essays-1-第64章
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Were the new coving to range with the front edge of the old coving o； the obliquity of the new coving would commonly be too great；or the angle d c o would exceed 135 degrees； WHICH IT NEVER SHOULD DO；or at least never by more than a very few degrees。
No inconvenience of any importance will arise from making the obliquity of the covings LESS than what is here recommended； but many cannot fail to be produced by making it much greater； and as I know from experience that workmen are very apt to do this； I have thought it necessary to warn them particularly against it。
Fig。 11。 shows how the width and obliquity of the covings of a Chimney are to be accommodated to the width of the back； and to the opening in front and depth of the Fire…place； where the width of the opening of the Fire…place is less than three times the width of the new back。 As all those who may be employed in altering Chimneys may not； perhaps； known how to set off an angle of any certain numbers of degrees；or may not have at hand the instruments necessary for doing it；I shall here show how an instrument may be made which will be found to be very useful in laying out the work for the bricklayers。
Upon a board about 18 inches wide and four feet long； or upon the floor or a table； draw three equal squares A； B； C； fig。 12。 of about 12 or 14 inches each side； placed in a strait line； and touching each other。From the back corner c of the center square B； draw a diagonal line across the square A； to its outward front corner f； and the adjoining angle formed by the lines d c and c f will be equal to 135 degrees；the angle which the plane of the back of a Chimney Fire…place ought to make with the plane of its covings。And a bevel m； n； being made to this angle with thin slips of hard wood； this little instrument will be found to be very useful in marking out on the hearth； with chalk； the plans of the walls which are to form the covings of Fire…places。
As Chimneys which are apt to smoke will require the covings to be placed less obliquely in respect to the back than others which have not that defect； it would be convenient to be provided with several bevels；three or four； for instance； forming different angles。That already described； which may be called No。 1。 will measure the obliquity of the covings when the Fire…place can be made of the most perfect form：another No。 2。 may be made to a smaller angle； d c e；and another No。 3。 for Chimnies which are very apt to smoke at the still smaller angle d c i。Or a bevel may be so contrived； by means of a joint； and an arch； properly graduated； as to serve for all the different degrees of obliquity which it may ever be necessary to give to the covings of Fire…places。
Another point of much importance； and particularly in Chimneys which are apt to smoke； is to form the throat of the Chimney properly； by carrying up the back and covings to a proper height。 This； workmen are apt to neglect to do； probably on account of the difficulty they find in working where the opening of the canal of the Chimney is so much reduced。But it is absolutely necessary that these walls should be carried up five or six inches at least above the upper part of the breast of the Chimney； or to that point where the wall which forms the front of the throat begins to rise perpendicularly。 If the workman has intelligence enough to avail himself of the opening which is formed in the back of the Fire…place to give a passage to the Chimney…sweeper； he will find little difficulty in finishing his work in a proper manner。
In placing the plumb…line against the breast of the Chimney； in order to ascertain how far the new back is to be brought forward； great care must be taken to place it at the very top of the breast； where the canal of the Chimney BEGINS TO RISE PERPENDICULARLY； otherwise； when the plumb…line is placed too low； or against the slope of the breast； when the new back comes to be raised to its proper height； the throat of the Chimney will found to be too narrow。
Sometimes； and indeed very often the top of the breast of a Chimney lies very high； or far above the fire （see the figures 13 and 14； where d shows the top of the breast of the Chimney）； when this is the case it must be brought lower； otherwise the Chimney will be very apt to smoke。So much has been said in the First Chapter of this Essay of the advantages to be derived from bringing the throat of a Chimney near to the burning fuel； that I do not think it necessary to enlarge on them in this place； taking it for granted that the utility and necessity of that arrangement have already been made sufficiently evident； but a few directions for workmen； to show them how the breast （and consequently the throat） of a Chimney can most readily be lowered； may not be superfluous。
Where the too great height of the breast of a Chimney is owing to the great height of the mantle； （see fig。 13；） or； which is the same thing； of the opening of the Fire…place in front； which will commonly be found to be the case； the only remedy for the evil will be to bring down the mantle lower；or rather； to make the opening of the Fire…place in front lower； by throwing across the top of this opening； from one jamb to the other； and immediately under the mantle； a very flat arch；a wall of bricks and mortar； supported on straight bars of iron；or a piece of stone （h； fig。 13）。When this is done； the slope of the old throat of the Chimney； or of the back side of the mantle； is to be filled up with plaster； so as to form one continued flat； vertical； or upright plane surface with the lower part of the wall of the canal of the Chimney； and a new breast is to be formed lower down； care being taken to round it off properly； and make it finish at the lower surface of the new wall built under the mantle；which wall forms in fact a new mantle。
The annexed drawing fig。 13； which represents the section of a Chimney in which the breast has been lowered according to the method here described； will show these various alterations in a clear and satisfactory manner。 In this figure； as well as in most of the others in this Essay； the old walls are distinguished from the new ones by the manner in which they are shaded； the old walls being shaded by diagonal lines； and the new ones by vertical lines。 The additions； which are formed of plaster； are shaded by dots instead of lines。
Where the too great height of the breast of a Chimney is occasioned； not by the height of the mantle； but by the too great width of the breast； in that case； （which however will seldom be found to occur；） this defect may be remedied by covering the lower part of the breast with a thick coating of plaster； supported； if necessary； by nails or studs； driven into the wall which forms the breast； and properly rounded off at the lower part of the mantle。See fig。 14。
CHAPTER。 III。
Of the cause of the ascent of smoke。 Illustration of the subject by familiar comparisons and experiments。 Of chimnies which affect and cause each other to smoke。 Of chimnies which smoke from want of air。 Of the eddies of wind which sometimes blow down chimnies； and cause them to smoke。 Explanation of the figures。
Though it was my wish to avoid all abstruse philosophical investigations in this Essay； yet I feel that it is necessary to say a few words upon a subject generally considered as difficult to be explained； which is too intimately connected with the matter under consideration to be passed over in silence。 A knowledge of the cause of the ascent of Smoke being indispensably necessary to those who engage in the improvement of Fire…places； or who are desirous of forming just ideas relative to the operations of fire； and the management of heat； I shall devote a few pages to the investigation of that curious and interesting subject。And as many of those who may derive advantage from these inquiries are not much accustomed to philosophical disquisitions； and would not readily comprehend either the language or the diagrams commonly used by scientific writers to explain the phaenomena in question； I shall take pains to express myself in the most familiar manner； and to use such comparisons for illustration as may easily be understood。
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