Wrought-iron girders, whether tubular, plate, or lattice, are therefore simply modifications of what has already been done in the same direction, and provided careful attention is given to the quality of the material and soundness of the workmanship, we may rest assured of the security and permanence of every similar structure.

Many distinguished men of science have written treatises to elucidate the principles involved in the tubular and girder constructions, and among them may be mentioned Mr. Unwin, who has just published a report of his lectures on this subject, including a short treatise on the construction of iron roofs, delivered to the members of the Ordnance Corps at Chatham. As a lecturer on practical science a more efficient person could probably not have been selected, for-to a competent knowledge of mathematics applied to constructions-Mr. Unwin has had the advantage of five years' practical experience with Sir William Fairbairn of Manchester, and he therefore enters upon the discussion with a full knowledge of the subject on which he treats.

In the first lecture Mr. Unwin fully proves the advantages he has gained, by the competency with which he enters upon the investigation of stress and strain, bringing to his aid many apt illustrations, and by the graphic way in which he treats the principles of load and molecular resistances, greatly for the benefit of the student and those interested in the accuracy of iron constructions. To show the distinction between stress and strain, which means the force of the former applied to a material body, and the alteration or the resistance of the latter as a result, Mr. Unwin states, that the "strain is sensibly proportional to the stress for a range of about only one-third of the whole stress which may be applied before rupture ensues, if the bar has not previously been strained, and for a range of, perhaps, two-thirds of the breaking-stress, if the bar has not previously been loaded with nearly the whole breaking-weight. But in either case, with loads near the breaking-weight, the strain is not proportional to the stress, and the condition of perfect elasticity is not fulfilled. Hence laws derived from the consideration of a perfectly elastic material will not give accurately the ultimate resistance of structures of wrought iron."

The second lecture is devoted to the intensity of stress on bridges, and the methods of estimating the load and its limits of safety. It also relates to the weight of the structure and its load, or the dead weight and its live or rolling load. In this, as in the former lecture, Mr. Unwin gives some useful examples and illustrations of highly practical value to the student and engineer.

The third lecture treats of tubular, tubular-girder, and plategirder bridges, showing the ratio of the top and bottom flanges to span, the depth of girder to span, and the method of designing, &c.

Lectures IV. 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 advantages 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 Lancaster. 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 inappreciable.

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 supporting 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 principals. The stone-vaulted inner roofs of some of the older churches are structurally identical with masonry bridges. Timber rooftrusses 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 features 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, according to a proposal of MM. Lehaitre and De Montdesir, to be pressed into the service of the roof-builder."

Of the construction of roofs many examples are given, and the strains on the parts of different forms are carefully worked out in spans varying from 15 to 240 feet. The methods of determining the strains for differently formed roofs are exceedingly well adapted for obtaining perfect security, and the clear and distinct manner in which the subject is treated must be highly edifying to the student and practical engineer.

We might enlarge on this, but it could not be expected that a subject of such importance as Iron Roofs and Bridges could be successfully treated within the limits of a few lectures. There is, however, sufficient matter contained in the work before us to recommend its perusal to the consideration of the practical architect and engineer; and looking at the clear and graphic style in which it is written, we feel indebted to the author for this addition to our knowledge of practical science.

Habit and Intelligence, in their connection with the Laws of Matter and Force: a Series of Scientific Essays. By JOSEPH JOHN MURPHY. 2 vols. London: Macmillan & Co., 1869.

THE intricate problems of the genesis of animal and vegetable life, and of the connection between the human mind and its material abode, will ever be among those which engage and fascinate our intellects of the highest order. The doctrine of a gradual evolution of life, as opposed to that of distinct specific creations, with which the great name of Darwin is associated, and which has been elaborated by Spencer, Wallace, and Hooker, has of late years received an extraordinary impulse; while the parallel theory of a "physical basis of life" has obtained the sanction of some of the highest names in natural science. While these theories are doubtless founded on a substratum of truth unknown to our older naturalists, they are probably mixed up with a considerable amount of error and over-statement, which further investigations will remove. therefore cordially welcome a work in which the problems of organic life are treated with so free and independent a hand, and with such close reasoning applied to a wide knowledge of facts, as we find in the volumes before us. While adopting the view of the evolution of all living organisms by descent, with modifications, from a few, if not from a single germ, Mr. Murphy holds that the Darwinian theory of Natural Selection from spontaneous variations is inadequate to account for the major part of these modifications, and altogether rejects the Huxleyan principle that the phenomena of life can be accounted for and explained on purely physical principles.

We

The first volume of this series of Essays is occupied with

a consideration of the laws which govern the material world, and a comparison of them with what we know of those which prevail in the domain of organic life. With great acuteness and force of illustration, Mr. Murphy points out that while it is a feasible hypothesis that many of the structures which we see in the animal and vegetable worlds are attributable to Natural Selection, or "the Survival of the Fittest," acting through a long course of generations, there are others which it is impossible to conceive can have become developed through the operation of this law, or of that promulgated by Herbert Spencer, depending on the mechanical adaptation of structure to function by the force of external agency. To this category belong such organisms in the vegetable kingdom as the hard, woody shell that protects the nut, but still more conspicuously all the most complex organs of the higher orders of animals. We will give the argument in Mr. Murphy's own words:

"There are structures for the origin of which it is, I believe, utterly impossible to account by any merely physical theory. I refer to such organs as the eye and the ear. If it is certain, as I think it is, that the flow of the nutritive fluids through cellular tissue, for successive generations, must have a tendency to form a rudimentary circulating apparatus, it is at least equally obvious that the action of light falling on the eye for any number of generations, can have no similar tendency to produce the optical apparatus of the eye. Nor can the constant exercise of the eye in the act of seeing have any such effect. The exercise of the eye, within the limits of what is healthful, does, no doubt, tend to increase the sensitiveness of the retina; and I do not say it is impossible, though I do not admit it as probable, that the muscular arrangements to which the mobility of the eyeballs and eyelids is due, may have been produced by the effort to move them, continued through successive generations; and that the expansion of nerves over the retina may have been produced by the constant stimulation of the nerves themselves. But no such merely physical theory will account for the origin of the special complexities of the visual apparatus. Neither the action of light on the eye, nor the actions of the eye itself, can have the slightest tendency to produce the wondrously complex histological structure of the retina; nor to form the transparent humours of the eye into lenses; nor to produce the deposit of black pigment that absorbs the stray rays which would otherwise hinder clear vision; nor to produce the iris, and endow it with the power of partly closing under a strong light so as to protect the retina, and expanding again when the light is withdrawn; nor to give the iris its two nervous convexions, of which one has its root in the sympathetic ganglia, and causes expansion, while the other has its root in the brain, and causes contraction."

Admitting, then, as Mr. Murphy does, the premiss of the

common ancestry of all organisms, by what process is it possible to account for the gradual evolution of a being with so complex an organ as an eye from the primordial homogeneous and amorphous Amoeba or Gromia? He sees the explanation in the co-existence with the vital principle of an Organizing Intelligence, consciously present in the mind of man, unconsciously in all organized structure. He believes that "the wondrous fact of organic adaptation cannot have been produced by any natural selection, or by any unintelligent agency whatever ;" that "wherever there is life there is intelligence, and that intelligence is at work in every vital process whatever, but most discernibly in the highest." The recognition of this Organizing Intelligence running throughout organic nature, is the keystone of Mr. Murphy's system. The unconscious intelligence by which the bee stores up food for the sustenance of the larvae, and builds cells for its reception on mathematical principles, is the same principle as the unconscious intelligence which has given it the organs necessary to collect the honey; the conscious intelligence in the mind of man which has manufactured the microscope is the same principle as the unconscious intelligence which in his body has manufactured, or, to coin a word, has "mentefactured" the lenses of the eye. The reasoning that the eye cannot have been produced by the action of mere natural selection is strengthened by the forcible argument that this latter view presupposes that the same selection from a long series of spontaneous variations has taken place in three separate lines of descent, in the Annulosa, the Mollusca, and the Vertebrata, the higher forms of which can, on no plausible hypothesis, have descended directly from one another, or from a common eye-possessing ancestor. Spencer's theory that all structures have been produced by adaptation to function in the individual aided by natural selection in the generation, is combated by the equally powerful argument that "as we ascend in the scale of nature to higher and higher vital functions, and higher and higher organic forms, we find the relation of cause and effect becoming less traceable by our faculties (though no doubt it exists all through nature); while at the same time the relation of means and purpose becomes at once more traceable and more definite. Nowhere in the universe, as known to us, is the relation of means to purpose more clearly traceable and more perfectly definite than in the organs of special sense in the higher animals, especially in the eye and the ear; and nowhere is it more difficult (I would say, utterly impossible) to assign any physical cause for the facts, than when we inquire by what cause, or by what agency, such wonderful organs have been formed. This truth, that purpose is most clearly discoverable where cause is least so, has not received the attention it deserves."

In his second volume Mr. Murphy enters upon the recondite subjects of Psychology, and especially of the relation of the mind to