��THE POPULAR SCIENCE MONTHLY.
��the flame of the explosion had not extended thus far. The walls of the galleries had been swept clear of timber, and presented the appearance of having been brushed with a broom. Volumes of coal-dust had been driven along by the force of the blast, and lay in waves and drifts on the floor of the levels, into which the party sank to their knees. It was found that clouds of the finer particles had been carried to the shaft and beyond it into the main north level, where a secondary explosion had taken place. . . . Secondary explosions caused by extracted or generated gas are nearly always in the vicinity of the first one ; but here is a case where the second was half a mile from the first, with an intervening space of at least a quarter of a mile known to have been free from flame and presumed to be free from gas, because men were in it with lamps which showed no indications of its presence." The conclusion is drawn that the fine dry particles were driven on by the force of the first explosion across the shaft, where the dampness preserved them, iDto the "lamp-cabin," where they were readily ignited by the lamp which was kept burn- ing openly, and thus caused the second ex- plosion ; " and it is probable that the same agency was efficient in producing, or at least augmenting, the subsequent explosions that made it necessary to flood the whole mine." A competent explanation on chemical prin- ciples of the remarkable exhibition of force and heat accompanying dust-explosions is needed.
Significance of Ancient Masons' Marks.
Professor Franz Rziha, of Vienna, has published the results of investigations, to which he has devoted many years, into the origin and meaning of the masons' marks which arc frequent in the constructions of antiquity and the middle ages, and which he specifies as occurring in Grecian, Roman, and Syrian buildings, as well as in later ones. They have been regarded as mystic signs, arbitrary marks, private signs, or simply as letters. Professor Rziha believes that they had a far wider significance, and that they formed, before the now recog- nized laws of statics on which the building art rests were made known by Galileo, the means by which the master transmitted to
��his students the secrets of his art, and to his workmen the principles of his plan. Particular figures were employed as a kind of a graphic key to the conditions of a con- sistent structure. The circle, square, cross, and triangle furnished the four elements of graphic delineation ; the compass, rule, and square were the three tools absolutely nec- essary to guide the work, and were indis- pensable in securing the proportions best adapted to give strength to all parts of the building. The geometrical properties and relations of the figures used thus served to indicate the proportions that were to be ob- tained. Professor Rziha maintains that the figures were never arbitrary, but that they were always derived from and conformable to common geometrical types. A comparison of more than five thousand masons' marks which he has made shows that, however independently individual signs may have been chosen, the combination of the lines is subject to a law in the shape of a geometrical pattern, of which the lines, whether straight or curved, must be a part. He has traced out some of these original designs, and has been able to adapt the lines of the marks to them without any artificial straining in strong confirmation, he believes, of his theory. After reviewing the modifications which these marks underwent in time and in different countries, Professor Rziha shows how it is possible to ascertain from them the ages at which buildings were erected.
Physiological Immunities of the Jews.
The "Revue Scientifique " has drawn the conclusion, from a comparison of the vital sta- tistics of different countries, that the Jews nearly everywhere enjoy certain physiolog- ical immunities which distinguish them from the other inhabitants, among which are the following : their general fecundity (propor- tion of births to the whole number) is less ; while the relative fruitfulncss of their mar- riages to those of other races varies in differ- ent places ; a greater proportion of their chil- dren survive everywhere ; illegitimate births and still-born children arc more rare among them ; the proportion of males to females among the births is greater; their mor- tality is lighter, the mean duration of life is greater; they increase more rapidly by the excess of births over deaths ; while they
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