74 Notices of Scientific Works. [Jan., 



Lectures IY. and V. contain comparative examples of the Warren, 

 lattice, and other kinds of bridges ; showing also the direction of the 

 strains of the lattice as compared with the solid web connecting the 

 upper and lower flanges of the plate bridge. In this part of the 

 investigation the author seems to demonstrate the theoretical advan- 

 tages of the lattice or open web over that of the solid plate-girder, 

 since in that system the bars may be inclined to the direction of the 

 strains of tension and compression, which is not the case in the 

 plate-web. There are, however, compensating advantages in the 

 solid plate-web which appear to have escaped the author's notice 

 in the increased degree of stiffness which is obtained in both the 

 plate and the box girders. Many examples of this kind may be 

 shown in bridges of long and short spans, and probably one of the 

 best and most substantial of this sort is that over the Lune at Lan- 

 caster. The advocates for the open- web system have intimated the 

 saving of weight at 50 per cent., but that statement is out of all 

 question, as the only saving is in the difference between the open 

 bars connecting the upper and the lower flanges in the lattice-girder, 

 and the middle web connecting the flanges in the plate-girder, and 

 which in the very best iron construction of that description is much 

 nearer 6 or 7 per cent., and in some cases the difference is inap- 

 preciable. 



The question of joints has been fully discussed by many writers ; 

 but the author brings under the notice of engineers the different 

 modes of rivetting, and, without entering upon the merits of punched 

 and drilled holes, he gives a mathematical analysis of the different 

 processes which enter into the maximum strengths and forms of 

 rivetted joints. 



On Roofs. — Lecture VI. — Mr. Unwin states that " in the sup- 

 porting framework of roofs precisely the same mechanical problem 

 is presented as when a railway or roadway is to be carried over a 

 ravine or river. Hence it is that the successive combinations adopted 

 for bridges reappear, in essentially the same forms, as roof princi- 

 pals. The stone-vaulted inner roofs of some of the older churches 

 are structurally identical with masonry bridges. Timber roof- 

 trusses are simply awkward-shaped girders, or, like the great roofs 

 at King's Cross and over the transept of the first International 

 Exhibition, they are timber arches analogous to those frequently 

 erected as bridges in the earlier history of railways. Nor is the 

 case otherwise with iron. All iron roofs may be classed as girders 

 or as arches, with certain transitional forms which embody the fea- 

 tures of both classes. And to pursue the analogy farther, even the 

 suspension principle, which at first sight, from the nature of the 

 supports required, would seem inapplicable to the purpose, is, ac- 

 cording to a proposal of MM. Lehaitre and De Montdesir, to be 

 pressed into the service of the roof-builder. " 



