of the given materials will oppose to compression without permanent al

As in tension, material should never be loaded with more than onefourth of its utmost strength.

TRANSVERSE STRAINS.

Many experiments have been made to determine the breaking strain of different building materials. The average results are given in the following table, the pieces used in the experiment being uniform rods one foot long, and with ends one inch square, supported horizontally by standards at each end, and the weight applied perpendicularly at the center.

As this table expresses the breaking weight of each piece it would not be safe to permanently load them with more than one-fourth the weight given in the table.

Experiments have shown, and mathematical calculations demonstrated, that the strength of beams and girders varies, inversely as their length, and directly as their width, and the squares of their depths. Thus, a beam 8 feet long will be only one-half as strong as one of the same breadth and depth, and 4 feet long; and the latter will be 4 times as strong as one of the same breadth and depth, and 16 feet long. Two beams of the same length and depth will sustain a weight just in proportion to their width; by doubling the width the strength is doubled. If two pieces have the same length and breadth their strength will be as the square of their depths. If one has twice the depth of the other, it will sustain a weight four times as great. A 2x12 8 will bear four times the weight that a 2x6 8 will, the 2x6 8 will bear one-third the weight that a 6x6 8 will sustain. Therefore, a 2x12 placed on edge will bear a weight placed on its center

one-third greater than can be borne by a 6x6 of the same length. The above is not strictly true in regard to lengths, as the strength appears to diminish in a ratio greater than the inverse proportion of the length; caused, probably, by the tendency to bulge and twist in long pieces of timber. This error is provided for in the following rule :-To find from the foregoing table the breaking weight of any piece of timber, the length, breadth and thickness being given: Divide the breaking weight given in the table by the length in feet; subtract 10 from the quotient; multiply the remainder by the breadth in inches and that product by the square of the depth in inches

EXAMPLE:-Required the breaking weight of a hard pine scantling 2in. X12in. X10 feet supported at ends.-658÷÷10-65, 65-10=55, 55X2X144= 15.840-breaking weight, 15,840X4=3,960=greatest weight it should be required to support permanently.

EXAMPLE:-Required the breaking weight of a hard pine sill 6in. X6in. X10 feet, supported at the ends; 658÷÷10=65, 65-10-55, 55X6X36=10,880 -breaking weight, 10,880÷4=2,720=greatest weight the sill should be required to bear.

The following dimensions, taken from the Liverpool Building Act, may be considered as standard sizes of joists for ordinary buildings. The distances between centers being one foot, joists in floors, clear bearing. Exceeding 7 and not exceeding 10 ft. should be not less than 6x2 in.

As timber does not come in fractions of inches, whena greater width or depth is used than that indicated the distance between centers can be increased proportionally.

TORSION is very seldom to be considered in calculating the strains to which building material is subjected, and its discussion will, therefore, be omitted from this book. Its province is in machinery, and its effect on shafts, etc., must be considered, and made the subject of special study, by the mechanical engineer.

ROOF ELEVATIONS.

By the "pitch" of a roof is meant the relation which the height of the ridge above the level of the roof-plates bears to the span, or the distance between the studs on which the roof rests. Thus, in the following diagram, with a span of 24 feet, and with a roof made one-fourth pitch, the ridge is 6 feet above the plates, with one-third pitch, 8 feet above, etc.

The length of rafters for the most common pitches can be found as fol. lows from any given span:

If 4 Pitch, multiply Span by .559, or 7-12 nearly.

.6+,
.625, 5%
.71+,

"3-5

"7-10 66

.8+,

" 4-5

66

"18

To lengths thus obtained must be added amount of projection of raftes at the eaves.

As rafters must be purchased of even lengths, a few inches more or less on their lengths will make a difference to the pitch so slight that it cannot be detected by the eye.

EXAMPLE: To determine the length of rafters for a roof constructed one-half pitch, with a span of 24 feet-24X.71=17.04; or, practically, just 17 feet, as in cut. A projection of 1 foot for eaves makes the length to be purchased 18 feet.

HEATING AND VENTILATION.

There is no one subject of more vital importance to the public and particularly to those people who own homes, than that embraced in the above caption. In connection with the foregoing designs, and hints to builders, a brief chapter under this head is not out of place, and may be of no little value to the reader. By the term "builder" as used herein is meant more than is ordinarily included in the mechanical designation, "carpenter and builder “—i. e. one whose avocation it is to design and construct buildings. We mean all those who build for themselves homes in which their days are to be spent, and in the construction of which certainly no pains should be spared to secure pleasure, comfort and health. Toward the last and most important of these the following remarks are directed.

As in almost everything we do, there is a right way and a wrong way to build a house, which assertion is forcibly illustrated in the two diagrams which follow. If rooms be constructed the same as boxes, closed tightly at top and bottom with only the ordinary doors and windows for openings, they must necessarily be unhealthy, as no circulation of the air can take place. The two greatest and most indispensible agents to health furnished by nature are fresh air and sunshine. To exclude either or both is to invite disease and death. Unless some means are provided for the ingress and egress of pure air, by which that in a room is constantly changing, it becomes at once impure, and the more persons there are in the room the worse this condition will be. The cold, dead and impure air falls to the floor, and is a constant menace to the health and lives of children whose time is principally spent there. This is probably the least understood of all the evils that beset the path of infancy and early childhood. Not only does the little one live in the coldest air of the room, but the foulest. Anyone knows that carbonic acid gas is a deadly poison, and anyone also ought to know that it is constantly thrown off from the human lungs in the process of breathing. Being heavier than air it falls to the floor and there remains and accumulates until some means of displacing it by pure air are used. Some one has very truly said, "our breath is our greatest enemy." The stifling atmosphere of any unventilated room in which a crowd of persons are gathered amply proves this assertion, and accounts for the many instances where ladies or anyone physically weak are known to be carried out of such crowds in a fainting condition. To attain the highest perfection in ventilation the air nearest the floors of a room should be warmest, growing gradually cooler nearer the ceiling. This keeps the feet warm and the head cool. Under the old-fashioned way of building houses with wide, open fireplaces, this condition was partially secured by reason of the draft from

the chimney which necessarily exhausted more or less air from the bottom of the room. With the modern method, however, of building almost an air-tight house, and heating it by stoves, the question of properly supplying the rooms with fresh air becomes of vital importance.

The best time to consider all these questions is upon building a new house. Then, the expense of complying with these reasonable laws of nature and health is little or nothing, while to change over an air-tight box into a healthy and comfortable habitation would require quite an outlay. In the diagrams which follow Fig. 1 represents the common form of balloon-frame house, in which, as usually constructed, the air is not only bad, but the heat is wrongly applied, as the following explanation will show:

The usual sill is simply a 2x8 plank upon which rest the studding of the outer walls and also the first story joists which are spiked to the studding. Then come the second story and attic joists, both nailed to the studding. A glance at the diagram shows that the floor, laid up to the inside of the studding, is joined at that point with the plastering upon the walls which continues up to and over the ceiling and down to the floor again on the other side. Upon the outside of the studding, of course, come the sheathing and siding, leaving an open space all around each room between this and the plastering. Each room is a box, the bottom of which is one-inch flooring, while the sides and top are onehalf or three-quarter inch plastering. Tens of thousands of houses are standing to-day all over this country built in this way, in which a rat or mouse may start from below the first floor and make the whole circuit, going up one side between the studding to the attic, over and down the other side without interruption.

The moment a higher temperature is created in any room the walls of plaster begin to radiate heat into the spaces mentioned, and the air within becomes rarified and at once moves to the right or left and crawls up the walls to the attic. To take its place the cold air in the upper part falls down under the floors. If the temperature in the attic be zero, the air underneath each floor, also cooled by the frost that penetrates the siding, must be very nearly the same. Thus it appears that had it been the design of the builder to make the floors cold, he could not have hit upon a device more certain to produce that result.

The arrows in the diagram Fig. 1 indicate the way the air will circulate through it, may be from right to left or the reverse, as influenced by external currents. Heat this house as you may by furnace, steam coil or common stove, the upper part of each room must be hot while the floor will be cold. Any person who has not actually tested it with the mercury will be astonished at the results of a trial. Cut a hole in the floor of either story and drop a thermometer into it any winter day, and then