The little gallery in which we are now stumbling our way over blocks of stone and rubbish, only varied by long tracts of thick slush and pools of water, is the section excavated by the boring-machine—in dimension about twelve feet broad by eight feet high. The tramway which has accompanied us all the way is still continued along this small section. In the middle portion underneath the rails is the canal, inclined toward the mouth of the tunnel, for carrying off the water; and in this canal are now collected the pipes for conveying the compressed air to the machines, and the gas for illuminating the gallery. At the end of a few minutes, a rattling, jingling sound indicates that we are near the end of our excursion, and that we are approaching the perforating-machines. On arriving, we find that nearly the whole of the little gallery is taken up by the engine, the frame of which, mounted upon wheels, rests upon the main tramway, so that the whole can be moved backward or forward as necessary. On examining the arrangement a little closely, we find that in reality we have before us nine or ten perforators, completely independent of one another, all mounted on one frame, and each capable of movement in any direction. Attached to every one of them are two flexible tubes, one for conveying the compressed air, and the other the water which is injected at every blow or stroke of the tool into the hole, for the purpose of clearing out the debris and for cooling the point of the "jumper." In front, directed against the rock, are nine or ten tubes (according to the number of perforators), very similar in appearance to large gun-barrels, out of which are discharged with great rapidity an equal number of boring-bars or jumpers. Motion is given to these jumpers by the direct admission of a blast of compressed air behind them, the return stroke being effected by a somewhat slighter pressure of air than was used to drive them forward. We will suppose the machine brought up for the commencement of an attack. The points most convenient for the boring of the holes having been selected, the nine or ten perforators, as the case may be, are carefully adjusted in front of them. The compressed air is then admitted, and the boring of the holes commences. On an average, at the end of about three-quarters of an hour, the nine or ten holes are pierced to a depth of two feet to two feet six inches. Another ten holes are then commenced, and so on, until about eighty holes are pierced. The greater number of these holes are driven toward the centre of the point of attack, and the rest round the perimeter. The driving of these eighty holes to an average depth of two feet three inches, is usually completed in about seven hours, and the second operation is then commenced.
The flexible tubes conveying the compressed air and the water are detached from the machines, and placed in security in the covered canal. The perforating-machine, mounted on its frame or truck, is drawn back on the tramway behind two massive folding-doors of wood. Miners then advance and charge the holes in the centre with powder, and adjust the matches; fire is given, and the miners retire behind the folding-doors, which are closed. The explosion opens a breach in the centre part of the front of attack. Powerful jets of compressed air are now injected, to clear off the smoke formed by the powder. As soon as the gallery is clear, the other holes in the perimeter are charged and fired, and more air is injected. Then comes the third operation. Gangs of workmen advance and clear away the debris and blocks of stone detached by the explosion of the mine, in little wagons running on a pair of rails placed by the side of the main tramway. This done, the main line is [{70}] prolonged to the requisite distance, and the perforating engine is again brought forward for a fresh attack. Thus, we have three distinct operations—first, the mechanical perforation of the holes; secondly, the charging and explosion of the mine; and thirdly, the clearing away of the debris. By careful registers kept since the commencement of the work, it is found that the mean duration of each successive operation is as follows: for the perforation of the holes, seven hours thirty-nine minutes; for the charging and explosion of the mine, three hours twenty-nine minutes; for the clearing away of the debris, two hours thirty-three minutes; or, in all, nearly fourteen hours. Occasionally, however, the three operations may be completed in ten hours, all depending upon the hardness of the rock. It has been found practically more expeditious to make two series of operations in twenty-four hours.
Whatever may be the nature of the rock, if it is very hard, the depth of the holes is reduced; that is, the perforation is only continued for a certain given time—about six and a half hours—which, for the eighty holes with ten perforaters, gives us about three-quarters of an hour for each hole. The rock is generally of calcareous schist, crystallized, and exceedingly hard, traversed by thick veins of quartz, which often break the points of the boring-tools after a few blows. Each jumper gives about three blows per second, and makes one-eighteenth of a revolution on its axis at each blow, or one complete revolution every six seconds. Thus, in the three-quarters of an hour necessary to drive a single hole to the depth of twenty-seven inches, we have four hundred and fifty revolutions of the bar, and eighteen hundred violent blows given by the point against the hard rock, and that under an impulse of about one hundred and eighty pounds. These figures will give us some idea of the wear and tear of the perforating-machines. It is calculated that on an average one perforating-machine is worn out for every six metres of gallery, so that more than two thousand will be consumed before the completion of the tunnel. The total length completed at the Bardonnêche side at the present time is just two thousand three hundred metres, or nearly a mile and a half.
At the north or Modane end, the mechanical perforators are precisely the same as at Bardonnêche, as also is the system of working in the gallery. The machinery for the compression of air, however, is very different, more simple, and in every way an improvement upon that at the South end. Not finding any convenient means of obtaining a head of eighty-four feet of water sufficient in quantity for working a series of compressors, as at Bardonnêche, there has been established at Modane a system of direct compression, the necessary force for which is derived from the current of the Arc. Six large water-wheels moved by this current give a reciprocating motion to a piston contained in a large horizontal cylinder of cast iron. This piston, having a column of water on each side of it, raises and lowers alternately these two columns, in two vertical tubes about ten feet high, compressing the air in each tube alternately, and forcing a certain quantity, at each upward stroke of the water, to enter into a cylindrical recipient. There is very little loss of water in this machine, which in its action is very like a large double-barreled common air-pump. It is a question open to science whether the employment of compressed air for driving the perforating engines in a work such as is in operation at the Mont Cenis, could not be advantageously and economically exchanged for the employment of a direct hydraulic motive force, the ventilation of the tunnel being provided for by other means. The system, however, employed at Modane has many advantages, which it is impossible to overlook, and its complete success has given a marked and decided impulse to the modern science of tunnelling through hard rock.
Translated from the Civiltà Cattolica.
ON THE UNITY OF TYPE IN THE ANIMAL KINGDOM.
I.
The generation of a human creature takes place neither by the development of a being which is found in the germ, sketched as it were like a miniature, nor by a sudden formation or an instantaneous transition from potential to actual existence. It is effected by the true production of a new being, which pre-exists only virtually in the activity of the germ communicated by the conceiver, and the successive transformation of the potential subject.
This truth, an a priori postulate of philosophy, and demonstrated by physiology a posteriori, was illustrated by us in a preceding article. Here we must discard an error which has sprung from this truth. For there have been materialists who maintained that there was but one type in the whole animal kingdom, that is, man, as he unites in himself in the highest possible degree perfection of organism and delicacy of feelings; and that all the species of inferior animals were so many stages in the development of that most perfect type. This opinion is thus expressed by Milne-Edwards in his highly esteemed lectures on the Physiology and Comparative Anatomy of Man and Animals:
"Every organized being undergoes in its development deep and various modifications. The character of the anatomical structure, no less than its vital faculties, changes as it passes from the state of embryo to that of a perfect animal in its own species. Now all the animals which are derived from the same type move during a certain time in the same embryonic road, and resemble each other in that process of organization during a certain period of time, the longer as their zoological relationship is closer; afterward they deviate from the common road and each acquires the properties belonging to it. Those that are to have a more perfect structure proceed further than those whose organization is completed at less cost. It results from this that the transitory or embryonic state of a superior animal resembles, in a more or less wonderful manner, the permanent state of another animal lower in the same zoological series. Some authors have thought right to conclude from this that the diversity of species proceeds from a series of stages of this kind taking place at different degrees of the embryonic development; and these writers, falling into the exaggerations to which imitators are especially liable, have held that every superior animal, in order to reach its definitive form, must pass through the series of the proper forms of animals which are its inferiors in the zoological hierarchy; so that man, for instance, before he is born, is at first a kind of worm, then a mollusk, then a fish, or something like it, before he can assume the characters belonging to his species. An eminent professor has recently expressed these views in a concise form, saying that the embryology of the most perfect being is a comparative transitory anatomy, and that the anatomic table of the whole animal kingdom is a fixed and permanent representation of the movable aspect of human organogeny."