PREFACE TO SECOND EDITION. Xlll 



even essential to depart from the plan of the work as laid 

 down, and to supplement it largely by various examples illus- 

 trative of the general principles and bringing out as clearly and 

 repeatedly as may be necessary the various points noticed in 

 the text. 



In the work four separate and distinct methods of solution 

 are given for such structures as bridge and roof trusses, and the 

 student should become familiar with all. Thus there are two 

 methods by diagram, viz., by resolution of forces (Maxwell) and 

 by the equilibrium polygon (CWraaw?i); and two correspond- 

 ing methods by calculation, viz., by composition and resolution 

 of forces, and by moments (fiitter). To give these in such 

 manner and with such emphasis that the student shall be con- 

 versant with all, and able to use them with discrimination 

 where in any case they best apply, we recommend the follow- 

 ing order of perusal : 



FIRST. Chap. I., which gives the first method by diagram 

 (Maxwell's), and such of the Appendix as relates to this chap- 

 ter, is read. The class may then go into the drawing-room, 

 and under the supervision of the teacher actually solve a vari- 

 ety of roof trusses from simple to more complex, both for dead 

 load and wind force in each case. In each and every case also 

 the results should be checked by calculation by the method of 

 moments (Hitter's), at first thoroughly and in detail, and after- 

 wards only a few test pieces to check the accuracy of the dia- 

 gram. From Roof Trusses we then pass on to Bridges, and 

 here also a series of selected examples of every class used in 

 practice may be solved, and the method of tabulation of apex 

 weights referred to in Art. 12 and Appendix to Chap. I., brought 

 out repeatedly until the student has thoroughly mastered it, 

 and appreciates fully the fact that for each form of truss the 

 strains due to only two weights are really necessary to be found, 

 and that the others may then readily be found directly from 

 these. Here also each example should be checked by calcula- 

 tion by the method of moments. In the case of curved flanges 

 the various lever arms may first be measured directly to scale 

 from the frame, and then trigonometrically computed. At 

 this point the student is then already in possession of two inde- 

 pendent methods of solution for any kind of framed roof or 

 biidge which occurs in general practice. 



There are in fact only two framed structures, the continuous 



