these have the advantage of greater durability and THE VALUE OF GELATIN AS FOOD. safety from leakage, there are many disadvantages HE general appreciation of articles of food more-such as their greater weight, their liability of getTor less exclusively gelatinous, such as isin- ting out of shape, the expansion and contraction by glass, jellies, cod sounds, calves' feet, cow-heel, heat and cold, thus having a larger capacity in sumcalf's head, &c., was a few years since denounced mer than in winter, and, finally, their expense. by some persons as an error; and, in opposition to These disadvantages are so great that there is no the general experience and common sense of man- likelihood of iron taking the place of wood in the kind, they maintained that gelatin and gelatinous manufacture of barrels for this purpose. articles of food were of no value whatever, and that the benefit of, say, half a pint of strong calf's-foot jelly solely depended on the tablespoonful of wine which it did or did not contain.

The grounds on which this extraordinary opinion was based were chiefly twofold; firstly, that gelatin is not found in the blood; and, secondly, that an animal fed on nothing else than gelatin speedily dies of starvation. The absence of gelatin from the blood does not prove that it is not nutritious. Milk is the model food, the only substance expressly created for the nourishment of animals, and which has no other use in the economy of nature. Now the albuminoid, or flesh-forming substance, in milk is the curdy matter, or casein; no one can deny its value, except perhaps the persons who say that gelatin is not nutritious; for casein, like gelatin, is never found in the blood. If we turn to the latest, and certainly the most exhaustive, treatise on physiology which has been recently published, namely, that of Marshall, we shall find that the value of gelatin is thus stated:-"Gelatin is not found in the blood itself, but when digested is converted into a gelatin-peptone, and so becomes absorbed as we have seen, but in what state is not yet known. Nor is its destination in the nutritive processes of the body certain. Either it may serve for the direct nutrition of the gelatin-yielding tissues, or-and this is very probable-it may by itself, undergoing oxidation, conserve other more important tissues, and at the same time maintain the temperature of the body. Its efficacy, as administered in jellies, &c., in cases of sickness, especially indicates its importance as an article of diet."

soup,

Dr. Edward Smith, F.R.S., in the fourth edition of his valuable "Practical Dietary," states that gelatin "exists very largely in the skin, horn, hoofs, tendons, and bones of animals, and in a less amount in the flesh. Hence we obtain it from calves' feet and cow-heel when boiled for that purpose, from the shin and other parts of beef when prepared for and from bones which have been broken and boiled for many hours in water. Whether, therefore, as jelly or in soup, this substance is largely eaten; and yet it is affirmed even to this day that it is innutritious, and therefore worthless as food. My own experiments have proved that gelatin, like albumen, is transformed within the system, and leaves the body as urea, and hence it must have played its part in nutrition; but whether its nutritive value is quite equal to that of albumen, is another question. I believe gelatin to be a valuable food, and every one knows that, with the addition of wine and other substances, it is a very agreeable one. It is however, very probable that it is not an economical food as it is ordinarily prepared, but it may be cheaply obtained in soup from bones."

In Russia, where, with the exception of the western provinces, casks of oak-wood are obtained with great difficulty, a substitute has at last been discovered, and quite a trade is done in barrels of pine and deal, enamelled on the inside. The enamel prevents the liquid from coming in contact with the wood, and fills the cracks and prevents leakage. These take the place of oak barrels for many pur poses, especially for lager-bier barrels. They are mostly bound with wooden hoops. The enamelling is done as follows:-The barrel is made and hooped, leaving out the bang stave, through this aperture the enamel is applied to the inside and then the bung stave is put in, in the ordinary way. The enamel is made of thirty-three pounds of carpenter's glue, three pounds of finely ground and sifted gall nuts, six pounds of pulverized and sifted glass, and five pounds of sifted cement or fine unslacked lime. The glue is softened with sweet milk instead of water, and boiled down quite thick, then the powdered gall-nuts are added, and the whole boiled for half an hour, then the glass-powder, and lastly the lime is thrown in. When these ingredients have been thoroughly mixed, five quarts of good linseedoil varnish is added, and the addition of a few pounds of sulphur is also desirable. It is then boiled until it is sufficiently thick, being well agitated all the time to prevent the glass and cement from settling at the bottom. When boiled sufficiently the kettle is immediately placed in ice, which causes the mass to solidify rapidly, so that the undissolved particles will remain in a finely-divided condition in the glue. The enamel is used in a semi-fluid state, and it is best not to prepare very large quantities at once, though it can easily be rendered fluid by heat when it becomes hard.

In

The casks to be enamelled must be perfectly clean
and dry, and just before putting on the enamel
they should be heated by burning a little alcohol
in them. Three coats are given, each one being
allowed to dry before putting the next one on.
the case of very large barrels it is necessary to have
the bottom well sapported, as, when full, the bottom,
if not supported, will give way a little, and the
enamel not being elastic will crack, which will
cause the barrel to leak.

Liquors placed in enamelled barrels will not be
coloured or changed in taste by the coating, as is
the case with oak barrels, which, when new, invari-
ably turn the alcohol brown. This enamel, how-
ever, will not stand water, therefore when the
barrels become dry they must be allowed to float
on the water instead of putting water in them.

A NEW SPHYGMOSCOPE.

I now come to the second so-called argument by which the uselessness of gelatin is attempted to be maintained. It is said that an animal fed on gela-IN the Centralblatt of the 25th of June, M. Lantin dies rapidly of starvation and inanition. This dois gives an account of a new sphygmoscope, is perfectly true; but the fact is, that no simple substance given alone will support life. The most nutritive materials, such as albumen, fibrin, gluten -to say nothing of such substances as starch or sugar-all equally fail to support life when given alone. Good wheaten bread will support life any length of time; but separate the starch, gluten, and other constituents, and give them singly, and, if the argument above adduced holds good, you could prove each of these substances to be totally without value as food, as neither will support life when used alone. I believe that this false opinion respecting the valueless character of gelatin arose from some experiments on the feeding of convicts in some of the French prisons. They were supplied with a very small quantity of gelatin in place of other articles of food, and soon showed symptoms of starvation. The fact that dogs, though they die rapidly when fed on perfectly pure gelatin, will live any length of time if kept on soft bones, which consist almost exclusively of gelatin and bone earth, is a convincing proof that the gelatin is a valuable article of diet.-W. B. TEGETMEIER, in the Food

Journal.

ENAMELLING LIQUOR BARRELS. WING to the extensive trade in liquor the demand for barrels is constantly increasing, and the growing scarcity of oak timber renders it necessary to find some substitute. This want is likely to be supplied by the process of enamelling, some observations on which are made by Mr. Krupsi in a German periodical from which we translate the following:

Up to the present time, these barrels have been made of oak-wood, but as this article becomes rarer and more expensive, it became necessary to find a substitute. Iron has been proposed, but, though

which so far differs in construction from those that
have been hitherto suggested, that the movements
of the pulse act on a column of gas, the undulations
of which can be recognized by the movements of
the flame resulting from the ignition of the gas as it
issues. M. Landois observes that the nature of the
dichrotic curve or secondary elevation in the pulse
so commonly depicted has not as yet been satisfac-
torily determined; some believing it to be a real
undulation, others that it is to a certain extent an
artificial curve, and due to a kind of recoil of the
instrument. None of the tracings obtained by the
various forms of instruments hitherto suggested,
are free from this peculiar undulation, whether we
take the mercurial column, as in the instrument of
Chelius; the column of water, as in that of Nau-
mann; the elevation of a lever, as in that of Vierordt;
or, lastly, a spring, as in that of Marey. The instru-
ment suggested by M. Landois appears, says the
Lancet, to be a small metal chamber, the edges of
which fit closely on the wrist or elsewhere, whilst a
small space is left between the skin and the inside
of the chamber through which either ordinary gas
or hydrogen is transmitted from a gasometer at a
very low pressure. The gas issues by a capillary
glass tube attached to the other extremity of the
chamber, and can easily be ignited. It is interest-
ing to find that the movements thus rendered visible
agree precisely, both as regards the principal curve
and the secondary mudulation, with those exhibited
by means of Marey's sphygmograph.

WATERPROOFING STONE.-At the recommendation

of Professor Abel, Chemist to the War Department, the
process invented by Messrs. R. Gay and Co., of Alton,
buildings, i, by order of the Office of Works, to be ap-
Hants, for waterproofing and preserving stone and other
plied to portions of the decayed stonework of the
Houses of Parliament.

LETTERS TO THE EDITOR.

[We do not hold ourselves responsible for the opinies

of our correspondents. The EDITOR respectia requests that all communications should be do up as briefly as possible.]

All communications should be addressed b EDITOR of the ENGLISH MECHANIC, 31, Tavisa street, Covent Garden, W.C.

All Cheques and Post Office Orders to be mak able to J. PASSMORE EDWARDS.

"I would have every one write what he know as much as he knows, but no more; and that s this only, but in all other subjects: For such the nature of such a person or such a fountain, may have some particular knowledge and exper to other things, knows no more than what does, and yet to keep a clutter with this littls of his, will undertake to write the whole he physicks: a vice from whence great incomes derive their original."-Montaigne's Essays.

In order to facilitate reference, correspo when speaking of any Letter previously inserted v oblige by mentioning the number of the Lette, a well as the page on which it appears.

ASTRONOMICAL, MECHANICAL, AND

OPTICAL.

[317] SIR,-The diaphragm described and figured by Mr. H. W. Reveley, p. 540, in answer to the query " Unit" (4495), p. 502, is ingenious enough, and effective; but it is not the "Iris Diaphragm," which is of much more simple construction. It merely consists of two brass rings connected by an india-rubber cylinder of their own diameter. By twisting one of these round it is perfectly obvious that the cylinder will assume an hour-glass figure; and it is equally apparent that the aperture may be entirely closed if the twisting be sufficiently continued, or that upon allowing the tabe to assume its original form the whole aperture of the rings will be available. The annexed sketch shows

The details may

the principle of this arrangement.
be studied by "Unit" in the stage of R. & J. Beck's
large microscope should he be able to obtain access to
one of those instruments. I would venture to point out te
Mr. Gould (235), p. 518, that I gave a perfectly straight
forward answer to his question in my letter (47), on
395, in the only practical form in which it admit
of a reply, and that without any reference, proxim
or remote, to what he calls "the tenuity of his on
cerevisia." As a matter of fact we have no right to a
sume that the air at the equator, or anywhere ele,
moves merely as a rigid shell attached to the earth's
surface; but conceiving for our present purpose that
it does, Mr. Proctor has very clearly laid down the condi-
tions of the problem to be investigated in his communi-
cation (258), p. 540. I may be, however, permitted to
influence of the air on shot sufficiently perfect to justify
say that we have really no experimental data as to the
Mr. Gould be fond of integrating, and cares to obtai
us in employing them, otherwise than tentatively. If
the Mécanique" of M. Duhamel, he will find spe
material of the best available sort to assist him in
investigations.

In this connection, too, I must ask "M. L." (296),
p. 518, to forgive me for saying that the trade wind does
not blow at the equator at all, but north and south of
it,-the region of calms and rains, or what sailors call
"the doldrums," extending for some six degrees of
latitude, and travelling north and south with
the sun and with the trades themselves. I would further
observe that your correspondent's idea of a ball which
is to be projected 3,970 miles, reminds me a little of the
story of the Cambridge undergraduate, who being asked
in an examination paper to explain the theory of
putting a cup into a pie, did so quite correctly, but
added, somewhat needlessly, by way of a rider, that
to have heard a few hundreds of guns fired in my life-
"the cup must not be more than 32ft. high." I happen
time, but should certainly, with my experience of
birden nee, shy one which was guaranteed to project a

ball to the height of half the earth's radius. I have nothing to add to what Mr. Proctor (257), p. 540, has said to "Etudiant" (251), p. 522. Like Mr. P., I wait for more definite data.

Your engraver, in his reproduction of my drawing of Saturn on p. 529, has made the inner, or crape, ring C very much too light, in fact, has engraved it of the same tint as the outer one A. Any one who wants a facsimile of my original sketch had better darken this inner ring with Indian ink until it is just fairly die

tinguishable from the black sky between it and the body of the planet.

"R. T." (260), p. 540, may make the requisite table of Algol's variations for himself when he has once caught it at its maximum, by remembering that those variations are performed in 2d. 20h. 48m. 55s. It only remains as a fourth magnitude star for some 15m. or 20m. I have not the most distant idea when its next minimum is to take place, and could only ascertain it in a way equally open to "R. T." himself, that of watching the star until I saw it begin to diminish. A very rudimentary acquaintance with the doctrine of probabilities will show that the odds are very heavy indeed against Algol being caught in its wane at any particular time; and that nothing but repeated and sedulous watching, undertaken with that sole object is likely to be rewarded.

I have no doubt whatever, that the undulation observed by the friend of "a Ursa Minoris " (4516), p. 526, was referable to a current of heated air rising from the house roof of which he speaks. The ground, trees, or houses, all radiate heat rapidly into space on a clear night, and the rippling of the limb of the moon or a planet, when at a small altitude, is one of the cominonest of all common appearances. The owner of the stand need not accuse it of unsteadiness in the absence of better evidence than that referred to.

I shall not condescend to argue with Mr. Firth (279), p. 545. My faculty of prevision being strictly limited I am utterly unable to conceive what I shall say thousands of years' hence." What I did say, and what, with or without Mr. F.'s leave, I shall continue to say, was, that on the 1st July, 1870, the sun was in the constellation Gemini. Were I about to continue so childish a discussion (which I certainly will neither waste your space nor your readers' time in doing) I might ask Mr. Firth "on what page of the Nautical Almanack' it is stated that the sun is in" any sign of the zodiac on the 1st July, 1870 ? (Since writing this sentence I have seen Mr. Proctor's very kind letter (290), p. 563. With reference to this it is only necessary for me to say that if my unpretending Notes" satisfy a man of his great and admitted eminence, I need not distress myself much as to the effect they produce upon such a critic as Mr. F.)

Mr. Preston (288), p. 546, is quite right about the phenomena of diffraction being exhibited irrespective of the employment of any telescope. This is a subject of very considerable interest, and might, I think, be profitably treated in these columns. Diffraction phenomena are very beautiful, and may be exhibited with little or no apparatus. With regard to your correspondent's other observation about keeping his disengaged eye open while observing with the telescope I may just say that a little practice will enable any one to dispense with a shade or covering of any description. I never close my unoccupied eye while employing the telescope or microscope, but only attend to the image in the one engaged at the instrument. From use I can so abstract this image as to cease see or rather perceive anything with the other eye; albeit, of course, à picture must exist on its retina wholly differing from that with which my mind is absorbed.

Mr. J. T. Hill (4588), p. 550, will do good rather than harm by blackening with a dead black, the interior of any telescope tube, terrestrial or otherwise, the less light lost by internal reflexions obviously the more will pass through the lenses.

ORIGIN OF METEORITES.

[318] SIR,-"D. W." (letter 315, p. 570) asks a
question of enormous difficulty; one on which it is
difficult to offer anything more than a mere opinion.
Clearly, if Professor Brayley and others have guessed
rightly in supposing the earth to have been formed by
the aggregation of meteorites, a theory as to the origin
of meteorites involves an opinion as to the origin of the
origin of our earth.

I cannot believe that meteors formed part originally
of larger masses, notwithstanding the very remarkable
evidence founded on their structure and the occasional
presence of organic matter in their substance. My
reasons for not so believing are founded purely on
dynamical considerations. I can scarcely imagine
any circumstances under which a portion of a large
mass could ever become free to travel on an inde-
pendent orbit.
The whole question is, however, immensely difficult,
and most of the views hitherto presented are only
tenable by aid of a very energetic forgetfulness of a
portion of the evidence.

RICHARD A. PROCTOR.

COURTEOUS CORRESPONDENTS, MELTING
GLUE, BLACKING BRASSWORK, SILVERING
CLOCK-DIALS.

[319] SIR,-Allow me to say that the letter of "Sigma" in the ENGLISH MECHANIC for Aug. 26, must express the feelings of many worthy contributors who give their time and special knowledge for the benefit of all who like to ask questions, and many who would be able to give much useful knowledge are often deterred by the way that others treat their endeavours to impart information to them. All who ask questions should remember that an answer, however humble or incomplete, comes from one who wishes to oblige another whom he has never seen, and of whom he expects nothing but a kindly acceptance of the proffered aid. I feel fully assured that many more would answer the numerous questions, which involve a vast amount of useful information to many others of your readers than the questioner thinks, if a kind word were always substituted for a harsh one. And let me call the attention of your readers to the list of Unnoticed Queries; every one should look this over and refer to the queries he may think himself able to answer, and if he can answer any let him do so, and he may be sure that some one will be thankful even if the querist is not. And now to set a good example and be practical as well as theoretical, I will try to answer a few questions. First, 4602, as to properly melting glue. No one can do this except in a proper glue-pot, which is an outer and inner vessel-the outer to contain boiling water, and the inner the glue. To attempt to melt glue in a saucepan or single vessel over the fire is simply useless, and leads to burning the glue; but supposing we have a proper glue pot, I find this to be the most convenient plan of managing. When I have done with the glue I add some of the boiling water from the outer vessel to the glue, so as to make it too thin for use. I then put it away till wanted again, and I find that by the time the water in the outer vessel is boiled, the glue in the inner is ready melted and the proper thickness for use. If the glue is put away without the addition of some water, it dries and becomes hard, and takes some time to dissolve, and this will specially apply to small quantities of glue, which amateurs are most likely to keep for their use, and when used only at long intervals.

And now as to Query 4576. The black brasswork of microscopes, &c., is obtained in the following manner, which I have seen done many times :-Take lamp-black,

I am unable to give any definite answer to the question or questions propounded in (315), on p. 570, by "D. W." By very far the greatest proportion of meteorites are most undoubtedly cosmical; but of course it is possible that volcanic stones might be projected during a tremendous eruption to a sufficient height to enable them to be carried for some considerable distance by a rapidly moving current of air, say about a thimbleful-and put it on a flat stone or and a stray specimen or specimens of terrestrial origin smooth slate; add four or five spots of gold size, and might thus fall, apparently from space, but in reality well mix with a pallet knife, or any flat or weakfrom no very enormous distance above the earth's sur-pointed knife, and if it does not make the whole face. This is however (though, as I have said, possible) about as thick as putty add a spot or two more exceedingly improbable. Literally, nothing is known of gold size; well mix. The less gold size there is the as to the cause of the chemical constitution of these better, so that the lamp-black just sticks together; strange bodies. I do not remember to have heard or if too much gold size be added, the effect will be a read of Professor Newton's theory; but with reference bright black and not a dead black. Now add some to the late Professor Brayley, while fully recogniz- turpentine, abont twice its own volume, to the whole, ing the excellence of the old adage, de mortuis nil in a small vessel, and mix with a camel-hair brush, nisi bonum," I am constrained in candour to say that and apply to the brass-work: it will dry in a few no deference was ever paid to his opinions, nor was he minutes. Put on as little as possible, so as just to ever held to be an original investigator at all. cover, and do not attempt a second coat; and if that will The difficulty of "Hesperus" (4654), p. 574, will not do there is something done wrong; a little practice vanish when he reflects that the "Elements of will soon decide how much turpentine to apply. If there be too much the black is apt to rub off; if not of Saturn, to which he refers, as given on p. 452 of the "Nautical Almanac" enough, the black will look streaky when dry, and not for this year, have reference to the geocentric a dead, that is dull, even black. places of the moon and planet. In the particular case to which his question relates, if he were situated at the centre of the earth he would see the moon pass quite clear to the north of Saturn without occulting him at all; but (the horizontal parallax of the moon amounting to some 59' 6" at the time of the conjunction) were he to go up north to Greenwich, she would be apparently depressed so considerably as to cover the more distant planet. If "Hesperus" will turn to p. 509 of the "Nautical Almanac," he will find an easy and intelligible description of the table which has puzzled him.

Occultation

4299. SILVERING CLOCK-DIALS.-Take a tablespoon ful of cream of tartar (the best), and add about as much nitrate of silver (in crystals) as will lie on a shilling, dissolve in a very small quantity of water, and make it all into a thick paste, in fact, only just wet the cream of tartar, for if it be too thin with water the dark deposit complained of by "E. H." will appear. No metal of any kind must be brought in contact with it during the mixing. A beautiful effect on the clock dial greatly depends on the regular emery-clothing of the brass plate. There should be no scratches seen If "T. W. C." (4698) will turn back to p. 323 of your the emery cloth should be very regular and all one before the plate is silvered, the grain or the marks of current volume he will find a sketch and description way; the emery cloth should not be too fine; the whence he will be able to construct quite as good a spec-paste of cream of tartar and nitrate of silver to be troscope as it is within the capacity of an ordinary rubbed on with the clean fingers. amateur to manufacture. If, however, he will take my advice he will go or write to Mr. John Browning, of 111, Minories, London, who will do infinitely better for

him than he can possibly do for himself.

BATTERIES.

VULPECULA.

glad to see in the MECHANIC the sketch John Legg kindly offers.

As a new subscriber, I begin to feel out of my element when I read "Sigma's" answers (pages 501 and 546) to my very first query, but cannot help thinking that the amount of space occupied by his two letters addressed to myself would have completely answered my query (4450), and also furnished information to the numerous new subscribers who must have become readers through the incorporation with the MECHANIC of other journals, and who may perhaps not feel disposed to buy the back numbers.

I find that I do not stand alone in my opinion that "Sigma" might be a little more explicit (316, page 570). He addresses us as if we were all philosophers, and, in my opinion, thereby detracts greatly from the value his writings would otherwise possess. I am still of opinion that his description of the sulphate of lead battery (page 482) would be insufficient to guide any one to make one, and as "Sigma" intends ultimately reproducing his papers in the form of a book, I should think he would be only too glad to invite criticism or receive suggestions. However insignificant the queries on this subject may appear to "Sigma," they evidently interest some one, for of what use are all the stores of learning in our scientific libraries to the individual if he cannot find there the knowledge he is in quest of? M.B.C.S (A New Sub.)

IRIS DIAPHRAGM.

[321] SIR,-I am greatly indebted to Mr. Revelcy for his reply and sketches, also to you, Mr. Editor, for the But I am afraid the engravings in the last number. diaphragm there described would not meet my wants. I think I have seen one made use of in an American photographic lens, which would be nearer what I want, but I have entirely forgotten its construction, and should feel exceedingly obliged if Mr. Reveley, or any other of our talented friends, could supply me with a drawing or description of an iris diaphragm, which could be applied in front of the object-glass of a telescope and reduce the aperture, retaining the circular form or nearly so, from 6 or 7in. to 2in. diameter. Or perhaps some friend could suggest another description of screen or diaphragm, which would accomplish the same object without being unsightly or clumsy? At the same time, however, I should prefer, if possible, a circle expanding or contracting iris diaUNIT. phragm.

WINDOW CLEANING.

[322] SIR,-Amongst the numerous inventions applicable to the saving of labour there has not yet appeared one-which is a real necessity-for sparing the lives of women servants. The article needed is a wooden hand and arm, to hold a cloth and move to clean the outside panes of glass in bedroom windows. In the country women servants constantly do this, and sit on the sill holding by the window frame, with the body outside. Can it be difficult to arrange an instrument to work upwards, downwards, and to the right and left, so as to clean the glass and spare the risk of life and limb by the common act of the neat tidy housemaid? The inventor would find remuneration. Every mistress would rejoice to possess what would do the work and spare her from anxiety. Tidy servants never will take warning or obey if ordered not to sit outside the window. ISLE OF WIGHT.

HEIGHT OF CLOUDS.

The

[323] SIR,-Referring to the letter of your correspondent, Mr. J. Steel, under the above title, in your columns of August 26, I beg to say that I have no described was a modification of the Aurora Borealis. doubt the phenomenon which he has very clearly In former years I have seen many of these luminous arches, whose appearance exactly tallied with his account, and whose real nature was evident from their progressive changes or their connection with some other simultaneous form of auroral display. earliest mentioned in my note-book was a very fine white one, very generally observed on September 29, 1828; the most remarkable, a crimson one on February 18, 1887, which I have described as "one of the most wonderful and awful spectacles I ever beheld." A brilliant moon, upwards of thirteen days old, prevented its being more generally noticed, or it might have occasioned wide-spread consternation among those who were ignorant of its true character. bability the luminous clouds in the old observations of Magnan and Riccioli may have had a similar origin. T. W. WEBB,

In all pro

PROPELLING A VESSEL BY A WINDMILL. [824] SIR,-In the year 1828 I built a small vessel about 4ft. long, and launched her on the Blackwater in Ireland. She certainly did go head to wind, and pretty fast too, and for a considerable distance. After this, I read in the Courier newspaper of such a vessel having been successfully constructed in America. But the new power of steam having beaten her, I never heard more of her. I believe that a vessel with two masts would go head to wind, in spite of the friction mentioned by "W."

R. J. MORRISON.

THERMOMETERS.

A FELLOW OF THE ROYAL ASTRONOMICAL SOCIETY. the information, and (with your permission) I shall be Opinion my observations of a thermometer 2ft. below

[320] SIR,-Thanks to John Legg and "R. M." for [325] SIR,-I have for some months sent Scientific

determining the lengths of the link and excentric rods,
but its influence on these is not absolute, but sub-
ordinate to considerations of space in the engine, and
the judgment and experience of the designer.

The "travel" of the valve is the length of its excur-
sion to and fro, and it is not necessarily equal to the
"throw" of the excentric, for in valves moved by the
link motion their "travel" is variable and under the
control of the engine-driver, who can at any moment
alter its amount, whilst the throw of the excentric is of
course constant. The proper amount of the travel of
the valve is 3 times the breadth of the steam port. In
the present case it is 14in. x 8in. = 48in. the travel
of the valve, and that is the greatest travel the gearing
should give it. In stationary engines, when the valve
is moved by one excentric, whose rod is connected
direct to the valve spindle, the throw of the excentric
and the travel of the valve are equal; but if a "rock-
ing shaft" and "levers" are introduced between the
excentric and the valve, then the travel of the latter
may be either equal to greater or less than the throw
of the former, according as the lengths of the levers
are proportioned.

The lap" of the valve is the amount which its "faces" project or lap" beyond the "admission edges," A A, of the steam perts, when it is laid evenly over them, as in Fig. 1. The amount of the lap is equal to one-fifth of its travel, and in the present case it is in., which is one-fifth of 4gin.

the breadth of the port is the quotient produced by centre" (see Fig. 2), in which the dark lines show the and the "valve spindle block" or "link block," and a

The "lead" of the valve is the amount of opening which is given to the port for the admission of steam to the cylinder when the crank is on the "dead position of the valve when the crank is on the outer dead centre pointing from the cylinder, and the dotted lines show its position when the crank is on the inner dead centre pointing towards the cylinder (the crank is supposed to be at the left-hand side of the figure). The lead in both positions is shown at L° and L, and its amount is equal to one-fifth of the lap, or nearly 3-16in.

A valve of the above proportions-viz., 4gin. of "travel," in. of "lap," and 3-16in. of lead, or lap one-fifth of travel, and lead one-fifth of lap-whether driven by the link motion or by a single excentric, will cut off the steam from the cylinder when the piston has made 80 per cent. of its stroke, and will allow it to exhaust at 94 per cent., and this holds universally true, no matter what the actual dimensions are, so long as the above proportions are maintained.

I believe I have now answered all Mr. Watson's questions with regard to the slide valve and ports, so I shall pass on to the consideration of the "link" and