Part 3

The above brief account may suffice to give a general idea of what the cortical jacket covering our timber is, and how it comes about that in the normal case the thickening of the cylinder is rendered possible without exposing the cambium and other delicate tissues: it may also serve to show why bark is so various in composition and other characters. But it is also clear that this jacket of coherent bark, bound together by the elastic sheets of cork, must exert considerable pressure as it reacts on the softer, living, succulent parts of the cortex, trapped as they are between the rigid wood cylinder and the bark; and it is easy to convince ourselves that such is the case. By simply cutting a longitudinal slit through the cortex, down to near the cambium, but taking care not to injure the latter, the following results may be obtained. First, the bark gapes, the raw edges of the wound separating and exposing the tissues below; next in course of time the raw edges are seen to be healed over with cork--produced by the conversion of the outer cells into cork cells. As time passes, provided no external interference occurs, the now rounded and somewhat swollen cork-covered edges of the wound will be found closing up again; and sooner or later, depending chiefly on the extent of the wound and the vigor of the tree, the growing lips of the wound will come together and unite completely.

But examination will show that although such a slit wound is so easily healed over, it has had an effect on the wood. Supposing it has required three years to heal over, it will be found that the new annual rings of wood are a little thicker just below the slit; this is simply because the slit had released the pressure on the cambium. The converse has also been proved to be true--_i.e._, by increasing the pressure on the cambium by means of iron bands, the annual rings below the bands are thinner and denser than elsewhere.

But we have also seen that the cambium is not the only living tissue below the bark: the cortical parenchyma (_pa_) and the cells (_c_) of the inner cortex (technically the phloem) are all living and capable of growth and division, as was described above. The release from pressure affects them also; in fact, the "callus," or cushion of tissue which starts from the lips of the wound and closes it over, simply consists of the rapidly growing and dividing cells of this cortex, _i.e._, the release from pressure enables them to more than catch up the enlarging layer of cortex around the wound.

An elegant and simple instance of this accelerated growth of the cortex and cambium when released from the pressure of other tissues is exhibited in the healing over of the cut ends of a branch, a subject to be dealt with later on; and the whole practice of propagation by slips or cuttings, the renewal of the "bark" of cinchonas, and other economic processes, depend on these matters.

In anticipation of some points to be explained only if these phenomena are understood, I may simply remark here that, obviously, if some parasite attacks the growing lips of the "callus" as it is trying to cover up the wound, or if the cambium is injured below, the pathological disturbances thus introduced will modify the result: the importance of this will appear when we come to examine certain disturbances which depend upon the attacks of fungi which settle on these wounds before they are properly healed over. In concluding this brief sketch of a large subject, it may be noted that, generally speaking, what has been stated of branches, etc., is also true of roots; and it is easy to see how the nibbling or gnawing of small animals, the pecking of birds, abrasions, and numerous other things, are so many causes of such wounds in the forest.

(_To be continued._)

SIBLEY COLLEGE LECTURES.--1887-88.

BY THE CORNELL UNIVERSITY NON-RESIDENT LECTURERS IN MECHANICAL ENGINEERING.

III.--THE EVOLUTION OF THE MODERN MILL.[3]

By C. J. H. WOODBURY, Boston, Mass.

BELT TOWERS.

The distribution of power has not always received the judicious treatment which its importance deserves. There are but few references to this question in the books on the subject, and these treat of methods that are not in accordance with the application of the art in its present state.

[3] Continued from SUPPLEMENT, No. 647, page 10331.

The lecture was illustrated by about fifty views on the screen, which cannot be reproduced here, showing photographs of mills and mechanical drawings of the methods of construction alluded to in the lecture.

The early form of the distribution of power consisted in placing a vertical shaft extending through the whole mill and distributing the power at each story by means of beveled gears, generally of skew-beveled form. The mechanical defects of such a method of distributing power, with regard to protection, repairs, and necessary care, are readily apparent, and there have also been many severe accidents caused by the breaking of teeth in these gears.

The present method of distributing power in this country is entirely by lines of belts extending up through what is known as a belt tower, which constitutes an element of great fire hazard to a mill. In some cases the belts are carried from story to story, covered by a casing of wood, and in other instances the tower forms a flue which may be the means of the rapid spread of fire throughout the building.

Before the introduction of automatic sprinklers there was not, I believe, a single instance of a fire entering the lower portion of a belt tower during working hours without accomplishing the destruction of the mill. Since the equipment of such places with automatic sprinklers, there have been several fires of this nature extinguished with nearly nominal damage. That is to say, the hazard of fire starting in such places is beyond the capacity of any apparatus other than automatic sprinklers to cope with it.

It would be impossible to arrange the distribution of power in many mills to conform to conditions of safety without reorganizing the whole plant, which would, of course, be impracticable. But in many instances modifications can be introduced which will diminish the hazard to a great degree. When the pulleys and belting are covered with sheathing in each room, the continuity of these flues can be broken by removing this sheathing down to the height of four or five feet above the floor, so that the covering will merely constitute a physical protection to any one approaching the belting.

The best method of arranging the belt tower has been in the case of a mill at Fall River, which was erected upon the ruins of a building destroyed by a fire originating in the belt tower. The machinery is driven by a steam engine situated in an ell projecting from one side at about the middle of the mill; and the main belt communicates to pulleys in a stone masonry tower located directly inside the walls of the main mill; and thence, from pulley to pulley, the power is communicated to each floor by shafting passing through holes left in the tower, and in no instances by means of belts.

There is a separate stairway inside of the tower for lubricating the journals, etc., and the top of the tower is covered with skylights protected underneath by a wire netting. In case of a fire in the belt tower, the heat will readily break the glass at the top, and the fire will tend to go up and out of the tower rather than through the mill.

MILL FLOORS.

The questions involved in designing the floors of a mill are of great importance, contributing in no small measure to elements concerned in the successful operation of the mill, and to a greater extent to its standing as a fire risk, and therefore affecting the constant expense of insurance.

In the case of a building designed merely for sustaining of loads, as in a storehouse, a floor would naturally be designed on the basis of considering the breaking strength of the timber. But in the case of a mill, the limitation is the amount of flexure allowable under the circumstances; and therefore the floors of the building are made more nearly rigid than would be required merely from the consideration of the ultimate strength of the structure.

The books on the subject, repeating over a constant which was first, I believe, given by Brunel in testimony before a parliamentary commission, have held that one four-hundredth of a span is the proper ratio of flexure. This may have been a very good rule to give to the parliamentary commission, but it is hardly the practical method of limitation for a matter of engineering construction, because the flexure of a loaded beam is in the form of a curve, and therefore its law is that of a curvilinear function, and not of a straight line. I have examined a great number of precedents of good construction in this connection, and for mill use have deduced the formula for deflection in inches, _d_ = 0.0012 L², in which L is the length of span in feet. It will be readily recognized that the true constant of deflection of span is measured by the radius of curvature which will give a uniform and allowable distortion to the floor in either direction to the limit of the radius upon which this formula is based, which is 1,250 feet.

I do not propose to offer to you on this occasion any remarks in regard to the treatment of the mathematics of the problem of applied mechanics concerned in the questions of transverse stress, knowing that you have certainly received instruction upon these subjects. But referring to the questions of mill floors, I would state that Southern pine beams of solid timber twelve by fourteen up to fourteen by sixteen inches are used; and instead of attempting the use of one piece of timber, it is preferable to use two pieces of the same depth and of half the breadth. These should be bolted together, with a space of an inch or so between them left by placing small vertical pieces of wood between the timbers when they are bolted together. In this manner one is more sure of sound timber, and in the process of seasoning there is less liability of dry rot in the interior, or of injurious checking, warping, or twisting.

The end of the beams should rest upon iron plates in the masonry, and should be secured by means of a tongue upon the plate entering a groove across the lower side of the beam. It is not feasible to make this groove to a close fit with the tongue; but it is cut a great deal larger, and the whole brought to a firm bearing by means of pairs of wedges or quoins driven into the groove each side of the iron tongue.

The outer end of the plate contains ribs or tongues reaching down into the brickwork. In this manner the timber is securely fastened to the brickwork; and yet in time of accident or of fire the falling of the beam in the middle of the mill will raise it up sufficiently so that it will clear the tongue and fall without tearing the wall down, which is the case whenever the beams are secured by bolts entering the end of the beam from the face of the wall.

At the points of support in a line of columns, the beams should be free from all compressive stress, transmitted through the lines of columns from floors above, by means of iron pintles between the cap of one column and the floor of the next one carrying this load.

A faulty method of construction, quite frequently used, consists in covering each column with a bolster of timber, four or five feet long, reaching out under the floor beams.

The transverse contraction of wood in seasoning after it is in position in the mill varies from three-eighths of an inch to double that quantity per foot; and the aggregation of such shrinkage amounts to a very considerable distortion or settling of the floor in a mill of several stories. Moreover, the resistance of timber to transverse crushing has been shown by experiments on the testing machine at the United States arsenal at Watertown to be about three times the resistance to longitudinal crushing.

Iron columns for mills have been entirely displaced by those of timber, as it was found that the latter were more reliable in resistance to fire, were freer from defects in construction, and possessed less tendency to vibration. A series of tests on full-sized mill columns of various forms of construction and age, made in the experiments referred to, at the Watertown arsenal, showed that resistance to crushing of Southern pine columns was about 4,500 pounds to the square inch, and remarkably uniform as to the different results. In white oak there was a wider range, owing to the difference in the grain of the various samples, the generality of the specimens being of somewhat less resistance than that of Southern pine.

It was furthermore found by these experiments, on comparing the crushing resistance of a full-sized column with that of a portion of the same, perhaps two feet in length, that the results were practically identical, likewise that within the limits of construction used for these columns the question of flexure did not enter at all in the problem, but they gave way by direct crushing, and that the resistance to crushing was proportional to its load upon the minimum cross section.

The precedents of safe construction in this matter show that wood columns in mills have successfully sustained for many years a load of six hundred pounds to the square inch without deterioration. As the resistance of such columns is proportional to the cross section, the results of these experiments have changed the practice of mill engineers in the matter; and square columns are of almost universal use, which interfere with no greater area on the floor than the round column of the same diameter, while they furnish an increased resistance of a little over twenty per cent. in excess.

Along the axis of such columns a hole of about one and one-half inches in diameter is bored, and near each end a couple of transverse holes, generally half an inch in diameter, furnish means of ventilating the inside of the column for the prevention of dry rot and also checking, due to contraction and seasoning.

There are several methods of laying the floor plank upon these beams, which are placed from eight to ten feet apart, according to the dimensions of the machinery to be placed in the mill. The first floor of three-inch plank, planed on one side and grooved on both edges, is laid planed side down, and the hardwood splines are inserted into the grooves before the planks are pressed up and spiked to the beams. An agreeable finish is sometimes arranged underneath by plowing a rabbet in each of the corners, and inserting a bead in the groove thus formed, which is secured by nails driven diagonally into the plank on one side only, because if the nails were driven into both sides, the bead would be split by the contraction of the plank.

These planks should be cut to sufficient length to cover two bays of the mill; and their transverse resistance is that of a beam fixed at one end and supported at the other, or one and three-fifths as much as a plank of the same size but half the length would support; but it should be remembered in this connection that, if evenly distributed on the floor, five-eighths of the load would be carried by every alternate beam unless the planks are so laid to break joints at convenient intervals of about three feet.

The top flooring is generally laid directly upon the floor plank, with one or two thicknesses of roofing paper interposed; but the preferable method, which deadens the sound and vibration, and also greatly increases the fire-resisting qualities of the structure, is to lay a coat of mortar on the floor plank, preserving the uniform thickness by means of furring placed about sixteen inches apart, and then to lay the upper floor upon this.

For these upper floors hardwood plank, one and one-fourth inches thick, and not over four inches wide, is used. The black birch is considered by many to possess the greater resistance to wear; and Southern pine is ranked next, although the latter wood gives trouble by stringing, especially when trucks are rolled over it. White maple forms an excellent top floor, although not so hard as others, especially where the floor is likely to be exposed to water, as in paper mills and bleacheries.

ROOFS.

Benjamin Franklin once said that next to a good foundation a good roof was the most important feature of a building. Although the constructive features of mill roofs are well defined, yet with regard to roof covering there is a wide diversity of experience and opinion.

The present form of factory roofing resembles a floor in its construction, being made, in a similar manner, of plank laid upon beams which project through the walls, where they act as a bracket to the cornice, the ends being sawed after any suitable ornamentation. The inclination for such roofs is about three-fourths of an inch to the foot. Where a mill is narrow enough for a single beam to reach from the wall to the ridge, they form cantilevers, the second point of support from the wall being by the columns one-third of the distance across the mill, and the ends of the beams are further secured together by means of iron dogs. For mills of greater width, the beam would reach only to the row of columns, and over the middle of the mill a beam is placed, usually horizontal on the under side, and hewn down from the middle to each end, so as to preserve the same slope on the upper side of the beam as for the roof.

In many instances mills are built with brick cornices, without any of the wood projection from the side; and in other buildings the walls are carried above the roof, which slopes toward the center, and all water falling on it or melted from the snow is conducted from it by pipes leading down through the middle of the mill.

It is not desirable to place gutters around the edge of the mill, as they serve no useful purpose, and are in continual need of repairs. By leaving the edge of the mill plank square and protecting it by sheet metal flashing, the rain falling from the roof can be received by a concave walk of coal tar concrete placed on the ground around the building. Suitable porches over doors, or some guard on the roof at these points, will prevent people who may be passing in at doors from being unduly wet by water from the roof.

There are numerous forms of roof coverings, the use of the different varieties being to a great extent local; that is, the sheet iron coverings used in the Middle States are almost unknown in New England; and in the latter place the ordinary tinned iron roofing is universally painted, while in the Dominion of Canada it is laid obliquely and never painted.

It is conceded by all that sheet copper forms the most desirable method of covering a roof; and, if one could be assured of the permanence of the structure, irrespective of the necessity for making changes every half year in order to keep pace with the march of invention, it would doubtless be shown that under such conditions of permanency copper would form the cheapest roof.

The most widely used roofing materials for this class of buildings are the asphalt and the coal tar roof, the latter being the most widely used in New England. There are numerous varieties of these composition coverings, which are applied by various methods. Some of these are of the most satisfactory character, while others are poorly designed and unskillfully applied, and are a constant source of trouble and expense to the occupant of the building.

One of the leading manufacturers, the efficiency of whose work for many years over a large amount of mill property I can vouch for by personal knowledge, uses the following method of applying the roofing. Three layers of roofing felt are placed on the plank parallel to the eaves, and continued by lapping each additional layer two thirds of its width upon the preceding one, and in this manner covering the roof with three thicknesses of the felt, breaking joints. This is secured to the roof by nails through tin washers and coated with a melted composition, and then two additional layers of felt are placed over the whole. Another coat of composition is then applied and gravel is placed over the whole while soft.

This maker does not approve of the practice of cementing each sheet of felt when it is laid, because it does not allow the felt freedom to yield from the expansion and contraction of the roof. When tin is applied to roofs, resin-sized building paper should first be laid on the roof plank, and the sheets of tin should be painted on the lower side before being laid.

Of late years cotton duck has been applied as a roof covering, and has been watched with a great deal of anticipation, although it has been used for similar purposes in covering ships' decks for many years. But the two uses are not strictly comparable, because the ship's deck is calked tight, and therefore the covering is free from the application of moisture underneath, while the roof is never tight, and the warm air underneath, heavily charged with moisture, which permeates the cracks between the planks, becomes chilled and condenses as it nears the top, carrying on a process of distillation.

As an example of the extent to which this can be carried on, I have known of instances where people presumed they were making a good roof by leaving slight air spaces by means of the furring laid between the roof plank and the top boarding. The circulation of air in these spaces deposited sufficient moisture to rot the boards.

A mill manager, wishing to have a roof over a very warm room, which should be both tight and a very perfect non-conductor, made a roof containing a space of about sixteen inches, which was filled with sawdust, and the roof boarding on top of this was covered with tar and gravel in the usual manner. In a few weeks the water began to drip through the ceiling as if the roof was leaking, although there was no snow on the top of the roof. Investigation showed that within that short time a sufficient amount of water had condensed with the sawdust to saturate the whole.

I would say in this connection that three inches of plank afford an ample protection against condensation over any ordinary process of manufacture, although four inches of plank have been used as a roof over paper machines in order to be safe beyond peradventure; but it is necessary that nails should not be driven into the bottom of this roof plank, because the point of a nail will reach to a lower temperature near the outside of the roof in the winter, and being a better conductor, it will cause moisture to condense upon the head of the nail.

Tin roofing is so general in use as not to require any allusion to methods of application, but the only course to reach economical and satisfactory results for a term of years, especially for locations near to the sea shore, is to use the best quality of dipped roofing plates of some brand which can be relied on as conforming to the standard and free from "wasters" or imperfect plates.