Page:Popular Science Monthly Volume 17.djvu/435

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much of the same form as the greyhound, and which it could hardly be questioned was the source from which it sprang. Assuming that the origin of the dogs could be traced to these sources, the more modified forms of the domestic animal were simply the result of the selected breeding which had given rise to similar modifications in dogs as it had done in the case of pigeons. Referring to the origin of dogs in general, Professor Huxley noticed the discovery of a fox-like animal of the Pleiocene period, which was found near the Lake of Constance. An animal, the cynodictus, lived in the upper Eocene period whose dentition was substantially that of the dog, and which appeared to bridge over the wide interval that separated the bears and animals of that order from dog-like forms. Beyond that period there was no distinct trace of doglike animals. By the application of ordinary common-sense reasoning, which was verified every day by experience, they were driven to the conclusion that they could only attribute the origin of these animals to causes which operated in the existing course of nature. This left them to the simple alternative of the doctrine of evolution. He believed that small differences of form—slight modifications of one main plan—were amply sufficient to give rise to the existing dog-like animals, and that these modifications had actually taken place, starting from the cynodictus.

Efficiency of Lightning-Conductors.—Direct evidence as to the efficiency of lightning conductor is afforded in a government report from Schleswig-Holstein, which is referred to in a recent work on the subject by Mr. Richard Anderson. Thunderstorms are said to be more numerous in Schleswig Holstein than in any other part of central or northern Europe, and the danger from lightning is correspondingly increased. The attention of the government insurance-office was called to the fact that, in four out of 552 cases of claims on account of damage from lightning arising in eight years, conductors of approved design had been in use, and an expert, Dr. Holtz, of Greifswald, was appointed to inquire into the causes of failure. He found that, in every case where a building provided with a conductor had been struck by lightning, the conductor was not in an efficient state. Sometimes the point of the rod was needlessly ornamented with gilding, while the underground connection with the earth, the very element of safety, was neglected. In the absence of a proper ground connection, the lightning-rod, instead of being a protection, may prove the means of attracting the discharge into the building. A measure for the periodical testing of conductors is suggested, for the detection of defective constructions, interruptions of conductivity by rusting or displacement, or of other faults that may arise from time to time.

Atmospheric Currents and Carbonic Acid.—M. Marié Davy, of the Observatory of Montsouris, France, has made a report of observations which he has taken for four years on the proportion of carbonic acid in the air as it is related to the grand atmospheric movements. The quantity of this gas is found to vary from twenty-two to thirty-six parts in one hundred thousand parts of air. During the earlier observations, extending to December, 1877, the proportion of carbonic acid was below the mean, and sometimes fell very low. In a second period, from December, 1877, to September, 1879, the proportion was considerably above the mean. A third period began in October, 1879, which was characterized like the first period by a relative weakness of proportion. The weakness became remarkable in December. The second period, in which the carbonic acid was superabundant, was characterized by moist weather with a predominance of the equatorial current over France, and embraced two years of short crops. The first period was characterized by an inferior extension of the equatorial current, by less wet weather and better crops. During all of the time of the observations, the proportion of carbonic acid showed no variation to correspond with the changes of the wind or the indications of the barometer, thermometer, and hygrometer. The fall of rain had no definite effect on the proportion. It thus appears that the proportion depends on the general predominance of the equatorial current, and not on the temporary changes of weather.