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 larvæ, 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

the physical system and to the vital organization. The same mode of reasoning is applied as to the facts of organic life, and an analogous conclusion is the result. As the phenomena of animal and vegetable life cannot be referred to the operation of Natural Selection, or of any unintelligent agency whatever, so Mr. Murphy maintains that in all mental intelligence there is an element not derived from habit, and not resolvable into any unintelligent force; and is hence at issue with the psychological school represented in this country by Mill, Bain, and H. Spencer. In other words, "life, intelligence, and the moral sense is each incapable of being resolved into anything lower than itself." We cannot follow Mr. Murphy over the oft-trodden ground of the existence or non-existence of Innate Ideas-which he believes to be the inherited experience of the race, the reality of our belief in an external world, the origin of our conceptions of time and space, and other cognate speculations, on some of which he contrives to throw new light; but we wish rather to comment on one portion of his scheme which we take to be erroneous. Mr. Murphy points out clearly the difference between conscious and unconscious Sensation, and between conscious and unconscious Thought, the greater part of our thought being unattended by consciousness; but he often confounds, as we think, between conscious and unconscious Volition. Now we would maintain that nothing in our mental constitution is clearer than that the Will is often, and indeed generally, exercised without any consciousness of its action. The movements of the limbs in walking we presume Mr. Murphy would call, and we think erroneously, consensual action, the result of habit. The motion of the heart, of the eyelids, of the chest in breathing, we hold to be truly either consensual or reflex; and the test we would apply is that they cannot be arrested, or only to a very inconsiderable extent, by the action of the Will. In walking, on the contrary, we can stop at any moment we please; and whatever can be arrested by the Will must have been set in motion by the Will. The view has been held that in the motion of the limbs in walking, a certain storage, as it were, of voluntary action, is set at work at the commencement, which is continually flowing forth at every step without any fresh volition. But this idea, we think, will not bear a careful scrutiny. Take the instance of the slight inclination of the body to one side necessary in turning a corner; this cannot be done without the exercise of the Will, and yet we are perfectly unconscious that any such motion is performed. Or we may illustrate our argument by the familiar example of a flight of steps, say twenty, which we are accustomed daily to descend, and which has been shortened by one step at the bottom of the flight. We all know the unpleasant jerk given to the body by the foot coming into contact with the ground with greater force than was expected. We cannot suppose that an amount of voluntary energy was stored up when we commenced the descent