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MECHANICAL MOVEMENTS, olick T whose pivot is set in tbe frame , preventsto the crank-pin, and the piston-rod is kept up

the larger ratchot from falling back, and во the spring S S' still drives the great wheel during the time the clock takes to wind, as it need only just keep the escapement going, the pendulum taking eare of itself tor that short time. Good watches have a substantially similar apparatus.

122. A very convenient construction of parallel ruler fordrawing,made by cutting a quadrangle through the diagonal, forming two right-angled triangles A and B. It is used by sliding the hypothenuse of one triaugle upon that of tbe other.

~123. Parallel rnler consisting of a simple straight ruler B, with an attached axle C, and pair of wheels A A. The wheels, which protrude but slightly through the under side of the ruler, have their edges nicked to take hold of the paper and keep the ruler always parallel with any lines drawn upon it.

124. Compound parallel rnler, composed of two simple rulors A A connected by two crossed arms pivotted together at the middle of theirlength, each pivotted at oue end to one of the rulers, and connected with the other one by a slot andslidingpin,as shown at B. In this the ends as well as the edges are kept parallel. The principle of construction of the several rulers represented is taken advantage of in the formation of some parts of machinery.

126. Parallel ruler composed of two simple rulers А В connected by two pivotted swinging arms С С

12G. A simple means of guiding or obtaining a parallel motion of the piston-rod of an engine. The slide A moves in and is guided by the vertical slot in the frame, which is planed to a true surface.

127 differs, from 12G in having rollers substituted for the slides on the cross-head, said rollers working against straight guide-bars A A, attached to the frame. This is used for small engines in France.

128. A parallel motion invented by Dr. Cartwright in the year 1787. Tho toothed wheels С Chave equal diameters and numbers of teeth; and the cranks A A have equal radii, and are set in opposite directions, and consequently give an equal obliquity to the connecting rods during the revolution of the wheels. The cross-head on the piston-rod being attached to the two connectingrods, the piston-rod is caused to move in a right line. •

129. A piston-rod guide. The piston-rod A is connected with a wrist attached to a cog-wheel В which turns on a crank-pin, carried by a p late С which is fast on the shaft. The wheel В revolve» around a stationary internally toothed gear D, oí double the diameter of B, and so motion ia given

right,

130. The piston-rod is prolonged and works in a guide A, which is in line withttie centre of the cylinder. The lower part of the connecting-rod is forked to permit the upper part of the pistou-rod to pass between.

131. An engine with crank motion, the crankwrist journal working in a slotted cross-head A. This cross-head works between the pillar guides D D of the engine framing.

132. A parallel motiou used for the piston-rod of side lever marine engine. F С is the radius bar, and E the cross-head to which the parallel bar E D is attached.

133. A parallel motion used only in particular cases.

(Го be continued.)

IMPROVED SCREW DRIVER.

THE device of which an engraving is given, secures increased leverage nt the will of the workman without increased length, and will for many kinds of light work also take the place of the bitstock or brace for> drilling, boring, &c.

In the engraving A represents a wood handle, having a recess to receive and retain, when not in use, a second and smaller poiut, B. This

and the counterparts of the clutch at D engage with each other. The shank will then turn with the handle, and may be used precisely like the ordinary screw-driver, except that when it is necessary to use the power of both hands in driving home a large screw, an increased leverage is gained by the curvature of the shank.

It will also be obviouä that bits properly formed may be placed on the end of the large screwdriver in the same manner as the supplementary point В above described, when the instrument will take the place of the ordinary bit-stock.

David Drummond, of McGregor Iowa, U.S.A., is the patentee.

THE LATHE AND ITS USES—THE SPHERICAL SLIDE-REST.

WE give this week very complete working drawings of the slide-rest for spherical and curvilinear work, described in the English Mechanic for November 19th. As a full general description of the rest was then given, it will suffice if we now simply explain the various detailed figures.

Fig. 1 gives a side elevation; Fig. 2 an end elevation; and Fig. 3 a plan of the slide-rest

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recess is indicated by the dotted line. This supplementary point has a sleeve, indicated by a dotted line, which slips over the poiut of the larger screw driver.

The shank С of the larger screw-driver is bent in the form shown. At D is a clutch, one portion of which is formed on the shank, and its counterpart on the handle, and underneath the ferrule.

The extremity of the shank С in the interior of the handle, has a turned groove, into which the point of the screw E enters, and holding the handle so that the two portions of the clutch cannot engage with each other, permits the shank С to be revolved like an ordiuary bit stock.

When it is desired to use the tool with one hand, the screw E may be turned out a little distance, when its point no longer enters the groove,

complete; Fig. 4 is a back-end elevation of the main slide; Fig. 5, the screw at back; Fig. 6, a side elevation, and Fig. 7 a front-end elevation of the main slide ; Fig. 8 is the side elevation of the slide in section, showing also the position and arrangement of tbe traverse screw; Figs. 9, 10, and 11 are elevations of the second slide ; Fiss. 13 and 13 show the nut for traversing this slide; Fig. 14 shows the second traverse screw; Figs. 16 and 1(1 give the nut for this screw; Figs, ш and 18 are elevations of the slide which carries tlfte tangent wheel; Fig. 19 is also an elevation of thtfe same slide, but in this the slide and tangent wbejjel s' shown in section. ThisFig.alsoshowsthcmetoi

I attaching the tangent wheel to the slide; is a plan of this slide and tangent wheel;

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one of the side stops; Figs. .19 ami 40 show the Ins:, which is attached to the tool slide; Figs. 41 and 42 are elevations of the tool receptacle; Fig. 43 is a plan of the tool receptacle, which shows aleo the opening for inserting; the holding nuts; Figs. 44 and 45 show the holding-down nut, of which there are two; Fig. 46 shows the nippingscrew for one of these nnts; Fig. 47 is the screw for the other nnt, the top part of which is differently shaped to receive the lever for moving the tool into cut—this lever is shown at Figs. 48 and 49; Figs. 50 and 51 show the two screws for rcgnlating the cutting depth for the tool ; Figs. 52 and 53 show the lng] nuts for these screws; Fig. 54 is the small screw which is u?ed for nipping the two regulating screws ; and Fig. 55 is a stud, which forms the fnlcrum for the forcing lever.

W. H. N.

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FRICTION.-VIII.
Вт С. Гкагвк, A.B., L.C.E.
(Continued from paga S3.)

Tub Scuew.—This mechanical power is generally so fatly described in elementary works on mechanics, that we may dismiss it with a few remarks.

As the screw with a rectangular thread may he described to be a cylinder round which is wrapped a triangular plate of nniform th ¡ckness, from which the lower portion is cut away so as to leave a projection of uniform breadth and depth whose upper and lower surfaces project at right angles to the surface of the cylinder at every point, we see that the screw is merely a modification of the "inclined plane," for it is clearly a matter of indifference, with regard to the forces engaged, whether we urge the body up the inclined plane or force the inclined plane under tbc body.

Suppose now the screw to be placed vertically, let t be the angle of inclination of the thread to the horizon, Í' the power applied horizontally at the circumference of the screw, and W the resistance overcome acting; vertically , then, reerring to Vol. X., page 528, we have

P' = W tan (i + Л)

Or if, as is always the case, P be applied at a distance R from

r the axis, we have, since P = F x —, В r Г = W x — tan. (i + ф\ R;

where r is the radius of the screw.

This formula can be nut into a form which contains The 1 ineal dimensioneof the screw alone. Let h he the " pitch " of the screw, or distance from any point of one thread to the corresponding point on the next, measured parallel to the

h

axis, then tan. i = , and since

«Ti

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In this formóla we пате supposed t and г to he definite quantities tor any given screw; hut this is not strictly the ease, since the angle of inclination, of the thread increases continually from the outside of the thread to the circumference oí the cylinder. We shall, therefore, consider » to be the inclination of the thread at half its depth, or midway between the cirouxoference of the cylinder and the outside of the thread, and r to be the distance of this point from the axis; these average values will give an approximation sufticicntly near the truth.

The ordinary screw press, a sketch of which is given in Fig. 15, affords a good example of the manner in which friction affects the working of machines. Before the lever P can be turned, the following frictions must be overcome in addition to the resistance of the body с с to be compressed :—1st. The friction of the thread of the screw A in the groove of the nut B, which may be divided into two parts, the friction of the upper or lower surfaces of the thread and groove, and the friction of the cylindrical surfaces of the thread and groove. 2nd. Between the vertical guides M and N and the moveable plate D; 3rd. Between the flat surface of the extremity of the screw and the piece D, which we shall shortly speak of under the head oí "friction of a Pivot."

Exahpl» 14.—The diameter of the screw of a screw press is Hin., the pitch is Jin., what effort can this press exert if a power of Sfilb. be applied at the end of a 3ft. lever, neglecting all friction but that between the screw and nut, which is most important, and of which the coefficient is J?

Iïere

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where l is the radios of the end or pivot.

Frequently we support a vertical shaft by means of acollar. as in Fig. 18. In this case the surface of contact between the

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collar and its support is clearly an annulus, whose external radius is the radius of the collar, and the radius internal radius of the shaft.

In Fig. 19, let С A, (he radius of the shaft, = r, and С В the radius of the collar, = R, then the element of area is in this case a trapezium А В D E, the pleasure in widen ocUthrough its centre of gravity, equal to which is situate at a distance RJ + В r + r'

§x

В + г

from the centre C- We have, then, in this case also the moment of the frietion of the collar equal to the sum of the moments of the frictions of all the elementary trapeziums, which make up the entire annulus.

3 X lSH-5

As this is a power applied between the fulcrum and resistance of a lever whose arms are 7 and 5; to find the force at the jaws we must reduce 8801 in the proportion of 5 to 7, *.*..

5 8901 x — = 2-8 tons nearly. As friction increases in direct

7 proportion to the pressure, it is manifest that the serew prese has this defect, that the work lost by friction goes on increasing during the process of compressing any substance, and that at the close, when the greatest effect is desired, we have the greatest loss from friction to contend against.

FaiCTtON or A Pivot.—A pivot may be defined as the support of the lower extremity of a vertical shaft. The pivot may be formed on the shaft itself or attached to it as in Fig. 15, or the pivot may form the support, and enter a socket in the end of the shaft, as in Fig. 16. In either case, we bave a circular surface pressed against another with a certain force, and capable ef turning round its centre.

The question is now, to find what force applied at the circumference in a tangential direction will be just sufficient to turn it round. In Fig. 17, let W be the force, which, acting along the axis of the pivot, urges the surface В against the fixed plane С D. We may assume that the pressure is uniformly distributed over the surface B, hence if we suppose the circle В to be divided into elementary triangles, such as В b c, the pressure ou each roust act vertically downwards through its centre of gravity, which is distant Jr from B. The moment of th* friction of this elementary triangle round В must therefore be the pressure on it multiplied by и and by jf r, therefore the moment of the whole friction of the pivot round В must be the snm of the moments of the frictions of all the elementan,- triangles which make up the entire circle, or the total pressure, W, multiplied by ц and hy $ r. But the moment of the required force is P r, therefore Pr= |/tWrorP = §pW.

Example 16.—The pressure on a pivot is 10001b., its diameter is 6in., and it makes б revolutions per second; if^

1

— —t find the work expended per minute on friction. A

10 pressure at the circumference of § ц W being equal to the friction, in every revolution of the pivot a pressure of $ ц W is overcome for the length of the circumference, or 2 тг r,

4 therefore, if я be the number of revolutions per minute —

3 W « /i тг r *Q1 be the number of units of work expended per minute on friction. Here

1
Y? = 1000, » = 300, ^ = — and r = 3ia. or ift.

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EXAMPI.1 1

gravity — 20j, 3ft. in diameter, and II; in height, rests о ronghhohzoutal plane; if the coefficient of friction = \y and the block be cajiahlc of a motion only of rotation round a vertical axis, passing through ita centre of tlgure, calculate the magnitude of the force, which, applied tangentially at ita circumterence, will suffice jnst not to more it.

ASTBONOMICil NOTKS TOR JfJNE. Вт A Feu-ow oi THi RoriL Ajuonoviical Soctxty THiRigbt Ascension of the Sun at Greenwich Mean Noon on the 1st of June is 4h. 36m. 34 47s , and his North Declination 2-2° 4' 12". He is therefore situated to tie North of Aldebaran and the Hyadcs, and not very far from the small star T Tauri (vide map of " Eastern Sky," Vol. X p. 641. His meridian altitude is now very conntlerable, attaining its maximum on the21st of the month, when at hi» culmination in London he will be nearly «2° big*. The Sun rise* in London on the 1st at 3h. 61m, and sets at 8h. 4m.; so that the day is ISh. 13m. long. On the 21at (the longest day) the Sun rises at 3h. 44m., and sets at Sh. 18m., so that he is 16h. and 34m above the horiion. From the cause explained on p. lie of the present volume, there is no real night in any part of the United Kiatjdom during the present month in fact, in the extreme North of Scotland, small print may be read easily enough now up to nearly 11 p.m. The equation of time on the first day of June is 2m. 29 71s., and is to be subtracted from the hour indicated by a sundial to obta» that which a properly-regulated clock ought to show. The equation diminishes to в-14*. on the 1 tth, after which it becomes additive, and on the last day of the month, 3m. 29 02«. must be added to apparent noon to obtain the mean time at the instant of ita occurrence.

The Moon enters her First Quarterat llh. l«-3m. on the night of the 6th -, is Full at lh. 472s. in the afternoon of the 13th; enter» her Last Quarter at 9h. 30 8m. on the evening of the 20th, and is New at llh. 33 2m. p.m. on the 28th. ShcisinPerigreeou the 11th at 1 p.m., and in Apogee at Noon on the 23rd. Пег age at Noon on the first is 21 days, at Noon on the 2nd 3 1 davs—and so on. At 2 o'clock in the afternoon of the 4th, Libration will bring an additional portion of her surface into view in her S.E. quadrant. At 4h. a.m. on the 22nd, more of her S.W. surface will come into sight from the same cause ; and finallv, at 15h. ou the 30th (or, speaking ordinarilv. at S o'clock ш the mornme; of the 1st of July), the South' Easterly portion of her surface will, for a similar reason, be once more seen in exce*. The Moon will be in conjunction with Uranus at 5h. 41m. on the afternoon of the 2nd. of course in brilliant daylight. At »9m. past 6 on the 15th she will be in conjunction with Saturn-, at 9 85 a.ra. on the 25th with Venu»; at »-22 ou the 26th with Mars, and 41m. later with Jupiter. She will be in conjunction with Mercury at 49m, past 2 a.m. on the next morning, and final I v. once more with Uranus at 14h.43ra. on the 29th. Three stars only, of the 6th magnitude, er larger, will be occulted during June. The first occultation will be of V Libraron the evening оГ the 11th. The star will disappear at the Moon's dark limb at 9h. 43m., and reappear at her bright limb at Mi. 51m. On the night of the litli. В. A. C. 6088 will disappear at the bright limb at llh. 55m., and reappear at the dark limb nt 12h. 38m. Lastly, on the 16th, ,i Capricorni will disappear at the Moon's bright limb at V2h. 50m. a in. and reappear at the dark limb at 13h. 58m.

Mercury is practically invisible for the first third of the month, being too close to the Sun. Ile Souths only about 19m. after Noon on the 1st. and is in inferior conjunction with the Sun on the 4th. Towards the end of June he becomes a Morning Star, and is on the Meridian about halfpast 10 a.m. on the 30th. Venus is a Morning Star throughout the month. She Souths on the lat at 3jm. past 9 am., and on the last 17m. past 9. She i» becoming less interesting as a telescopic object, and it û getting apparently smaller daily. She travels during June throngb Arie*, and the greater part of Taurus. Mars is atill a Morning Startoo, Southing at Wli. 43-4m. um. on thefirst day of the month hand at 10иЛ5т. a.m. on the 30th.He is passing through

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Taon*, «ai i» m conjonction with Jupiter at 7h. 12m. on the afternoon of the 27th. Jnpiter is a Morning Star likewise, Adj too cioM to the Sun to he well aces. Owing to this pronmitv the phenonena of hia satellites are invisible, lie ¡» on the' Meridian on the morniu" of the 1st only, about Sim before Noon, and on that of the SOtli about 9m. past lu. Jupiter will remain in Taurus during the entire month. Saturo, were he not so very low down, is now conveniently situated for observation. He is on the Meridian at 13h.2'9m. on the mght of the 1st, and about 11 on the last night of the month. He continues on the N.W. connues of Sagittarius. He ia in opposition to the Sun at am. past 4 in the afternoon ol the 16th. lint rlnfa being now practically at their greatest opeuing.be presents^ under faroorablc atmospheric conditions) a telescopic object of the very highest interest. Uranus, still in Gemini, and Neptune, on the confines of Puces and A.ncs, arc both too close to the Sua to be risible.

J une is the month ш which the smallest number of Shooting Stars has been observed; and would appear to be the one most free, of all the twelve, from the apparition of these phenomena.

LETTERS TO THE EDITOR.

£We do not bold ourselves responsible for the opinions of our correspondents. The Editor respectfully requests that all communications should be drawn up as briefly as possible.]

»#* All communications should be addressed to the Kditor of the English Mechanic, 31, Tavistockstreet, Covent Garden. W.C.

AU «beques and Post Office Orders to be made payable to J. Pabbmore Edward».

T would have every one write what he knows, and as much as he knows, but no more; and that not in this only, but in all other subjects: For such a person may have some particular knowledge and experience of the nature of euch a person or such a fountain, that, as to other things, knows no more than ■what everybody does, and yet to keep a clutter with tliiâ little pittance of his, will undertake to write the whole body of physicks: a vice from whence great inconveniences derive their originaL —Montaigne's Essay*.

THE USE AND CONSTRUCTION OF THE
TELESCOPE.

Sir,—If "Herbert,'* p. 2Í4, will look through the English Mechanic from its commencement, he will rind that his idea is by no means a novel one. It last appeared in its only practical form on pp. 73 and 153 of your tenth volume.

Mr. A. W. Btacklock (on the same pace) will perhapb forgive me for pointing out that he observed Z, Ursae Majori» with Win. of aperture—I with 1¿.

I must alio ask the forgiveness of "J. V. В.." p. 911, when I express my conviction that an account of a few words uttered by Mr. Buckingham at the Royal Astronomical Society, contained in some journal unnamed and unknown, can scarcely be held to be strictly :i "detinue report of the optical performance1' of his telescope.

It "Turton" (3797J. p. 213, wants merely a simple stand, be will find a detailed description of a rough but effective form of Jv.uatoreal mounting at p. 391 of your second volume. The best kind of Equatoreal with which I am acquainted is certainly that of Cooke, of York. The description of the water clock of Lord Jtoftse—then Lord Oxman town—will be found at p. 205 of vol. M of the iioyal Astronomical Society's "Monthly Notices."

1 may inform "E. M. li." that a panoratie eyepiece is nothing in the world but a common terrestrial one, with a provision for separating the double combinations at the eye-end from that at the object-end; the farther these pairs of leases are removed from each other the greater being the power acquired. If my querist will refer to Yol. III. of the Екоийп Mechanic, be will find a diagram in a letter by Mr. Moot (Fig. 2) on p. 381, which will illustrate this. Assuming him to have this diagram before him, 1 need only observe that when the combination OD ie poshed towards the combination А В the power is diminished. When, on the other hand, they are separated, tbe power is increased. This eyepiece would eerew into tbe adapter of a Newtonian telescope, just as an ordinary Huygbenian one does; but I should not expect it to be, in the least degree, a practically useful гони with that instrument. 1 may eay, in reply to your correspondent's second question, that the leases of tbe Ramsden eyepiece are of the ваше focus, but zw of the same diameter« the eye-lens being the smaller of the two. In an eyepiece of this construction now before me, the diameters of the eye and Held lenses are as 4: 7.

Turning now to the letter of "Scorpio" on p. 234, 1 und a mixture of queries which, with bis permission, I will reply to separately under their appropriate headings. Imprima, though, I should remarkably Like to know the exact value and meaning of the ч-wertton that "All Solomon's £5 telescopes are тлттшяЫ to show five stars in the trapezium 0 Orionis." Suppose (rather a wild supposition, by the way) that I bought a £5 telescope, and failed to sec the fifth star in the trapezium with it, should I have my money returned, or be told that my own eye was in fault, or what? Again,before proceeding to answer yoercorre.«pondent s questions, 1 must just remark that turning back to the page he specifies I certain./ find that I am there made to »peak of 2*5 as the limit of separating power of я 2|bx telescope, bat I wrote 2* 0, and the 0 must have got turned into a 5 in the press. Beginning now with bis third query, as the first having reference to tbe telescope. I may tell him that one great difference between the mounting of a reflector and that of a refractor, is that the latter is supported «.bout the middle of its length ; while all the weight of tbe former being at the bottom of the tube, it has to Ъе held near that part of it This necessitates a compacting and compressing of an Equatoreal mounting fur a reflector, bringing it all, so to speak, near, the 'ГТоЫ, and a general maesiveness in the details

which are rather drawbacks when applied to a refractor. Your back volumes contain some illustrations from which "Scorpio" may derive many useful bints. The simplest way to calculate the diameter of the field of a Huygbenian eyepiece is to set the telescope so that a star shall cross the field accurately on a diameter, and to note the seconds and fractions of a second which it occupies in doing so. Then this interval in time x lu x cos. star's declination = the angular diameter of tbe field. I append an example, as requited. It Is worked by logarithms to obviate the necessity for half a column of mere common multiplication. Suppose that your correspondent has observed the transit of Kegulus diametrically over the Held of his Huygbenian eyepiece, and has found that it occapied lm. 8'ls. by a sidereal clock in crossing it, then the calculation would stand thus :—

684a. log. ,-8300561

15 log. 11760913

Doc. N. of Kegulus 12° 35' log. cos. 0-9804128

log. of 1001**29 3-0003602 : that is

to say the angular diameter of the field of his eyepiece ie 1001**89 or 10' 4l"'2D. I bave employed seven {»faces of decimals, but five will be ample in practice. It will be further pretty evident that if we were observe a star on tbe Equator, the operation would resolve itself into multiplying the second** of sidereal time occupied in its passage by 15, to convert them into seconds of arc, as there is, obviously, no allowance to be made for declination. Stars like <T Orionis, rf Virginis, «y Virginia^ Virginis, a Aquarti or K i'i-eium, will furnish results quite sufficiently accurate for my querist*» requirement!*, when treated In this way. I need scarcely add that this determination must bo made afresh with each new object-glass to which the eyepiece is applied. Next, the webs in a transit eyepiece are enclosed in a tube, but are only "protected " by the саге of the observer. It is not considered de rigueur in the beet observatories to poke your lingers through them. "Scorpio"must read thereply^ivenabove to " E. M. B." with regard to the Pancratic Eyepiece. The Cambridge (U.S.) Refractor is of J4*02in. aperture, and 22"38ft. focal length. The Great Melbourne reflector, a CasBCgratnian of 30 5ft. focal length, and 4ft. In clear aperture, will be found described in great detail with engravings in Vol. VIII. of the English Mechanic (no. 104), pp. 2*7 to 252. "Scorpio" should certainly buy the volume containing the description referred to, If he be interested in this monster instrument.

The formula for obtain! о g the geometrical focus of a double concave lens is simply this, divide twice the product of its radii by tbeir sum. Or, more simply, in the case of a double concave lens of glass of equal curvatures, the principal focus is at the centre of the first surface. I cannot think of any method of determining such focus practically. Perhaps opticians have some way of doing thia, and one of them might enlighten us.

Mr. Bagutey (3853), p. 238, almost answers himself. The large telescope, from the spherical aberration of the object-glass remaining un cured, would give a quantity of overlapping images which must inevitably be fatal to sharp definition. It scarcely requires one to rise from the dead to announce authoritatively the fact that a small object-glass which is good, will always give clearer and erisper definition than a larger one which is bad.

Mr. Walter -unes (3890), same page, should really treat himself ">•• set of our Mechanics. He will find his q uestion answered over and over again, so far as it is susceptible of a reply, iu former volumes. A Fellow or The lio Val Astronomical Society.

ASTRONOMICAL.

Sir,—The" doctors" who "disagree," in tbe case to which "Ab Initio," refers on p. 207, are the Thanet Guaräia*i, and "The Nautical Almanac ;" from the la tier of which I obtained the data on which I founded my replies on pp. 525 and 570 of your last volume. The "Establishments of the Ports" for the different localities given on pp. 496 and 497 of tbe National Ephemerif*, are reckoned from Apparent Noon. The tides at London Bridge are given for Mean Noon; but as the Establishment of the Port for London is obtained in the same way as that for Margate, or anywhere else, the difference remains constant. I really do not see what I have to add to what I said on p. 570 of Vol. X. If I rightly remember the request of "Saul Rymea," and my present correspondent, it was that I should myself furnish tide table* ; but I really must u-k to be excused for declining to wade through a mass of the most merely mechanical computation. Why does not "Ab Initio u undertake it himself? He will find it a Tidy little Job.

The mention of the "Nautical Almanac" brings us back to "Scorpio" and his string of questions on p. 234. Noticing now those having more special reference to Astronomy. I may tell him that the "Nautical Almanac" is published by John Murray, of Albe marie-street, Loúdon, and that its

?rice is half- a-crown. Any bookseller will procure it. t contains the R. A. and Dec of 147 fixed stars. The object of your correspondent, however, in askingforparticulars of abook containing Right Ascensions and Declinations, is probably that he may be enabled to find objects of interest in the Heavens. If such be his intentiou let him get forthwith Webb's " Celestial Objects for Common Telescopes ;" it is published by Longmans and Co., London, for 7s.6d.,and is essentially the book which the incipient amateur will find Indispensable. "Scorpio" asks for a "moderately cheap " work, but if he will take my advice and buy the book I recommend him, I shall feel grievously disappointed if he does not consider it dirt cheap, when he sees what a mass of information it contains. The map of the Moon alone is honestly worth 5s. out of the 7s. fid Turning to a third question, I may briefly say that Kepler's third law Is not rigidly exact, because of the perturbations caused by tbe various bodies of the Solar system, inter se,- but that it is sufficiently nearly so to get an excellent approximation in any given case, where either the periodic time, or the mean distance ia known.

Surely the "Cluster" figured by "Aristotle" (387*7. p. 238, cannot be meant for the Pleiades? and yet, the Stagy rite speaks of it as a "cluster," and having "lately disappeared iu the West." With regard to my Map, on p. 64 of your last volume, I can only say that, when 1 drew it, the Pleiades occupied the precise position I have there indicated; and that although just now I walk by faith and not by sight in the matter, 1 have not the very slightest doubt that, at this present writing, they are in exactly the same place, as they were then.

A Fellow Of The Botal Astronomical Society.

THE EXGLTBJI MBCRANIC LIFEBOAT.

Sir,—Will you permit me once more toappeal to my fellow-subscribers for their more cordial co-operation lu the matter of the lifeboat? Though much disappointed, I cannot yet believe that they are really indifferent, and positively refuse to lend a helping-hand to this Godlike work. Comrades 1 will you longer shirk yuur responsibility? Can you allow a few of us to struggle on, hoping almost against hope, when n single united effort from you would make our hearts rejoice, and be the means of saving numbers of fellowcreatures' lives? Tbe refrains of two of the songs sung at our concert are still ringing in my ears :—

"Then three times round went our gallant ship,
And three rrmes round went ebo;
Forthe iraní of a lifeboat they both went down,
Aa she sunk to the bottom of the sea."

And

<! They're in the boat! they're all afloat!

Hurrah 1 they've gained the shore!
Bless the lifeboat 1 bleu the lifeboat!

Oh, God, thoult hear our prayer!
Bless the lifeboat! bless the lifeboat!

Nö longer we'll despair,"

Can there really be any amongst our body who would willingly be even in part responsible for the disaster detailed in the Mermaid, or think a few pence too much with which to purchase a share in the blessings which hundreds of grateful hearts may pour upon our lifeboat t

Our undertaking, though important, is not difficult. If only a reasonable proportion of subscribers will help. It is not even novel in itself, unless we make it во by either unprecedented success or signal failure. Other journals have their lifeboat«, and why should not we? 1 find RoutUdge's Magazine for Boys has subscribed £306 12s. 6d., and built one lifeboat; ¿he Duru ■■'•■ People's Journal has subscribed £822 15s. I'd., and built two lifeboats; and the Quiver has subscribed £1878 Is. lid., and built three lifeboats. It is my belief that the English Mechanic can do more than any of these, and with all my heart I will make one to eay that it shall.

Geo. Luff, Staunton Harold, Ashby-de-la-Zouch

P.S.—Success to the Mutual Improvement Societies! Country residents are almost powerless to aid in such matters, or I should be delighted to join in the movement. I know it would be a great boon to me could I discuss certain subjects appearing in tbe Mechanic with a few kindred (but more practical) spirits. May I ask If our mechanical friends at Derby could not attempt a branch?

btfl-oa

Sir,—In the black days of the Cotton Famine a celebrated " Launcashlre lud "—viz., the late Lord Derby —wrote to the Tunes in these forcible, if not elegant, words :—•' Can yo help us? We be clemming "—i. е., starving. I quote these memorable words of our "Rupert of debate " in behalf of "poor Jack," substituting "drowning" for " clemming."

It was said or sung by Dibdtn that

"A sweet little oherub sits up aloft.
And looks out for the life of poor Jack."

This may be quite true, although bow he manages to sit at all, with his peculiar conformation, is a mystery past my solving, assuming ye cherub's portrait In our old City churches to be accurate in detail; so I must leave it to Owen or Huxley. But as we can hardly expect the aforesaid cherub to don a cork Jacket and sit on the thwarts of a lifeboat, or any conceivable number of cherubs, to man it; and even If they could, having no arms, to row, it,would be impossible to them. I think it logically follows that instead of trusting to the assistance under difficulties which cherubs may afford, we ought to provide the lifeboat ourselves, trusting there will be no want of stout arms and stout hearts, able to sit in and row It on its blessed errand at need.

I am in во sense of the words a clever beggar, but however my friends may doubt the fact, and think it quite impossible that one so lamentably addicted to '* chatting " can be In earnest, I do assure my fellowreaders that I am terribly in earnest in the matter of the lifeboat, and do most earnestly entreat them to aid and assist this meritorious attempt to provide the means of saving the lite not only of poor Jack, but the ship's passengers also; and I truHt they will stamp it with their approval In the moat effectual manner possible by sending at least one—1 hope many more than one—stump of her Majesty's kind countenance impressed on gold, silver, or paper; for the precious metals are doubly precious when employed on works of mercy and benevolence. If the constable is legally empowered to call on all good men and true to aid and assist, in tbe Queen's name, to keep the peace, bow much more are we morally empowered to call on all good men and women also to aid and assist this good work.

1 have before said I am not a clever beggar—soot h to say. begging is not my forte—bnt I trust the want. of abüity in the solicitor may not In this instance cause any injury to the case. What is Impossible to one is easy to many; and we English mechanics are manyenough, surely, to do what Capt. Busk has done singlehanded—viz., to launch one lifeboat. It only wants the will—and I trust we have enough of that—to prevent the captain's princely liberality from " makin? us ashamed." The Harmonious Blacksmith.

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SMALL SLIDE SCREW CUTTING LATHE.

Sir,—We beg to send you above a photograph of a «mall description of screw cutting lathe, which we have lately constructed.

This lathe is constructed with bed on the triangular bar principle, cast hollow to receive the leading screw, which is thus protected from dust and injury.

The mandrel head is fitted in the usual way with back gear, the mandrel of the steel running in double conical collars of hardened steel. Compound slide rest. Full set of change wheels, and arrangement for cutting screws of right and left hand thread to antpitch. This lathe oomplete weighs about 2ewt, aud runs very easily with the foot wheel and treadle motion.

Booth, lines , Engineers, Dublin.

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Let A B C be objects which appear equidistant in the field of view of telescope. A is the hair line in the telescope; B is the signal 3 miles distant; C is the signal 6 miles distant. Join A A1, ABB1, and A C as represented by the dotted lines —the angles subtended by these lines are equal. Now join ABC, the points which are equidistant in the field of view. The line that joins them cannot be other than convex.

Verax.

THEAIRPUMP.-THE HIGHEST VACUUM.

Sir,—Can any of your seien tifie readers assist me in the following difficulty? I have a large doubled-barrelled air pump, which I bought in order to produce such vacuo as would exhibit the phenomena of the induced current in vacuo. I find, however, that I cannot url a vacuum good enough to show Gassiot's fountain, the Aurora tube, or the rotation of the electric light round the magnet. Now if some of your practically scientific readers can and will answer the following queries, I shall be deeply obliged. I say practically scientific, because, theoretically, I know as much about the subject as most, but I want the opinions and advice of someone who has done Ainu**//what he recommends and not merely taken it undigested from books, most of which I have read.

1. What is the highest vacuum, as indicated by the syphon gauge, that an ordinary, well-made doublebarrelled air pump, with oiled silk valves, ie capable of producing?

2. Is there any wayof adapting other valves, such as Babinet's plan, so as to produce a better result.?

3. The machine admitting It, could not an extra barrel on some other plan, such as Grove's or Newman's bo adapted so as to produce the higher vacua?

4. If so. which is the best, simplest, and most effective ? and would youreorreependentaescribeitsconstruction pructloaJly?

5. Whatdegreo of exhaustion is necessary to produce the phenomena mentioned in the former part of my letter?

Theoretically considered. Grove's plan seems to me the simplest and best Can anyone describe minutely (for I make all my own instruments) its construction? I know its general principle, as described in scientific books, but I want one to work by.

In return for such information, I can give your readers such information on the construction of induction coils as I know they cannot get anywhere else in England. I have apian of construction, so simple, and yet so effective, that I get such results as have never been yet obtained by any maker. I have one just containing only 4 miles of secondary, and yet with one email Grove's battery with the platina ouly 4in. by 2in. I produce splendid sparks iinAn length. Any information on the above subject to the purpose will greatly oblige. Inouctorium.

WRAYS 20-INCH O. G.

Sir,—Inquiries have been made in your columns as to the performance of the above glass now in the possession of Mr. Buckingham, and the only answer as yet given has been by "F.R.A.S.." to the effect that "with regard to ita optical quality we have no definite report."

In the absence of this, perhaps it may be interesting to some of your readers to have the maker's own report in a letter to the Astronomical Register in Aug., 1867. It is as follows :—" Sir,—Permit me through your pages to inform Mr. Darby that the new 201n. clear aperture object glass which I have just furnished for Mr. Buckingham shows y.' Andromeda, separated with all powere above 200, and that with a power of 450 the distance between the components is much wider than in proportion to Mr. Dawes's diagram, evidently showing that the separating power depends mainly on the size of the aperture, &c. To the 20in. object glass just finished T Herculls and g Cygni are astonishingly easy objects. We have not as yet had time to make much scrutiny, but the most difficult tests have hitherto, with the full aperture, been Instantly resolved even in an indifferent state of the atmosphere.—I am, Ac, W. Wray."

This account Is I think very interesting, and makes one earnestly wish to hear more. Mr. Buckingham can hardly think it other than complimentary to him to be asked for such information, and I am surprised that he withholds it, more especially as I think the wish for it has been strongly expressed at a meeting of the K. A. S.

G. T.

CHERRY STONING MACHINE.

SIR,—I send a clipping from the Scientific American illustrating a new cherry-stoner.

It is screwed to the table by a hand screw A. An upright B supports the body of the machine. The fruit is held in the left hand as shown in the engraving, and rolls down along B gutter C, and enters the email cups in the periphery of the annular wheel D.

K is a double crank from which a link F imparts vertical reciprocating motion to the cross bar G, and also to the recurved punching bar H. Each time the cross bar G rises, it stud l'engagea with one of the cups on the annular wheel D, turning it along oneeighth of a revolution, and bringing another cup directly under the point of the punching bar H.cnrrying with it the fruit which has fallen into it from the gutter C. Each of the cups has a hole through the bottom large enough to permit the passage ttf the pit: and when the punching bar descends by the rotation of thecrauk, it pierces'the fruit and forces the pit through the bottom of the cup, into the chute 3, whence it falls into a dish placed to receive It.

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Thus the pits may be removed almost as fast as a child can turn the crank, aud the operation is во rapid that the juice does not escape, and the fruit retains its natural shape and appearance.

This invention is manufactured by Geer, Stewart and Brother, Galeeburg, Illinois, U.S.

Kappa.

READINGS FROM THE GLOBES.

Sir,—One of the consequences of the rotation of the earth upon its axis (which we take tor granted as having been proved) Is that man, animals, trees, and atmosphere are carried round with it from W. to B, at different degrees of swiftness, according to their proximity to the equator; but the rotation is so rapid and silent with which we proceed, that none of us can feel it, for the earth rotates

"With inoffensive pace that spinning sleeps
On her Boft ax'e, while she paces even,
And bears thee soft with the smooth air along.'

By reference to Fig. It will be easily seen that whU'e Bho spins on her axis P P from west to east, tho ae who live at the equator E E must be carried around with much greater velocity than those living on the parallel, A A, and these again than those living at B В, while the pole will have scarcely any motion whatever. This will be readily understood by turning round a terrestrial globe, and having once ascertained the length of a degree on the earth's surface, we can then know at what rate per hour any point on the earth proceeds, and it is a matter of considerable nicety to determine the length of a degree, which has been found to vary a little, and to be longer near the pole than the equator. The importance of an accurate measurement of the number of miles. in a degree cannot Fj o- • / p be too highly estimated, for upon it the great Newton founded his theory of universal gravitation, and he was for some time prevented from propounding it from the inaccurate measurement of his age; but for the sake of illustration we will take it as commonly estimated No)\ miles in a degree. Now, as P

the globe completes its

rotation in 24 hours, it of course passes through la"' every hour, and by multiplying oil by 15 we get 1042 5 miles per hour, which is more than 1 mile per second. This is speaking of its actual rotation through 360°, or till it arrives opposite to the same fixed star from which it started, but in reality it revolves nearly 361°, because the aun has a motion of its own, having proceeded about this distance on its annual co i rae In the interval. This, then, le only a close approximation to the actuality. We have seen that the parallels of latitude differ in length as they approach the poles, and consequently London is carried round with less velocity than Quito. If we wish to ascertain its speed we must find the number of miles in the degree in this parallel—namely, .il ',, which is done by measuring the distance between the meridians 15° apart, which is equal to 9$, and then we have a simple proportion sum—15° : »i° : : Cl>{ miles : 43, which gives644 miles per hour as the rate at which London revolves. The most accurate way, however, of determining the number of miles in a degree in London's latitude, is by an easy formula in trigonometry—R :69J: : COB. Ы\ : x. Thus :— log. tin Г. = 1-841985 COB. 51° 30' = 9791150

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