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boiler. This was sufficient to account for the explosion. Below the fall the acid was diluted in the main stream; above, it was stealthily creeping along the bank. There are natural acid waters also. In them the acidity sometimes arises from sulphate of iron, sometimes from vegetable matter. There is one cure for acid water, and it is an easy one; the acidity must be neutralized by adding alkali in sufficient quantity.

This alkali may be lime or it may be soda, and in some waters as much as half a pound of soda carbonate is required for each 1,000 gallons of water; for some large boilers this means a daily addition of from 2 to 4 pounds of soda carbonate. Lime would neutralize double the quantity, but would then go into the boiler and form scale.

It seems reasonable to suppose that when an acid water must be used it would be well to pass it through a filter of limestone chips. These would neutralize the acidity and possibly only become changed by the water and not dissolved and carried away with it. In any case, to prevent rapid corrosion by the water it must be served with sufficient alkali to cause it to produce the blue reaction on litmus paper.

As regards the final removal of scale or mud, there are two opinions. One is that the removal of sediment after treatment should take place outside the boiler; the other, that the sediment should be removed from the inside of the boiler.

In the March number various methods will be explained and illustrated.

PAVEMENTS.

Benjamin F. La Rue.

WHAT CONSTITUTES A SATISFACTORY PAVEMENT THE WEARING-SURFACE, BASE, AND NATURAL FOUNDATION-QUALITIES ESSENTIAL TO AND MATERIALS SUITABLE FOR EACH.

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We all recognize a satisfactory roadway pavement when we ride over it, and a suitable footway pavement when we walk upon it, but we are not generally familiar with the conditions essential to a properly constructed and satisfactory pavement of either kind. Indeed, comparatively few of us understand of what a pavement really consists, further than that we are able to name the material in the surface of those pavements with which we are familiar. In the present article we will endeavor to learn something about the essential parts of, and the conditions requisite to, a suitable roadway pavement.

Pavements are for the purpose of improving the facilities for, and reducing the expense of, the transportation of merchandise and all industrial products, and for

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increasing the safety, speed, comfort, and pleasure of travel. Such are the purposes for which they are constructed, and for any particular case, the pavement that will meet these conditions most effectually and with the greatest degree of economy, will be the best and most suitable. should afford a smooth, even surface, offering the least possible resistance to traction and over which vehicles may pass with ease, safety, and comfort, and should, at the same time, furnish an impervious covering that will protect the soil of the natural foundation; it should also have strength to distribute the weight of the loads concentrated upon the wheels over sufficient areas of the natural foundation to be supported without destructive effect upon that foundation. This is necessary in order that the pavement may retain its smooth and even surface.

In the construction of a pavement we have to deal with three different, and more or less distinct, features, namely, the wearing-surface; the base, or artificial foundation; and the natural subsoil foundation, or roadbed. The first two are integral parts of the artificially constructed pavement, while the last generally consists of the bed of natural earth, or subsoil, upon which the pavement

rests, although, where the natural soil is very unstable, this foundation, also, is sometimes artificially prepared.

The wearing-surface is the upper layer of material which constitutes the finished surface of the roadway and which sustains the traffic; it is that visible portion of the pavement with which we are familiar, and by which the traffic is directly affected. In order that this wearing-surface may be satisfactory for public travel, there are certain qualities which it should possess.

It should be impervious to water or other liquids that may fall upon or flow over its surface, and its surface should be of such form as to promptly discharge them into the side gutters and drainage outlets.

It should be hard, tough and durable, so as to resist the wear of the traffic and the disintegrating effect of the elements.

It should be smooth and even, so as to offer the minimum resistance to traffic, and, at the same time, should be of such a character as to afford a secure foothold for horses, and one which will not become polished and slippery from use.

It should be comparatively noiseless, of such a character as to yield very little dust or mud, and should be easily cleaned.

It should be adapted to the grades and suited to the traffic.

There are a number of different kinds of material that fill the above requirements more or less perfectly, and that have proved reasonably satisfactory. Others have also been tried, but have not proved suitable. The following materials have been quite extensively used, namely: asphalt, bricks, stone blocks, wood blocks, cobblestones, and broken stone. These materials are named in about the order of their comparative merits, although the true value of any one of them depends largely upon the character of the traffic to which it is subjected. For instance, if the pavement is subjected to a constant and exceedingly heavy traffic, granite blocks will be the best material for the wearing-surface; for a residence street, having a moderate traffic of reasonably light character, with more or less pleasuredriving, asphalt may be the best. Most of the materials named above have proved reasonably satisfactory for the wearing-surfaces of properly constructed pavements where the traffic is of the character to which the material is adapted.

As the surface is the only part of a completed pavement that is visible, we very naturally designate the pavement by the

name of the material constituting its wearingsurface, and we are very apt also to consider the wearing-surface to be in all respects representative of the entire pavement. We not uncommonly attribute the condition of a pavement, and whatever of merit it may possess, wholly to the wearing-surface; if the pavement has proved satisfactory, we generally think that another pavement, having a wearing-surface composed of the same material, would be equally satisfactory; and if the pavement has not proved satisfactory, we are inclined to condemn all pavements having wearing-surfaces of the same material.

This idea, though quite common, is very erroneous. The wearing-surface is a very essential part of any pavement, and none can be satisfactory unless this portion of it is formed of suitable material and is properly constructed. Though it be ever so well constructed, however, the wearingsurface cannot satisfactorily sustain the traffic without being in turn supported by a firm and unyielding foundation, any more than the superstructure of a building can support its contents and retain its proper form unless it is upheld by a suitable foundation, or than a vehicle that is mounted on broken wheels can be a satisfactory means of transportation,

As a matter of fact, the wearing-surface of a pavement is properly little more than a surface, and, of itself alone, is not capable of sustaining traffic and distributing weight over a sufficient portion of the yielding soil beneath it. It is, therefore, necessary that the wearing-surface of any pavement should rest upon, and be sustained by, a foundation having sufficient strength to resist deformation from the concentrated loads of the traffic, and it should also distribute the loads over sufficient areas of the underlying soil so that the soil will not be unevenly compressed or distorted, but will sustain the loads without injury. In any pavement, the value and condition of the wearing-surface, and, consequently, of the pavement as a whole, will depend largely upon the efficiency of the foundation. It is thus seen that a well-constructed foundation of suitable material is very essential. The materials commonly used for the foundations are hydraulic cement concrete, bituminous concrete, broken stone, bricks, gravel, sand, and plank. Here, again, the materials are named in the order of their respective merits. Hydraulic cement concrete, composed of hydraulic cement, sand, and broken

stone, forms, when it has become thoroughly set, a solid, unyielding foundation that is also impervious to water; it very efficiently distributes the loads upon, and protects, the underlying soil of the natural foundation; the same is, to a less extent, true of bituminous concrete, which is com

the passage of all subsequent loads, thus disturbing and destroying the form of the wearing-surface, but also retain water and tend to further destroy the natural foundation. Plank foundations form at best but temporary expedients; under the usual conditions they soon decay and become even worse than useless.

The soil of the natural foundation should consist of suitable material. Hardpan, gravel, and sand are the best materials; clay is good if well drained; any ordinary earthy material is generally satisfactory if free from decaying vegetable matter; humus, or vegetable mold, is not suitable. Where the natural soil is unsuitable, it is usually necessary to excavate it to a considerable depth and fill in with suitable material, such as gravel, sand, or hardpan; a mixture of gravel or sand with clay forms a good material for this purpose.

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monly composed of coal-tar residuum, creosote-oil and broken stone. Bricks, broken stone, gravel and sand all form, under suitable conditions, reasonably solid, but not impervious foundations, while foundations formed of planks are neither unyielding nor impervious. Plank foundations, as usually constructed, are not capable of distributing the loads concentrated upon the wheels over sufficient areas of the underlying soil to prevent the latter from being unevenly compressed; depressions are formed in the subsoil by the passage of heavy loads, which not only allow the planks to spring with

e.

In the accompanying illustration is shown the cross-section of a brick pavement. The wearing-surface b is composed of bricks set on edge upon the layer of concrete c, which is spread upon the natural earth foundation The curbing k consists of thin slabs of stone set along the edges of the roadway to define its boundary and serve somewhat as a retaining wall for the adjacent earth. The flagstone f of the sidewalk rests directly upon the layer of sand s, which is spread upon the natural earth foundation e, and acts somewhat as a cushion to distribute and equalize the pressure.

THEORY AND PRACTICE.

THEORETICAL AND PRACTICAL FORMULAS-RULES OF THUMB NOT PRACTICAL FORMULAS-THEORY AS A TEST, SAFEGUARD, AND GUIDE-RIVETED JOINTS AS AN EXAMPLE.

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HE controversy between theory and practice in the machine-shop, or between the drafting-room and the shop, or the college-graduate engineer and the practical machinist, is as old as the art of machine-building. There is no doubt that machines were built long before theory gave us the reasons for the results obtained, and long before the age of formulas. Among the artisans of old, rules always existed, deviating from which would have been dangerous in many ways, and those rules were, in the majority of cases, so well established by the results obtained that their correctness could not and cannot to-day be questioned; they have stood the test of theoretical investigation since. But there have been other rules, long cherished ones, many of which are still lingering in the minds of some people, rules that theory has no explanation for. These are the "rules of thumb." While a purely theoretical solution of even the simplest problem in the mechanical arts must necessarily be incomplete on account of the utter impossibility of theoretically considering each and every secondary influence bearing on the final result, theory is the only test, safeguard, and guide for our practical rules. The "theoretical formula" forms as it were the skeleton upon which practical experience builds the flesh which must be added to complete the body, and the result is the "practical formula," which takes account of all minor details and allows for all those secondary influences which theory had overlooked, either purposely, to avoid too complicated a formula, or without purpose, in cases where the true cause of a certain effect had not been recognized. The more thoroughly theory investigates causes and subsequent effects, the more closely do its formulas tally with those obtained by practical methods, until finally the ideal 'theoretical" formula becomes at the same time the ideal "practical" one. Of these there are at present very few. It is hoped, however, that there will come a time when eternal peace will reign between theory and practice, and when there will

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be no more mysterious rules of thumb that cannot be made transparent even to the strongest X-rays that science can produce. In order to illustrate the above, there are given below a number of practical formulas and tables relating to riveted lap-joints-a subject that is generally among the first to be treated in text-books on machine design; and a few remarks pointing towards the theories involved are added. The tables are considered as embodying the best practice and are valuable, therefore, to the practical machinist in laying out such work.

In every case the thickness t of the shell is given, and upon this the other data are dependent. Commencing with the dimensions e, Fig. 1 (the distance of the center line

HOME STUDY.

FIG. 1.

of rivets from the edge of each plate) these must be such as to ensure safety against the breaking of the plate as indicated at A. A simple theoretical formula for calculating the values of e cannot well be given, as the strains and stresses at these points are complicated on account of calking. It is an ordinary practical rule to make e equal to 1 times the diameter d of the rivet; for thin plates this may be somewhat increased, say to 1 times the diameter of the rivet. Too wide laps make it difficult to calk the joints, but, nevertheless, some advise as wide a lap as 3 to 33 times the diameter. The diameters of the rivets should be such as to be secure against crushing as well as shearing. The diameters d given in Table I are safe against crushing, and if the proper pitch, or distance between rivets, is chosen, they will also be safe against shearing. The diameter of rivet can be found by theoretical deduction, taking into account the fact that there is a limit to the size of hole which it is possible to punch; or, in other words,

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HOME STUDY

FIG. 2.

To prevent the bursting of the shell along the center line of the rivets, the distance between the latter-or the pitch p-must be large enough. Theory says as follows: Take a strip A of the width p. (See Fig. 2.) The ends of this strip are held together at the joint by one rivet. Suppose sufficient force is applied to tear the joint apart, then, either the rivet will be shorn off or the plate will burst along the center line BC. To give equal chance to both elements, the shearing strength of the rivet must be equal to the tensile strength of the metal of the plate on both sides of the rivet-holes (p-d). Now it is very easy to make up the following equations:

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But it is a purely theoretical formula and, as is to be expected, does not allow for various minor influences which are at work on the final result. As will be seen from Fig. 2, it is necessary, first, to make allowance for the fact that the plate at a lap-joint is subjected, not to a straight pull, but to an oblique one and has therefore to resist a force which tends to bend it. We cannot therefore insert the full tensile strength in our formula, but must use a smaller amount. Thus, if a material has by test a tensile strength of 45,000 pounds, it will be well to substitute 40,000 in the formula. Punching the holes will also have some effect on the strength of the plate, the material being crowded up around the edge of the hole. Annealing rectifies that however to a great extent. Fig. 2 also shows that the rivet is not subjected to a shearing stress pure and simple, but to tension as well. It will be advisable, therefore, to modify the constant for S, also. It must further be remembered that it makes a great difference whether the rivet-holes are punched or drilled. 46,000 pounds per square inch is a good value for iron rivets in punched holes. With these values, then, our formula now reads a X 46,000 +d=p.

t X 40,000

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From this, Table III has been calculated. The assumption that SS, is not necessarily arbitrary, however, as, in cases where the holes are drilled in iron plates, this condition approximately exists. The figures in Table III are also better suited for steel plates and iron rivets.

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