiderable as we have stated, or is it not? Has it been deserted by the great body of the Mechanics of London, or has it not? Is it over head and ears in debt, or is it not? As long as none of these matter-of-fact questions can be answered in the negative, all the laudatory writing, which such persons as Fidelitas or A Member of the Committee can bring to the aid of Dr. Birkbeck, will not vindicate the Doctor from the charge of perpetrating a gross delusion on the public, in holding forth this ruined Institution, to be every thing that the warmest friends of popular instruction can desire.

A Real Friend to Dr. B. should have been taught discretion by the answer he received from The Times on Monday; and not have attempted to foist his advertising puff at second-hand upon us..

H. H. has overlooked the answer given to his request among the interim notices in No. 330. We have received a communication from a correspondent at Exeter, containing all the information desired by Homo, and other correspondents, respecting the Camera Lucida, and shall endeavour to find room for it in a week or two.

The promised notice of Mr. Gurney's SteamCarriage, though deferred, is not forgotten: we think we may safely pledge ourselves that it shall appear long before either "N," or any one else, sees the carriage itself in actual use. "W. S. Ignoramus" is requested to favour us with his address.

Communications received from Mr. Sims→→ Mr. Higgins-B.-Amator Scientia-Mr. SlyMr. Gibson-C. II.-Mr. Reilly.-S. P. W.

LONDON: Published for the Proprietor, by M. SALMON, at the Mechanics' Magazine Office, No. 115, Fleet Street; where Commu nications for the Editor (post paid) are requested to be addressed.

M. SALMON, Printer, Fleet-street.

306

A

Pile- Driving.

(Continued from p. 229.)

I have now arrived at that part of my subject which will treat more particularly of the machinery employed in the erection of bridges and other heavy works. The pile-engine, with its various modifications, is a subject of considerable interest. Your readers will probably recollect, that when the pile-driving question was agitated in your pages, a correspondent ventured to hope that as so much had been written about pile-driving, some one or other would give, through the medium of your pages, the various plans of machines for that purpose. The nature of these papers has fortuitously thrown upon me that task; and I shall from this cause perhaps occupy a greater space than I at first intended in describing this machinery.

The operation of pileing has been rarely dispensed with in bridges, quays, light-houses, locks, &c. On the Continent and in America many extensive tracts of land have been piled throughout; in some cases cities have been erected thereon-Amsterdam is an instance. The solidity gained by this mode of searching for the firm ground, time and experience have shown to be the most judicious and scientific. That ground or even rock apparently firm and secure is not at all times to be trusted, will appear by the following singular accidents, which no human foresight perhaps could have prevented.* Blondel instances the thick walls of the Church of Val de Grace, at Paris, which gave way on one side, though built upon a good bottom, because at some fathoms lower than their foundations there was afterwards discovered great cavities where had been formerly quarries of stone. Another instance, related by Gautier, occurred at one of the isles of Oleron or Re, where part of the walls of the fortification, though

The nature of the accidents would lead us to infer that all ground should be piled, an inference I conceive not far from the truth.-C.D.

erected upon a rock, were overthrown owing to a cavity therein. There are few architects who are naturally averse to pileing; but, from various causes, many very extensive buildings are destitute of this mode of security. Inverts or inverted arches under openings and in walls, are amongst the other precautions against settlement. It is well known, however, that the latter will sometimes fail; and where neither the one nor the other has been used, those contortions will appear in buildings which are so remarkable in the streets of this metropolis and the suburbs. Sir Christopher Wren was amongst those who only partially availed themselves of the aid of pileing. Even in his greatest work (St. Paul's Cathedral) he has in preference trusted to the arch in many parts of the foundations. At the same time he observed, that should a settlement take place, it would be on the south side. This proved actually the case; and it is likely that had not the brick cone* been so securely tied in by massive iron chains at every 10 feet, its natural tendency to spread would have caused a more serious rupture. Whether this was an original idea of Wren's is difficult to determine. Michael Angelo had carried up the superstructure of St. Peter's, at Rome, with the greatest care and attention to its stability (by enlarging the slender piers previously erected by Bramante to a very great size), as high as the tambour on which the cupola was to be raised; and he left at his decease a model of the dome, for succeeding architects to conduct the same according to his design. The cupola was completed, and failed; and to prevent farther accident, a cincture of iron was subsequently inserted in the work. All this was anterior to the same plan being adopted in St. Paul's Cathedral. Acting upon the principle of " stronger than strong enough," piles should have been driven till the ram rebounded. Calculations might then have been made to ascertain the

A misunderstanding seems to prevail about the thickness of the cone. It is true that it is of 2 bricks' thickness, but these bricks are 18 inches in length! The present size in all our buildings is 9 inches.-Vide " Hughson's London."

308

thenuse of the triangle is found by working from the hypothenusal column instead of that of longest legs.

I have written a small treatise explanatory of the tables, and with them forming a concise method of working all the several cases in right and obtuse-angled plain trigonometry, with its application to longimetry and altimetry; which treatise it was my intention to have published, but time of life and other avocations make me now not desirous of it. However, if any person would like to undertake it, he may see the manuscript by calling

B

5A

I plant my instrument, which we will say is 5 feet high, at A, and after having very carefully adjusted it (a circumstance to be strictly attended to), I take the angle of depression BCD, and find it to be 2 degrees, 30 minutes. I then refer to the tables at that angle, and see against it in the column of longest legs 22-914; therefore I say 1:54:22+⚫914: 114.57 CD. Then to find the height of the tower I take the angle of elevation DCE, which is 23 degrees, 15 minutes, and turning to it in the tables see against it in the column of longest legs 2:329; then I

CHIMNEY-SWEEPING APPARATUS.

Sir, As mechanical chimneysweeping is not universally practised, we are naturally led to suppose that there is something more than public prejudice in the way, and that the apparatus employed still needs improvement. The cause of humanity is so deeply concerned in endeavours to emancipate some hundreds of children from the degrading drudgery to which they are subjected in their capacity as climbing-boys-and the amelioration of their condition is an object so desirable-that I think no apology need be made for adding to the general stock of inventions intended for that benevolent purpose.

The apparatus I am now going to describe, whatever merit it possesses in other respects, is wanting in that of extensive application. It appeared so likely to answer in the most difficult cases, judging from the model, that I had a large one made, between 30 and 40 pieces long, and was at some pains to render it serviceable for straight or bent chimneys. In practice, however, it was found that when the machine entered an irregular flue, a certain degree of mechanical power was lost, and the woods soon jammed in