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galvanometer 81°, which mean3 a very large current, and at 81° it kept it for hoars. It remains ready for aotim at all times, and the heavy sulphate of zinc, as it is formed, sinks below the silver and leaves the solution active for a prolonged period, and may be drawn off by a syphon at intervals, and replaced with fresh acid solution without wasting the upper portion. This perfect cell has been too recently made to enable me to give all its results, but amuch smaller one, and one not so fully developed, and contained in a quart jar, gave 78-*, and after use for several ilays steadily in electrotyping, and three weeks irregular experimental work, gave 62", having had had nothing done to it meanwhile. So powerful is this cell, that after the introduction of a porous jar to contain the zinc, it gave 76°, and with common salt substituted for acid in the porous jar it gave 63"; for many purposes it might be so worked, as cast zinc might then be used, the loeal action being lessened; in such case, as wide a porous jar as possible should be used, and its contents replenished at intervals when required.

(To be continued.)

MECHANICAL MOVEMENTS.*

(Continued from paye 124.)

n-4 A COMBINATION movement, in which O JL • the weight moves vertically with a reciprocating movement; the down-stroke being shorter than the up-stroke. B U a revolving disc, carrying a drum which winds round itself the cord D. An arm C is jointed to the disc and to the upper arm A, so that when the disc revolves the t'rm A moves up and down, vibrating on the point U. This arm carries with it the pulley E. Suppose we detach the cord from the drum and tie it to a fixed point, and then move the arm A UP and down, the weight W will move the same distance, and in addition the movement given to it by the cord, that is to say the movement will be doubled. Now let us attach the cord to the drum and revolve the disc B, and the weight will move vertically with the reciprocating motion, in which the down-stroke will be shorter than the up-stroke, because the drum is continually taking up the cord.

02 and 63. The first of these figures is an end view, and the second a side view, of an arrangement of mechanism for obtaining a series of changes of velocity and direction. D is a screw in whioh is placed eccentrically the cone B, and 0 is a friction roller which is pressed against the cmie by a spring or weight. Continuous rotary

* Kxtrnctcrt from a compilation by Mr. U. J. Brown, Kill tor of the American Artisan.

motion, at a uniform velocity of the s'.rew D, carrying the eccentric cone, gives a ssries of changes of velocity and directum t) the roller C. It will be understood that during every revolution of the oone the roller would press against a different part of the cone, and that it would describe thereon a spiral of the same pitch as the screw D. The rolter C would receive a reciprocating motion, the movement in one direction being shorter than that in the other.

64. Two worm-wheels of equal diameter, but one having one tooth more than the other, both in gear with the same worm. Suppose the first wheel has 100 teeth and the second 101, one wheel will gain one revolution over the other during the passage of 100 X 101 teeth of either wheel across the plane of centres, or during 10,100 revolutions of the worm.

65. Variable motion. If the conical drum has a regular circular motion, and the friction-roller is made to traverse lengthwise, a variable rotary motion of the friction-roller will be obtained.

6C. The shaft hast *o screws of different pitches cut on it, one screwing into a fixed bearing, and the other into a bearing free to move too and fro. Rotary motion of the shaft gives rectilinear motion to the moveable bearing, a distance eqaal to the difference of pitches, at each revolution.

67. Friction pulley. When the rim turns in tbe opposite direction to the arrow, it give? motion to the shaft by mean3 of the pivoted eccentric arms; but when it turns in the direction of the arrow, the arms turn on their pivots, and the shaft is at rest. The arms are held to the rim by springs.

68. Circularinto reciprocating motion by means of a crank and oscillating rod.

69. Continued rectilinear movement of the frame with mutilated racks gives an alternate rotary motion to the spur-gear.

70. Anti-friction bearing for a pulley.

71. On vibrating the lever to which the two pawls are attached, a nearly continuous rectilinear motion is given to the ratohet-bar.

72. Rotary motion of the bevelled diso cam gives a reciprocating rectilinear motion to the rod bearing on its circumference.

73. Rectilinear into rectilinear motion. When the rods A and B are brought together, the rods C and I) are thrust further apart, and vice versa.

74. An engine-governor. The rise and fall of the balls K, are guided by tbe parabolic curved arms B, on which the anti fraction wheels L run. The rods F connecting the wheel L with the sleeve move it up and down the spindle C D.

75. Rotary motion of the worm gives a rectilinear motion to the rack.

| (To be cmtinMcd.)

LIEUTENANT-GENERAL H CTCIIINSON'S 1'ROPOSED NEW RAILWAY Si'STEM.

P>EFLECTION will show that coning of the * wheels of railway carriages is so far wrong in principle that it militates against progress in. proportion to the extent of the coning.

The plan was originally designed in order that at cur res centrifugal force should be met bywheels higher on the outer rail than on the inner, and that the wheels running over the greater distance (the outer curve) should virtually have a larger diameter than the inner wheels running over a less distance (the inner curve). The incorrectness of this reasoning is evinced by the fact that, owing to centrifugal force, the tendency of the carriage is to run in a tangential direction (calculated from the centre of the carriage), thereby forcing the outer leading wheel against the outer rail—the inner near wheel against the inner rail.

That coning wheels is mathematically opposed to progression, and occasions much waste of steam power, is manifest (for it is a drag on every carriage) when we consider that the wheels of all railway carriages could be coned to such an angle with the horizon that the lateral pressure againtt the rail would exceed the vertical pressure on it, thus producing a jamb, and that the less the au,:le the less would be the jamb—in other words, the less the coning the less the resistance to progress. Il thus becomes evident that no wheel would so little resist progress »s the cylindrical.

With wheels coned merely to .an angle of 5° with the horizon, the increase of draught =

W 0873 —, W representing the weight of the car

2 riage.

When it curves, or from swaying to the right or left, the carriages shift to the part coned to 45° near the flange, the draught is doubled. The pressure (the horizontal pressure) varies as the tangent of the angle that the coned surface makes with the horizon.

Moreover, this constant resistance to progress from constant lateral pressure is yet further increased when the carriages, by swaying from aide to side, cause one wheel to run np hill, tbe opposite wheel down hill, thereby creating jerks and oscillations, especially at high velocities, detrimental to comfort, and as injurious to the permanent way as to the rolling stock—exacting', moreover, unnecessary demands on the steaxn power—" unnecessary" because they would not in: exacted by unconed wheels.

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These considerations predisposed us to wish success to any proposition for safely substituting cylindrical for coned wheels, and we feel bound to say that we think the inventor has designed a plan that will be found of great advantage to future lines. By his making the carriages run on many wheels, the rails may be made lighter than is usual, for it is nowhere strained by a jjreatweight on any one spot, and by his giving (as shown on the curved line of rail, Fig. 2 in the woodcut) an increased breadth to the central wheels, proportionate to the severity of the curve, carriages can travel at the greatest speed round a circle whose radius (as is well exemplified in a model he has made) little exceeds their wheel base—for the outer edge of the broad wheels is at the severest curves supported by the rail, thus ensuring safety by virtually enlarging the base. In olher words, the more centrifugal force acts upon the carriage, the wider is the part of the base which resists its displacement.

The chord (movable) of the curve may be regarded as the true rigid wheel base of the carriage. All the axles of the wheels are at right angles to the chord, and in every instance half the breadth of each wheel equals in length the part of the line, in the vertical plane of the axle, intercepted between the chord and the arc. To the full breadih must be added, as before -' .iteil, the breadth of the rail, and about lin. for " plav."

To ascertain the exact breadth of the wheels, let the breadth of half the axles of the wveral wheels (exclusive of the allowance for width of rail and " play ")' be respectively represented by D D, D,„ &c., placed respectively at distances C Ci di, &c, from the centre of the chord (the centre of the wheel base).

Let G represent the width of the gauge,

R represent the radius of the curve of rail.

Then

D = ^ C Ra (R — G)

Di = */Ci' — tt» — (U — G)

D = .y C„* It' — (R — G)
&c., tos., &c

Only the wheel base (the chord) lying within the curve has been spoken of. It is evident that the wheel base can be extended at both ends on the prolongation of the chord beyond the curve. Wheels placed on this prolongation prevent any overhang. The less abrupt the curves the greater might be the number of wheels.

It is mathematically correct that, regarding the part of the radius intercepted between the centre of the chord and the aro as unity of any measure, the diameter of the circle is equal to the tquare of half the chord expressed in the same measure plus 1.

The breadth given to the wheels would not lead to increased width at cuttings. At extremely severe curves the rail would be laid down close to the outer edge of the curve. With the curves

of the ordinaiy radius of 20 chains, the brearith of the broadest wheels needs be no more than i; u beside the width of rail and the usual imh for "play." The projection of the broadi at wheel outside the rails could never exc(cd tytn.

The less acute the sharpest enrve, the lets need be the width given to the wheels, and the greater might be the length assigned to the wheel base within the curve, as well as without the curve.

The constructor of the locomotive would have to devise a somewhat different arrangement than is usual for some of its parts, but one quite feasible, to give room for the wide tires of the central wheels. It would be simpler than any double "bogie" carriage, from all the axles being parallel.

The parallelism of the axles would never have been found objectionable bad it not been for the coning, and the grinding of the flanges against the rails. In many countries the whole of the oommon carts still continue a fixture to the axle, yet with heavy loads tuey easily turn corners where the outer wheel has to travel over a space many times exceeding the distance traversed by the inner. With the very unusual sharp cur\e of a 5 chain radius(110 yards)measured from the outer wall—in a curve of J of a circle—the gauge being 1J ft. between the centre of the rails, the outer wheels only run over 235G2 yards more than the inner. Manifestly, therefore, the evil arising from the difference in travel of the wheels of untlanged cylindrical wheels is more imaginary than real.

When a carriage is stationary, the perpendicular line in the vertical central longitudinal plane of the axle intercepted between the point of contact of the cylindrical wheel with the rail, may bo regarded as (.be spoke of a wheel (tins spnko on which the carriage rests). During one revolution of tho wheel such imaginary spokes are endless in number—ever of the same length in cylindrical wheal*, hot of constantly varying length in coned wheel*, as well as of varying inclination from the perpendicular, ac«ortlingto the degree of coning.

These points of contact (especially on the inventor's suggested light, deep, narrow rails) must be very small, therefore the change of the imaginary spokes from their one plane at right >u)gles to the axle, to the unresisted spiral rotation necessary when the direction of the rail alters, cannot retard the rotation of the wheel (cannot retard progress), for oach successive spoke still revolves in its own plane at right angles to the axle. On straight 'lines of rail, and at regular curves, all the imaginary vertical axles of each wheel rotate ,in one plane ever at right angles to the axle.

The 10 pair of wheels shown (Fig 1) in the representation of the locomotive (which is about double the ordinary length) are driving wheels—■ all have the same diameter, and all the pistons the snme travel. The area of the piston of the central cylinder is of course about 5 larger than tho pistons of the cylinders at the extremities, as it, drives 4 pair of wheels—the others only 3 pair. Should there be a practical difficulty, which is not apprehended in making the one piston (with ite double rod) both pnll and thrust, and the 4 pair of wheels work harmoniously together, there might be two pistons in one cylinder, or two cylinders.

As regards progression, it seems undeniable that cylindrical wheels must demand less haulage than coned, whether or not they be arranged In "bogie" frames, and the inventor retains them on the line by means a9 little opposed to progress as can be imagine))—viz., by guide wheels that have na other duty to perform than, on a straight line, to occasionally touch and rotate in a vertical plane against the vertical side of the rail, and at curves to act against the sido in the direct manlier that the cylindrical supporting wheels act on the upper surface.

These deeply-flanged guide wheels (Fig. 8) are placed at the extremities of the wheel base (within the enrve), and sustain no more of the weight of the carriage than suffices, by means of a screw, so to press them down, that however sudden may be aag>sinking in the rail, the wheels follow it,a<idcaonotgetdisplaced. Through a horizontal movement (eqnal to the extent required by the severest curve) they, readily adjust themselves to the coarse of the rail', for the great length of the stem (playing in the tube strongly fixed to t he carriage) saves them from any severe lateral pressure at top. In eases of difficulty this wheel seems much safer than the ordinary wheel, which, owing to the coning, has a bias to run upwards over tho rail.

Tho wheels 6 and i are supposed to have flanges, which, however, are never called upon to act unless through some unlikely chance the guide wheels should get out of order, when as regards safety the carriage would be on a par with those now running.

It is evident that tho diminntion of hau'agc, consequent on tho employment of cylindrical unrlmiged wheels solely to support, and of guide wheels solely to direct, must effect a valuable saving of. steam for the substitution is made in every class of carriage.

The inventor says that the correctness of the principle would he inexpensively tested on any line by firmly joining three passenger carriages together supported onunconcd wheels, and fixing vertical guide .wheels (Fig. 8) at the points of junction. This triple carriage to be connected with the adjoining by the buffer bar described in the patent.

The inventor has designed other gnido wheels (Fur.. 9), which lie in a plane iaclined to the horizon, and icidily pass through the openings at guard rails and switches, but we are of opinion thux the vertical will be the most liked.

The reservoirs containing a lubricant are marked r.

It is part of the proposed system that the rails on sleep inclines shall hoof great breadth, and that t!.e friction (holding pjwer) between them and

the broad wheels shall be further increased by a constant escape of sand between their surfaces during ascents.

As tho carriages run either end foremost, should it so happen in a line of railway that all thesharpest curves between the two termini btnl the same way, it evidently would diminish the width necessary for the broad wheels, were the wheels so fixed to the carriages that, when running on a straight liae, the/sentre of the wheels would not be on tne rail, but lie a little outside of it towards that mde of the rail where the principal corvee bentf owtwarrls.

The corves on most of the railways In England are so wide that (without altering the rails), by giving the proposed additional breadth to the wheels, a greatly increased length could be easily given to all the carriages, therefore to the boiler, with a central fire, consequently augmented steam power to the locomotive; besides the increased holding power acquired by being enabled to place a greatly increased weight over the many driving wheels, and go making every locomotive a tank engine.

The working portion of the locomotive towards1 the centre and rear conld be covered in to prevent injury from dust, not that it injures the machinery of the Fairlie, though its parts are more complicated.

As nearly £500,000,00© have beeo expended on railways in tho United Kingdom, we cannot think, however great might be any suggested! improvements, that a rapid change would be made in a system on which such vast sums have been spent. We are, however, mistaken if the present proposal be not gradually adopted in this country, and if those on whom will devolve the responsibility of constructing future linos abroad, is the most safe, economical, awl durable manner do not follow a plan which we hold to be correct in. principle, and may be improved on in the minor details. It is as applicable to the narrowest as to the widest gangs, and as it gives the means of ascending very steep gradients, and turning the sharpest curvwa, there are few countries where the rails could1 not be laid down as a surface line.

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COLOUR AND COLOtTH BLINDNESS.

I5r •* OmenoN."

(Continued from page 122.)

IN the undulatory theory of light, difference of colour is accoiaMed for by the frequency of the vibrations, evtie number ot impulses made on our nerves in a given time by the ethereal molecules next in contact with them, precisely as in the doctrine of sound the frequency of the aerial pulses, or the n umber of excursions to and from its point of rest made by each molecule of the air, determines the pitch or note.* The shortest wave produces violet light j the longest, red. Sir John Herschel has composed the following table, in which he gives the length, and the rapidity of the coloured rays of the spectrum, assuming the velocity of light to be 192,000 miles per second :—

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From this table we see that tho sensibility of tho eye is confined within much narrower limits than that, of the ear, the ratio of the extreme vibrations being nearly 1*58 :1, and therefore less than an octave, and about equal to a minor sixth. That man should be able to measure with certainty such minute portions of space and time is not a little wonderful, for it may be observed, whatever theory of light we adopt, these periods, and these spaces, have a real existence, being in fact deduced by Newton from direct measurements, and involving nothing hypothetical, but the names here given them. What marvellous mechanism must pervade Nature, to see in the flashing of an eyo the innumerable and varying hues with which the earth is carpeted, the birds gorgeously plumed, and that distinguishes the races of animals and man!

But au acute discrimination of colours is not given to all alike, and we do not speak here of the effect that education in colour produces on the eye, but the natural inability that exists with

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• orne individúala ,to detect certain colours, and » hich no amount of education, or acquaintance xwi h coicur, will entirely obliterate. Among eufferer« from colour blindness, a wide range of relative degrees of ¡nappreciation to colour exists. The first, and the most severe ease, is fortunately rare. It occurs when the sufferer сап discriminate between no two colours ; when Oie entire spectrum such as we have described ».rising from the decomposition of light by a prism presents merely gradations of light and shade, of black and white. In Taylor's "Scientific Memoirs," 184-6, several cases of this strongest form of what is properly called colonr blindness are placed on record, some of which we shall quote in the sequel. Another form in which colour blindness (and herd the term is inappropriately applied) manifests itself is constituted by an inability to discriminate between the nicer shades of the more composite colours, such as browns, greys, and neutral tints, and there exists a last class, with whom it is difficult to distinguish between the primary colours, red, blue, andyellow, or between these and secondary colours.

When we speak of the existence'of an eye, perfectly formed, that wus totally incapable of detecting any difference in colour, and with whom all colours resolved themselves into intermediate shades between black and white, we do not mean to be understood that perfect achromatism has етег been met with. The subject has never been treated with that oare by medical mon to enable us to assert that any eye, perfectly formed, is under all circumstances insensitive to colour. The apparatus for polarising light has enabled the philosopher to produce the most brilliant and purest tints, and till an eye has been subjected to every test that can be suggested shall we be warranted in asserting that a case of absolute achromatism has been produced? "And even if a human eye were altogether nnimprcseible to colour, it seems exceedingly doubtful whether we should be able to discover that it was во. We have it not in our power to subject an eye simply to the influence of colour. Every luminous ray is a bundle of colour-giving, heat-giving, and chemical or actinic rays, mingled in unequal proportion. Each colour ray, therefore, carries with it to the retina a dif

A peculiarity is often noticeable in the colour-1 amusing ease of total colour
blind, by the ease with which they can detect person of a house painter
objects assisted by the least possible light. A'
case is mentioned in Wartmann's paper on the
Colour-Blind, of a woman who could detect no
difference of colour in

the prismatic spectrum, and yet could read a book in a completely darkened room. This may be accounted for by supposing that tho perception of different coloured tints to a normal eye induces a certain amount of fatigue to the retina, and renders it less sensitive to faint light, from which the colour-blind are free. Nor is it to be supposed that in cases where a normal eye perceives an object by the difference of colour from the objects surrounding it, that a colour blind eye see's the object of a smaller size. For instance, in the spectrum thrown upon a white screen, a normal eye is sensible to its appearance by the variation of colour, from the red extremity to the violet; to the colour-blind the spectrum is equally long

f rent number of heat rays and chemical rays. Thus, if for simplicity's sake, we assume only three colours to exist, red, blue, and yellow, then the red r$j of the spectrum is, in reality, when tried by the thermometer, much hotter than the blue or yellow. The blue, which is the coldest and darkest of the three primary colours, greatly excels the red and yellow in the number of chemical rays which accompany it, whilst the yellow, which is totally destitute of such rays, and contains very few heat rays, must necessarily consist almost entirely of colour rays, and further excel» red and blue in luminosity. The retina, accordingly, in so far as it is affected by heat, will be most influenced by the red ray, in so far as it is susceptible of chemical change, by tho blue ray; and in so far as it is influenced by luminosity by the yellow ;ray, apart from the special impressions made upon it by each ray in virtue of the colour." When, therefore, we ask any ene to gaze at a rainbow, and tell us what he sees in it, we ask him to report on a series of complex sensations, to the production of which the brightness, heat, colour, and chemical force present in the light by which he sees it, all do, or at least may, contribute* It is not probable, therefore, that if we were to paint a rainbow in streaks of black and white paint mixed in due proportion, that we should be able to reproduce before a colour-blind person the exact effect that a rainbow presents to him, because there might be a certain chemical quantity wanting, which in the original rainbow produces an impression on the eye of the colour-blind, thongh it is not such an impression as is produced on a normal eye by colour. For instance, Mr. Pole, whose eye is undoubtedly blind to red. and who distinctly asserts that "there is a hue of red which to me is colourless," could not form a red by merely mixing black and white in due proportions, but who to make red mixed one part of lampblack with five or six of chrome ¡/libre, so as to darken it to the required tone. Carmine to his eye was produced by mixing nine part« of lampblnok with one of chrome yellow.

Or. O. Wilson on "Colour BUndnees."

and tho eye is made sensible of its existence by the different degree of illumination that each colour presents, and if the colour-blind person had given to Chase different shades the terms red, blue, green, &c, it would be impossible to discover, from hie description of what the spectrum presented, that he was a sufferer from colour blindness. This fact is particularly shown by the description of the spectrum by persons affected with colour-blindness in the second degree, that is, when two colours present precisely the same appearance, so thnt one colour cannot be detected from the other. Dalton was very particular on this point. Tho spectrum appeared as long to him as it did to Sir David Brewster, only Dalton saw one colour twice over. ^ An instance of this severe deprivation of sensitiveness to colour is described in the "Philosophical Transactions" for 1777, in a letter from a Mr. Huddart to Dr. Priestley. The subject was a shoemaker, named Harris, living in Cumberland. The following is the account he gives of himself :—" That ho had reason to believe other persons saw something in objects which he could not see; that their language appeared to mark qualities with precision and confidence, which he could only guess at with hesitation, and frequently with error. His first suspicion of this arose when he was about four years old. Having, by accident, found in the street a child's stooking, he carried it to a neighbouring house to inquire for the owner; he observed that the people called it a red stocking, though he did not understand why people gave it that denomination, as he himself thought it completely described by being called a stocking. This circumstance, however, remained on his memory, i and, together witli subsequent observations, led him to a knowledge of his defect. He also observed that when young, other children could discern cherries on a tree by some pretended difference of colour, though he could ouly distinguish them from the leaves by the difference of their size and shape I

believe he never could do more than guess the name of any colour, yet he could distinguish white from black, or black from any white or bright colour. In general, colours of an equal degree of brightness, however they might otherwise differ, he confounded together." * There can be little doubt but that this is one of the few reliable cases of a complete insensibility to colour, and it is also related in the narrative of Mr. Huddart, that this man had two brothers similarly affected, and two other brothers and two sisters whose vision presented no peculiarities. The fact of this defect manifesting itself in more than one member of a family is not uncommon ; and there is an instance of a gentleman who had seventeen relatione that suffered more or less from tliis false vision, and often it appears that only the malo members of a family exhibit this peculiarity. Indeed, the phenomena is much rarer among females than among males, and our ingenuity may be tortured to account for this fact. It is however certain, from the facts that Dr. Wilson collected, that when a female is marked by this peculi arity she exerts herself to the uttermost to hide it, which render statistics concerning female colour-blindness very difficult to acourotely ascertain, and lead us to suppose that the number is loss than is really the fact.

Mr. Inglis, of Edinburgh, reports a rather

and perfect in every particular except colour, -either the first or last. In this class the miscorn

blindness in the The explanation of his prosecuting a calling for which apparently he was so unfit is found in the fact that he was an excellent draughtsman, with a goo'd eye for form, and skilful in designing. He trusted to his wife to keep him right in mixing and selecting his colours. On one occasion when she was out of the way, and workmen were scarce, he took a part himself in painting a public building, which he had been employed to put in order. He mixed the colours himself, and believed that he had produced a »Une tint, with which ho proceeded to colour the walls. After he had gone over some square yards, he was informed that he was painting the building blue. " *

The second division that we have made among colour-blind people is marked by the inability to discriminate between the nicer shades of the more composite colours, and this claes comprehends a far greater number of subjects than

"Philosophical Transactions," 1777.

prehension of colour is one of degree, and not of totality, and Dr. Wilson remarks that this defect seems to be rather the rule than the exception—at least among the male sex of this country. A defect of vision of this nature is probably to be overcome in some degree by education, and continual attention to oolour. The inability to name a colour does not necessarily prove that the difference of tho colour from any other is not marked on the retina. The scanty nomenclature employed by men generally in describing colour might lead one to suppose that they appreciated far fewer shades of colour than is actually the case. Even educated men ore to be found who would call scarlet crimson, and magenta indiscriminately red, although they would be distinctly aware of difference between the colours arranged side by side. To judge, therefore, by the inability to name colours, we should conclude that colourblindness was much more common than it fortunately is. In teeting, therefore, for this slight degree of non-discernment of colour, it is necessary to dispense with names of tints. Give to a person a number of squares of cloth, or skeins of wool, and request him to arrange them so that all of the вато colour and shade may bo placed together, in gradual succession from dark to light. We ourselves have submitted this test to many of our acquaintance, ' and our experience is very similar to that of Dr. Wilson. Many will have no difficulty in arranging the full bright tints of the primary and secondary colours, but when the fainter tints come to be arranged, great hesitation will be often evidenced as to which bundle they ought to be placed in. The most frequent mistakes occur in blue ana" green (of course the test is made in full day light), while pink and pale blue arc a stumbling block to many who am able to arrange all the other shades, and the pink and pale yellow are often interchanged. "In truth we are all by birth, coloui-blind to some extent, and painters know how long it is before the most susceptible eye acquires its maximum sensibility to colour."

For the general purposes of life, the slight error in the discernment of colour that this class exhibits is not likely to produce any very ill effect ; not so, however, in the third and last division, in which the sufferers soe no difference between tho primary colours, red, blue, and yellow, or between these and secondary colours, or where the difference is so slightly marked that hesitation and confusion between these colours exist in the mind of the beholder. Dr. Wilson has devoted a large portion of his book to the consideration ot oircamstances in which colour is made to convey a certain amount of information to the mind, as in the case of railway signals and ships' lights. It is an nnfortanate circumstance that, in the case of people who confound colours, red and green are tho two colours that most often present a similar appearance to the eye, and the importance of these two colours in railway signalling is well known, and Dr. Wilson wisely doubts, considering the general prevalence of colour blindness, bnt that among the army of guards and signalmen of this country nnd ' tho continent some cases may be found where the difference of red and green cannot be detecttHl, and the danger of travelling is consequently increased. To those who nre interested in the question wo recommend his book, whore they will find many amusing and instructive pages.

(To be continued.') * L)r. Wilson, Op. Clt. 4G DRESSING MILLSTONES WITH THE

— DIAMOND Oil "BOUT."

■j"' . (Continued from page 128J

'•) ГрНЕ first who appear» to have thought that

c JL the action by which the glazier produces a

I'l' rectilinear crack along the surface of glass with

r< his diamond might he availed of for producing в

ni; series of fine grooves or cracks on the surface of

itr » millstone, so as t > form a suitable drees thereon,

mi. was D. If. Childs, of Rochester, United States,

ft and a pateHt was obtained for his invention in

or this country by A. B. Childs, on 23rd August,

in' 1861, No. 2107. In his provisional specification

w Mr. Childs says :—

¡hi "The particulars of this invention relato to the

r application of the diamond or other mineral-cnt

te ting substance in the production of the small Hues

el (or what is generally known as the cracking), in

is any required form upon the face of millstones,

sn The diamond or other mineral cutting substance

^e (of which one or more may be used) is to be drawn

ba by the hand or by mechanical agency, so that the

al cutting edge or edges mayjeut into the face of the

if millstone, the fine line or cracks, instead of the

i» ordinary metal mill-bill now used. Although I

w prefer the diamond to be drawn by the hand or

3v by mechanical agency, I do notconfiue myself to

h< that particular mode in the application of the

si diamoud for the aforesaid purpose."

cc In his final specification, however, he makes no

▼' reference to "mechanical agency." Fig. 3 is a

tl • view of the instrument he uses, which is described

in the following extracts from his specification, я along with other interesting details :—

£ "The tool is composed of one or more dia

T monds inserted in a bit or handle, and when two

( or more diamonds are used, they are to be firmly

set in a row at equal distances apart. The furrows or cracks arc produced by running with adequate force the point» or diamonds over the face of the stone where the lines are intended to be cut, and such operation may be conveniently performed by running the tool containing the diamond along the edge of aetraight rule. Curved lines are made by operating the same diamond points by the side of a curved ruler. The width of the space separating the lines upon the grinding surface of the stone is regulated by holding the points more or less diagonally across the line» of motion.

"In order to show the precise nature of my improvements I will explain the ordinary modes of dressing millstones. In theordinary mode, the pick, being brought down «pen the face of the stone, produces the stellated fracture, and thereby weaken« or disintegrates the stone as far as the fracture extends, and the edges of the crack which have been thus weakened by the blow from the pick soon crumble away, and the particles thus detached are thrown oat and wear the face of the stone. These disadvantages are prevented by my improvements in dressing millstones.

"The line cut by the diamond npon a glossy surface, which has never been detonated or disintegrated by a blow from a pick, is clear and distinct, having its edges sharp and fine. Millstones will be more perfect whon dressed again according to my invention, as the diamond will cut below the bruises occasioned by the old mode of dressing with a pick.

"In the old mode of dressing with a pick the furrow is rough and nearly flat at the bottom, whereas furrows cut with the diamond are smooth, having the side upon which the grain rises a regular smooth inclined plane. There is no perceptible moisture generated in the operation of grinding with millstones dressed according to my process, and the sprouts are clean and dry, which I attribute to the fact that the grain is kept back or iu towards the eye until well ground, and while the motion is less rapid, and consequently less liable to heat. When the flour has reached the extreme of the breast circle it is rapidly thrown from the stone with few, if any, inequalities to retard its egress, and it will be found that much less power is required for grinding, when my mode of dressing is employed, to produce the same amount of a superior article of flour."

There cannot be a doubt but that millstones can be dressed by Mr. Childs' process in a practically satisfactory manner, and with much better results than by the steel pick ; and it is to be regretted, as Mr. Childs was the first to nse the diamond, that he received so little encouragement, and did not think it worth while to maintain his patent beyond its third year. Recent experience, however, makes l pretty pretty cloar that his want ofeucccss was

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in a great measure owing to his using the common translucent glazier's diamond, instead of bort, which latter is not only lese expensive, but. in every way better suited for the parpise, as we shall shortly explain. It is true Mr. Childs included "other mineral substances" in his patent, but we understand his actual trials were coufined to the common diamond.

The second to use a diamond for millstone dressing was J. DickinsDn, of New York, who invented a kind of parallel ruler for guiding the hand in dr.iwing the diamond along the surface of the stone. His invention was patented in this country in the name of J. A. Knight, on 16th January, 1862, No. 118. The ruler was provided with a ratchet-feed motion, by which the distancée opart of the lines were rendered regular; but the ruler covered only a very small portion of the stone at a time, and in no way tended to preserve its truth or level.

In 1864, J. D. Jobin obtained a patent (No. 148) for a great variety of machine tools -specially adapted for applying bort or carbon as outters in working stone, marble, and other hard substances, and some of the combinations could have been nsed for producing the fine-groove dress upon millstones. It docs not appear that they have as yet been used for that purpose, but there ein be no doubt that this patent covers any use of bort as a substitute for steel covers. Indeed, the importance of this patent will necessitate our again referring to it.

To Samuel Golay, of Paris, undoubtedly belongs the credit of first introducing a practically successful method of dressing millstonee otherwise

than by the old handbill, and his merit is in no way diminished by the fact that since his plan was made known, others of at least equal efficiency, and possessing some advantages over his, have made their api>earance. Golay's appara:« was shown in the Paris Exhibition, but attract no attention. It was patented in this country in the name of G. Davies, on 30th April, 1867 (No. 1245). The British patent was purchased at the commencement of 1868 for, we believe, something less than £4000. The invention was practically tested, and it was decided to charge millers a royalty of £50 per pair of stones for the right to nse it, several millers valuing it so much as to be quite willing to pay that sum. The original purchasers of the patent were fortunate in finding numerous parties willing to take off their hands the trouble of dealing individually with the multitude of millers scattered over the country, and by the middle of the same year they had sold licenses for various districts for prices amounting altogether to close on the amazing sum of £200,000. .

Our readers will be curious to know the detailof a machine occasioning such extensive transactions in so short a time, and we need no excuse in transferring from the specification to our columns tbe following full description :—

"It is well known that for seveaal years th? employment as a tool of hard stone, and especially of the diamond, has been adopted in various trades. Different applications have been made of these stones in working stones less hard, such as granite and others, and it is by a new application o£ this principle that the inventor has arrived at

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