Page:Cassells' Carpentry and Joinery.djvu/55

Rh lines are struck. The log or balk is now turned over, and longitudinal lines corresponding with the vertical lines are struck. To make an impression that may be clearly seen, the top and end lines are struck with a string that has been passed through a mixture of red ochre and water of the consistency of thin paste. The string used for lining the under side of the timber is passed through whiting. A red or dark line can be better followed by the top sawyer, while from underneath a white line can be best seen. The ochre is placed in a little box (see Fig. 156) and water added. There is a handle at, and a notch at. The string is placed in the box and drawn through the notch. A thin piece of wood, as Fig. 157, is placed on the string while it is being pulled through the notch, otherwise it would be necessary for the finger and thumb to guide the string, and to remove the surplus ochre that may be on it.

These particulars are given in the accompanying table.

The safe load in tension and compression (columns 3 and 4) would be from one-tenth to one-fifteenth of the amounts given. The safe bearing pressure across the grain of timber as at the ends of a beam will be about one-fifth of the amounts given in column 6. Column 5 gives the coefficient in the formula  = bd$2$ ÷, and the safe load would be about one-sixth of  for temporary work, or one-tenth for permanent loads.

Fig. 158.—Beam 6 in. x 6 in.

Fig. 159.—Beam 6 in. x 3 in.

Fig. 160.—Beam 12 in. x 3 in.

The strength of solid timber beams varies as the square of the depth, directly as the width, and inversely as the span. Thus, in a beam 6 in. square (Fig. 158), multiplying the width by the square of the depth gives 6 x 6² = 6 x 36 = 216; and if this beam was sawn down the middle, there would be 3 x 6² = 3 x 36 = 108 (Fig. 159). Another case is that of a beam 12 in. deep and 3 in. wide (Fig. 160), and the corresponding figure then is 3 x 12² = 3 x 144 = 432. The ordinary formula for the strength of a beam lying loose on the bearings at each end, and with central load (Fig. 161), is as

Fig. 161.—Loose Beam with Central Load. follows, when = breadth in inches, ² = square of depth in inches,  = length of bearing in feet,  = constant, for which Barlow and Tredgold give a value for Riga of  = 4 cwt. This constant is obtained from the results of trials, but it must be noted that such tests vary considerably. The strength of timber will vary in the same cargo, and allowance must be made for the difference in the growth and fibres of the