Chapter 29

y=a+bx+cx2+dx3+ex4

gives, on increasingxalways by unity, a set of values for which the fourth difference is constant. We can, by an arrangement like the above, with five clocks calculateyforx= 1, 2, 3, ... to any extent. This is the principle of Babbage's difference machine. The clock dials have to be replaced by a series of dials as in the arithmometers described, and an arrangement has to be made to drive the whole by turning one handle by hand or some other power. Imagine further that with the last clock is connected a kind of typewriter which prints the number, or, better, impresses the number in a soft substance from which a stereotype casting can be taken, and we have a machine which, when once set for a given formula like the above, will automatically print, or prepare stereotype plates for the printing of, tables of the function without any copying or typesetting, thus excluding all possibility of errors. Of this "Difference engine," as Babbage called it, a part was finished in 1834, the government having contributed £17,000 towards the cost. This great expense was chiefly due to the want of proper machine tools.

Meanwhile Babbage had conceived the idea of a much more powerful machine, the "analytical engine," intended to perform any series of possible arithmetical operations. Each of these was to be communicated to the machine by aid of cards with holes punched in them into which levers could drop. It was long taken for granted that Babbage left complete plans; the committee of the British Association appointed to consider this question came, however, to the conclusion (Brit. Assoc. Report, 1878, pp. 92-102) that no detailed working drawings existed at all; that the drawings left were only diagrammatic and not nearly sufficient to put into the hands of a draughtsman for making working plans; and "that in the present state of the design it is not more than a theoretical possibility." A full account of the work done by Babbage in connexion with calculating machines, and much else published by others in connexion therewith, is contained in a work published by his son, General Babbage.

Fig. 4.--Slide rule.Fig.4.

Slide rules are instruments for performing logarithmic calculations mechanically, and are extensively used, especially whereSlide rules.only rough approximations are required. They are almost as old as logarithms themselves. Edmund Gunter drew a "logarithmic line" on his "Scales" as follows (fig. 4):—On a line AB lengths are set off to scale to represent the common logarithms of the numbers 1 2 3 ... 10, and the points thus obtained are marked with these numbers.As log 1 = 0, the beginning A has the number 1 and B the number 10, hence the unit of length is AB, as log 10 = 1. The same division is repeated from B to C. The distance 1,2 thus represents log 2, 1,3 gives log 3, the distance between 4 and 5 gives log 5 - log 4 = log 5/4, and so for others. In order to multiply two numbers, say 2 and 3, we have log 2 × 3 = log 2 + log 3. Hence, setting off the distance 1,2 from 3 forward by the aid of a pair of compasses will give the distance log 2 + log 3, and will bring us to 6 as the required product. Again, if it is required to find 4/5 of 7, set off the distance between 4 and 5 from 7 backwards, and the required number will be obtained. In the actual scales the spaces between the numbers are subdivided into 10 or even more parts, so that from two to three figures may be read. The numbers 2, 3 ... in the interval BC give the logarithms of 10 times the same numbers in the interval AB; hence, if the 2 in the latter means 2 or .2, then the 2 in the former means 20 or 2.

Soon after Gunter's publication (1620) of these "logarithmic lines," Edmund Wingate (1672) constructed the slide rule by repeating the logarithmic scale on a tongue or "slide," which could be moved along the first scale, thus avoiding the use of a pair of compasses. A clear idea of this device can be formed if the scale in fig. 4 be copied on the edge of a strip of paper placed against the line A C. If this is now moved to the right till its 1 comes opposite the 2 on the first scale, then the 3 of the second will be opposite 6 on the top scale, this being the product of 2 and 3; and in this position every number on the top scale will be twice that on the lower. For every position of the lower scale the ratio of the numbers on the two scales which coincide will be the same. Therefore multiplications, divisions, and simple proportions can be solved at once.

Dr John Perry added log log scales to the ordinary slide rule in order to facilitate the calculation ofaxorexaccording to the formula log logax= log loga+ logx. These rules are manufactured by A.G. Thornton of Manchester.

Many different forms of slide rules are now on the market. The handiest for general use is the Gravet rule made by Tavernier-Gravet in Paris, according to instructions of the mathematician V.M.A. Mannheim of the École Polytechnique in Paris. It contains at the back of the slide scales for the logarithms of sines and tangents so arranged that they can be worked with the scale on the front. An improved form is now made by Davis and Son of Derby, who engrave the scales on white celluloid instead of on box-wood, thus greatly facilitating the readings. These scales have the distance from one to ten about twice that in fig. 4. Tavernier-Gravet makes them of that size and longer, even ½ metre long. But they then become somewhat unwieldy, though they allow of reading to more figures. To get a handy long scale Professor G. Fuller has constructed a spiral slide rule drawn on a cylinder, which admits of reading to three and four figures. The handiest of all is perhaps the "Calculating Circle" by Boucher, made in the form of a watch. For various purposes special adaptations of the slide rules are met with—for instance, in various exposure meters for photographic purposes. General Strachey introduced slide rules into the Meteorological Office for performing special calculations. At some blast furnaces a slide rule has been used for determining the amount of coke and flux required for any weight of ore. Near the balance a large logarithmic scale is fixed with a slide which has three indices only. A load of ore is put on the scales, and the first index of the slide is put to the number giving the weight, when the second and third point to the weights of coke and flux required.

By placing a number of slides side by side, drawn if need be to different scales of length, more complicated calculations may be performed. It is then convenient to make the scales circular. A number of rings or disks are mounted side by side on a cylinder, each having on its rim a log-scale.

The "Callendar Cable Calculator," invented by Harold Hastings and manufactured by Robert W. Paul, is of this kind. In it a number of disks are mounted on a common shaft, on which each turns freely unless a button is pressed down whereby the disk is clamped to the shaft. Another disk is fixed to the shaft. In front of the disks lies a fixed zero line. Let all disks be set to zero and the shaft be turned, with the first disk clamped, till a desired number appears on the zero line; let then the first disk be released and the second clamped and so on; then the fixed disk will add up all the turnings and thus give the product of the numbers shown on the several disks. If the division on the disks is drawn to different scales, more or less complicated calculations may be rapidly performed. Thus if for some purpose the value of sayab³ √cis required for many different values ofa,b,c, three movable disks would be needed with divisions drawn to scales of lengths in the proportion 1: 3: ½. The instrument now on sale contains six movable disks.

Continuous Calculating Machines or Integrators.—In order to measure the length of a curve, such as the road on a map, aCurvometers.wheel is rolled along it. For one revolution of the wheel the path described by its point of contact is equal to the circumference of the wheel. Thus, if a cyclist counts the number of revolutions of his front wheel he can calculate the distance ridden by multiplying that number by the circumference of the wheel. An ordinary cyclometer is nothing but an arrangement for counting these revolutions, but it is graduated in such a manner that it gives at once the distance in miles. On the same principle depend a number of instruments which, under various fancy names, serve to measure the length of any curve; they are in the shape of a small meter chiefly for the use of cyclists. They all have a small wheel which is rolled along the curve to be measured, and this sets a hand in motion which gives the reading on a dial. Their accuracy is not very great, because it is difficult to place the wheel so on the paper that the point of contact lies exactly over a given point; the beginning and end of the readings are therefore badly defined. Besides, it is not easy to guide the wheel along the curve to which it should always lie tangentially. To obviate this defect more complicated curvometers or kartometers have been devised. The handiest seems to be that of G. Coradi. He uses two wheels; the tracing-point, halfway between them, is guided along the curve, the line joining the wheels being kept normal to the curve. This is pretty easily done by eye; a constant deviation of 8° from this direction produces an error of only 1%. The sum of the two readings gives the length. E. Fleischhauer uses three, five or more wheels arranged symmetrically round a tracer whose point is guided along the curve; the planes of the wheels all pass through the tracer, and the wheels can only turn in one direction. The sum of the readings of all the wheels gives approximately the length of the curve, the approximation increasing with the number of the wheels used. It is stated that with three wheels practically useful results can be obtained, although in this case the error, if the instrument is consistently handled so as always to produce the greatest inaccuracy, may be as much as 5%.

Fig. 5.--Amsler's Planimeter.Fig.5.

Planimeters are instruments for the determination by mechanical means of the area of any figure. A pointer, generally called thePlanimeters."tracer," is guided round the boundary of the figure, and then the area is read off on the recording apparatus of the instrument. The simplest and most useful is Amsler's (fig. 5). It consists of two bars of metal OQ and QT,which are hinged together at Q. At O is a needle-point which is driven into the drawing-board, and at T is the tracer. As this is guided round the boundary of the figure a wheel W mounted on QT rolls on the paper, and the turning of this wheel measures, to some known scale, the area. We shall give the theory of this instrument fully in an elementary manner by aid of geometry. The theory of other planimeters can then be easily understood.

Fig. 6.--Theory of Planimeter.Fig.6.

Consider the rod QT with the wheel W, without the arm OQ. Let it be placed with the wheel on the paper, and now moved perpendicular to itself from AC to BD (fig. 6). The rod sweeps over, or generates, the area of the rectangle ACDB =lp, whereldenotes the length of the rod andpthe distance AB through which it has been moved. This distance, as measured by the rolling of the wheel, which acts as a curvometer, will be called the "roll" of the wheel and be denoted byw. In this casep=w, and the area P is given by P =wl. Let the circumference of the wheel be divided into say a hundred equal partsu; thenwregisters the number ofu's rolled over, andwtherefore gives the number of areaslucontained in the rectangle. By suitably selecting the radius of the wheel and the lengthl, this arealumay be any convenient unit, say a square inch or square centimetre. By changinglthe unit will be changed.

Fig. 7.--Theory of Planimeter.Fig.7.

Again, suppose the rod to turn (fig. 7) about the end Q, then it will describe an arc of a circle, and the rod will generate an area ½l²θ, whereθis the angle AQB through which the rod has turned. The wheel will roll over an arccθ, wherecis the distance of the wheel from Q. The "roll" is noww=cθ; hence the area generated is

and is again determined byw.

Fig. 8.--Theory of Planimeter.Fig.8.

Next let the rod be moved parallel to itself, but in a direction not perpendicular to itself (fig. 8). The wheel will now not simply roll. Consider asmallmotion of the rod from QT to Q′T′. This may be resolved into the motion to RR′ perpendicular to the rod, whereby the rectangle QTR′R is generated, and the sliding of the rod along itself from RR′ to Q′T′. During this second step no area will be generated. During the first step the roll of the wheel will be QR, whilst during the second step there will be no roll at all. The roll of the wheel will therefore measure the area of the rectangle which equals the parallelogram QTT′Q′. If the whole motion of the rod be considered as made up of a very great number of small steps, each resolved as stated, it will be seen that the roll again measures the area generated. But it has to be noticed that now the wheel does not only roll, but also slips, over the paper. This, as will be pointed out later, may introduce an error in the reading.

Fig. 9.--Theory of Planimeter.Fig.9.

We can now investigate the most general motion of the rod. We again resolve the motion into a number of small steps. Let (fig. 9) AB be one position, CD the next after a step so small that the arcs AC and BD over which the ends have passed may be considered as straight lines. The area generated is ABDC. This motion we resolve into a step from AB to CB′, parallel to AB and a turning about C from CB′ to CD, steps such as have been investigated. During the first, the "roll" will bepthe altitude of the parallelogram; during the second will becθ. Therefore

w=p+cθ.

Fig. 10.--Theory of Planimeter.Fig.10.

The area generated islp+ ½l2θ, or, expressingpin terms ofw,lw+ (½l2-lc)θ. For a finite motion we get the area equal to the sum of the areas generated during the different steps. But the wheel will continue rolling, and give the whole roll as the sum of the rolls for the successive steps. Let thenwdenote the whole roll (in fig. 10), and letαdenote the sum of all the small turningsθ; then the area is

P =lw+ (½l2-lc)α. . . (1)

Hereαis the angle which the last position of the rod makes with the first. In all applications of the planimeter the rod is brought back to its original position. Then the angleαis either zero, or it is 2πif the rod has been once turned quite round.

Hence in the first case we have

P =lw. . . (2a)

andwgives the area as in case of a rectangle.

In the other case

P =lw+lC . . . (2b)

where C = (½l-c)2π, if the rod has once turned round. The number C will be seen to be always the same, as it depends only on the dimensions of the instrument. Hence now again the area is determined bywif C is known.

Fig. 11.--Theory of Planimeter.Fig.11.

Thus it is seen that the area generated by the motion of the rod can be measured by the roll of the wheel; it remains to show how any given area can be generated by the rod. Let the rod move in any manner but return to its original position. Q and T then describe closed curves. Such motion may be called cyclical. Here the theorem holds:—If a rod QT performs a cyclical motion, then the area generated equals the difference of the areas enclosed by the paths of T and Q respectively.The truth of this proposition will be seen from a figure. In fig. 11 different positions of the moving rod QT have been marked, and its motion can be easily followed. It will be seen that every part of the area TT′BB′ will be passed over once and always by aforward motionof the rod, whereby the wheel willincreaseits roll. The area AA′QQ′ will also be swept over once, but with abackwardroll; it must therefore be counted as negative. The area between the curves is passed over twice, once with a forward and once with a backward roll; it therefore counts once positive and once negative; hence not at all. In more complicated figures it may happen that the area within one of the curves, say TT′BB′, is passed over several times, but then it will be passed over once more in the forward direction than in the backward one, and thus the theorem will still hold.

Fig. 12.--Amslers Planimeter.Fig.12.

To use Amsler's planimeter, place the pole O on the paperoutsidethe figure to be measured. Then the area generated by QT is that of the figure, because the point Q moves on an arc of a circle to and fro enclosing no area. At the same time the rod comes back without making a complete rotation. We have therefore in formula (1),α= 0; and hence

P =lw,

which is read off. But if the area is too large the pole O may be placed within the area. The rod describes the area between the boundary of the figure and the circle with radiusr= OQ, whilst the rod turns once completely round, makingα= 2π. The area measured by the wheel is by formula (1),lw+ (½l²-lc) 2π.

To this the area of the circleπr² must be added, so that now

P =lw+ (½l²-lc)2π+πr²,

P =lw+ C,

where

C = (½l²-lc)2π+πr²,

is a constant, as it depends on the dimensions of the instrument alone. This constant is given with each instrument.

Fig. 14.--Recording wheel with a sharp edge.Fig.14.

Fig. 13.--Amslers planimeter.Fig.13.

Amsler's planimeters are made either with a rod QT of fixed length, which gives the area therefore in terms of a fixed unit, say in square inches, or else the rod can be moved in a sleeve to which the arm OQ is hinged (fig. 13). This makes it possible to change the unitlu, which is proportional tol.

In the planimeters described the recording or integrating apparatus is a smooth wheel rolling on the paper or on some other surface. Amsler has described another recorder, viz. a wheel with a sharp edge. This will roll on the paper but not slip. Let the rod QT carry with it an arm CD perpendicular to it. Let there be mounted on it a wheel W, which can slip along and turn about it. If now QT is moved parallel to itself to Q′T′, then W will roll without slipping parallel to QT, and slip along CD. This amount of slipping will equal the perpendicular distance between QT and Q′T′, and therefore serve to measure the area swept over like the wheel in the machine already described. The turning of the rod will also produce slipping of the wheel, but it will be seen without difficulty that this will cancel during a cyclical motion of the rod, provided the rod does not perform a whole rotation.

Fig. 15.--Early planimeter.Fig.15.

The first planimeter was made on the following principles:—A frame FF (fig. 15) can move parallel to OX. It carries a rod TTEarly forms.movable along its own length, hence the tracer T can be guided along any curve ATB. When the rod has been pushed back to Q′Q, the tracer moves along the axis OX. On the frame a cone VCC′ is mounted with its axis sloping so that its top edge is horizontal and parallel to TT′, whilst its vertex V is opposite Q′. As the frame moves it turns the cone. A wheel W is mounted on the rod at T′, or on an axis parallel to and rigidly connected with it. This wheel rests on the top edge of the cone. If now the tracer T, when pulled out through a distanceyabove Q, be moved parallel to OX through a distancedx, the frame moves through an equal distance, and the cone turns through an angledθproportional todx. The wheel W rolls on the cone to an amount again proportional todx, and also proportional toy, its distance from V. Hence the roll of the wheel is proportional to the areaydxdescribed by the rod QT. As T is moved from A to B along the curve the roll of the wheel will therefore be proportional to the area AA′B′B. If the curve is closed, and the tracer moved round it, the roll will measure the area independent of the position of the axis OX, as will be seen by drawing a figure. The cone may with advantage be replaced by a horizontal disk, with its centre at V; this allows ofybeing negative. It may be noticed at once that the roll of the wheel gives at every moment the area A′ATQ. It will therefore allow of registering a set of values of ∫axydxfor any values ofx, and thus of tabulating the values of any indefinite integral. In this it differs from Amsler's planimeter. Planimeters of this type were first invented in 1814 by the Bavarian engineer Hermann, who, however, published nothing. They were reinvented by Prof. Tito Gonnella of Florence in 1824, and by the Swiss engineer Oppikofer, and improved by Ernst in Paris, the astronomer Hansen in Gotha, and others (see Henrici,British Association Report, 1894). But all were driven out of the field by Amsler's simpler planimeter.

Fig. 16.--Hatchet planimeter.Fig.16.

Fig. 17.--Hatchet planimeter.Fig.17.

Altogether different from the planimeters described is the hatchet planimeter, invented by Captain Prytz, a Dane, and made by HerrHatchet planimeters.Cornelius Knudson in Copenhagen. It consists of a single rigid piece like fig. 16. The one end T is the tracer, the other Q has a sharp hatchet-like edge. If this is placed with QT on the paper and T is moved along any curve, Q will follow, describing a "curve of pursuit." In consequence of the sharp edge, Q can only move in the direction of QT, but the whole can turn about Q. Any small step forward can therefore be considered as made up of a motion along QT, together with a turning about Q. The latter motion alone generates an area. If therefore a line OA = QT is turning about a fixed point O, always keeping parallel to QT, it will sweep over an area equal to that generated by the more general motion of QT. Let now (fig. 17) QT be placed on OA, and T be guided round the closed curve in the sense of the arrow. Q will describe a curve OSB. It may be made visible by putting a piece of "copying paper" under the hatchet. When T has returned to A the hatchet has the position BA. A line turning from OA about O kept parallel to QT will describe the circular sector OAC, which is equal in magnitude and sense to AOB. This therefore measures the area generated by the motion of QT. To make this motion cyclical, suppose the hatchet turned about A till Q comes from B to O. Hereby the sector AOB is again described, and again in the positive sense, if it is remembered that it turns about the tracer T fixed at A. The whole area now generated is therefore twice the area of this sector, or equal to OA. OB, where OB is measured along the arc. According to the theorem given above, this area also equals the area of the given curve less the area OSBO. To make this area disappear, a slight modification of the motion of QT is required. Let the tracer T be moved, both from the first position OA and the last BA of the rod, along some straight line AX. Q describes curves OF and BH respectively. Now begin the motion with T at some point R on AX, and move it along this line to A, round the curve and back to R. Q will describe the curve DOSBED, if the motion is again made cyclical by turning QT with T fixed at A. If R is properly selected, the path of Q will cut itself, and parts of the area will be positive, parts negative, as marked in the figure, and may therefore be made to vanish. When this is done the area of the curve will equal twice the area of the sector RDE. It is therefore equal to the arc DE multiplied by the length QT; if the latter equals 10 in., then 10 times the number of inches contained in the arc DE gives the number of square inches contained within the given figure. If the area is not too large, the arc DE may be replaced by the straight line DE.

To use this simple instrument as a planimeter requires the possibility of selecting the point R. The geometrical theory here given has so far failed to give any rule. In fact, every line through any point in the curve contains such a point. The analytical theory of the inventor, which is very similar to that given by F.W. Hill (Phil. Mag.1894), is too complicated to repeat here. The integrals expressing the area generated by QT have to be expanded in a series. By retaining only the most important terms a result is obtained which comes to this, that if the mass-centre of the area be taken as R, then A may be any point on the curve. This is only approximate. Captain Prytz gives the following instructions:—Take a point R as near as you can guess to the mass-centre, put the tracer T on it, the knife-edge Q outside; make a mark on the paper by pressing the knife-edge into it; guide the tracer from R along a straight line to a point A on the boundary, round the boundary,and back from A to R; lastly, make again a mark with the knife-edge, and measure the distancecbetween the marks; then the area is nearlycl, wherel= QT. A nearer approximation is obtained by repeating the operation after turning QT through 180° from the original position, and using the mean of the two values of c thus obtained. The greatest dimension of the area should not exceed ½l, otherwise the area must be divided into parts which are determined separately. This condition being fulfilled, the instrument gives very satisfactory results, especially if the figures to be measured, as in the case of indicator diagrams, are much of the same shape, for in this case the operator soon learns where to put the point R.

Integrators serve to evaluate a definite integral ∫abf(x)dx. If we plot outIntegrators.the curve whose equation isy=f(x), the integral ∫ydxbetween the proper limits represents the area of a figure bounded by the curve, the axis ofx, and the ordinates atx=a,x=b. Hence if the curve is drawn, any planimeter may be used for finding the value of the integral. In this sense planimeters are integrators. In fact, a planimeter may often be used with advantage to solve problems more complicated than the determination of a mere area, by converting the one problem graphically into the other. We give an example:—

Fig. 18.--Use of Planimeter as Integrator.Fig.18.

Let the problem be to determine for the figure ABG (fig. 18), not only the area, but also the first and second moment with regard to the axis XX. At a distanceadraw a line, C′D′, parallel to XX. In the figure draw a number of lines parallel to AB. Let CD be one of them. Draw C and D vertically upwards to C′D′, join these points to some point O in XX, and mark the points C1D1where OC′ and OD′ cut CD. Do this for a sufficient number of lines, and join the points C1D1thus obtained. This gives a new curve, which may be called the first derived curve. By the same process get a new curve from this, the second derived curve. By aid of a planimeter determine the areas P, P1, P2, of these three curves. Then, ifxis the distance of the mass-centre of the given area from XX;x1the same quantity for the first derived figure, and I = Ak² the moment of inertia of the first figure,kits radius of gyration, with regard to XX as axis, the following relations are easily proved:—

Px=aP1; P1x1=aP2; I =aP1x1=a²P1P2;k² =xx1,

which determine P,xand I ork. Amsler has constructed an integrator which serves to determine these quantities by guiding a tracer once round the boundary of the given figure (see below). Again, it may be required to find the value of an integral ∫yφ(x)dxbetween given limits whereφ(x) is a simple function like sinnx, and whereyis given as the ordinate of a curve. The harmonic analysers described below are examples of instruments for evaluating such integrals.

Fig. 19.--Amsler's integrator or moment-planimeter.Fig.19.

Fig. 20.--.Fig.20.

Amsler has modified his planimeter in such a manner that instead of the area it gives the first or second moment of a figure about an axis in its plane. An instrument giving all three quantities simultaneously is known as Amsler's integrator or moment-planimeter. It has one tracer, but three recording wheels. It is mounted on aAmsler's Integrator.carriage which runs on a straight rail (fig. 19). This carries a horizontal disk A, movable about a vertical axis Q. Slightly more than half the circumference is circular with radius 2a, the other part with radius 3a. Against these gear two disks, B and C, with radiia; their axes are fixed in the carriage. From the disk A extends to the left a rod OT of lengthl, on which a recording wheel W is mounted. The disks B and C have also recording wheels, W1and W2, the axis of W1being perpendicular, that of W2parallel to OT. If now T is guided round a figure F, O will move to and fro in a straight line. This part is therefore a simple planimeter, in which the one end of the arm moves in a straight line instead of in a circular arc. Consequently, the "roll" of W will record the area of the figure. Imagine now that the disks B and C also receive arms of lengthlfrom the centres of the disks to points T1and T2, and in the direction of the axes of the wheels. Then these arms with their wheels will again be planimeters. As T is guided round the given figure F, these points T1and T2will describe closed curves, F1and F2, and the "rolls" of W1and W2will give their areas A1and A2. Let XX (fig. 20) denote the line, parallel to the rail, on which O moves; then when T lies on this line, the arm BT1is perpendicular to XX, and CT2parallel to it. If OT is turned through an angleθ, clockwise, BT1will turn counter-clockwise through an angle 2θ, and CT2through an angle 3θ, also counter-clockwise. If in this position T is moved through a distance x parallel to the axis XX, the points T1and T2will move parallel to it through an equal distance. If now the first arm is turned through a small angledθ, moved back through a distancex, and lastly turned back through the angledθ, the tracer T will have described the boundary of a small strip of area. We divide the given figure intosuch strips. Then to every such strip will correspond a strip of equal lengthxof the figures described by T1and T2.

The distances of the points, T, T1, T2, from the axis XX may be calledy,y1,y2. They have the values

y=lsinθ,y1=lcos 2θ,y2= -lsin 3θ,

from which

dy=lcosθ.dθ,dy1= - 2lsin 2θ.dθ,dy2= - 3lcos 3θ.dθ.

The areas of the three strips are respectively

dA =xdy,dA1=xdy1,dA2=xdy2.

Nowdy1can be writtendy1= - 4lsinθcosθdθ= - 4 sinθdy; therefore

whence

where A is the area of the given figure, andythe distance of its mass-centre from the axis XX. But A1is the area of the second figure F1, which is proportional to the reading of W1. Hence we may say

Ay= C1w1,

where C1is a constant depending on the dimensions of the instrument. The negative sign in the expression for A1is got rid of by numbering the wheel W1the other way round.

Again

dy2= - 3lcosθ{4 cos²θ- 3}dθ= - 3 {4 cos²θ- 3}dy

which gives

and

But the integral gives the moment of inertia I of the area A about the axis XX. As A2is proportional to theroll of W2, A to that of W, we can write

I = Cw- C2w2,Ay= C1w1,A = Ccw.

I = Cw- C2w2,

Ay= C1w1,

A = Ccw.

If a line be drawn parallel to the axis XX at the distancey, it will pass through the mass-centre of the given figure. If this represents the section of a beam subject to bending, this line gives for a proper choice of XX the neutral fibre. The moment of inertia for it will be I + Ay². Thus the instrument gives at once all those quantities which are required for calculating the strength of the beam under bending. One chief use of this integrator is for the calculation of the displacement and stability of a ship from the drawings of a number of sections. It will be noticed that the length of the figure in the direction of XX is only limited by the length of the rail.

This integrator is also made in a simplified form without the wheel W2. It then gives the area and first moment of any figure.

While an integrator determines the value of a definite integral, hence aIntegraphs.mere constant, an integraph gives the value of an indefinite integral, which is a function ofx. Analytically ifyis a given functionf(x) ofxand

Y = ∫cxydxor Y = ∫ydx+ const.

the function Y has to be determined from the condition

Graphicallyy=f(x) is either given by a curve, or the graph of the equation is drawn:y, therefore, and similarly Y, is a length. ButdY/dxis in this case a mere number, and cannot equal a lengthy. Hence we introduce an arbitrary constant lengtha, the unit to which the integraph draws the curve, and write

Now for the Y-curvedY/dx= tanφ, whereφis the angle between the tangent to the curve, and the axis ofx. Our condition therefore becomes

Fig. 21.--Integraph.Fig.21.

Thisφis easily constructed for any given point on the y-curve:—From the foot B′ (fig. 21) of the ordinatey= B′B set off, as in the figure, B′D =a, then angle BDB′ =φ. Let now DB′ with a perpendicular B′B move along the axis ofx, whilst B follows they-curve, then a pen P on B′B will describe the Y-curve provided it moves at every moment in a direction parallel to BD. The object of the integraph is to draw this new curve when the tracer of the instrument is guided along they-curve.

The first to describe such instruments was Abdank-Abakanowicz, who in 1889 published a book in which a variety of mechanisms to obtain the object in question are described. Some years later G. Coradi, in Zürich, carried out his ideas. Before this was done, C.V. Boys, without knowing of Abdank-Abakanowicz's work, actually made an integraph which was exhibited at the Physical Society in 1881. Both make use of a sharp edge wheel. Such a wheel will not slip sideways; it will roll forwards along the line in which its plane intersects the plane of the paper, and while rolling will be able to turn gradually about its point of contact. If then the angle between its direction of rolling and thex-axis be always equal toφ, the wheel will roll along the Y-curve required. The axis ofxis fixed only in direction; shifting it parallel to itself adds a constant to Y, and this gives the arbitrary constant of integration.

In fact, if Y shall vanish forx=c, or if

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