Page:Popular Science Monthly Volume 30.djvu/295

Rh speaker had long been convinced of the correctness of this supposition, but it was only recently that he had been able to make a satisfactory demonstration of the principle.

Visions of the Blind.—Mr. J. Jastrow had an interesting paper, at the American Association, on "The Dreams of the Blind." Vision in dreams is connected with a recollection of sight, and the fact of its manifestation would mean that in the dreamer's brain there is developed a sight-center, the spontaneous activity of which is the material substratum of his dreams. Brain-centers, we know from observation and experiments on animals, are of slow growth. By asking what is the latest age at which a child may become totally blind and still retain dream-vision, we will be asking how long a time is necessary for the sight-center to develop, and sufficiently to enable it to function without further retinal stimulation. "Two hundred blind persons (mostly young) in the institutions for the blind at Baltimore and Philadelphia, were questioned in detail in regard to their dreams, and from their answers I conclude that the critical age is between the fifth and the seventh year. Those losing their sight before this age have no more vision in their dreams than if they were blind from birth. Those who become blind during this period may or may not lose dream-vision; while those whose eyesight is destroyed after this period find themselves quite on a par with seeing persons in dream-life. Only cases of total blindness are employed as a basis for this conclusion. With regard to cases of partial blindness it is found that the same period divides those whose dream-vision is brighter and more vivid than the partial sight of waking life from those whose waking life furnishes, though filled with imperfect sensations of sight, the material for dream-images."

The Cause of the Charleston Earthquake.—The theories of the causes of earthquakes are almost as various as the phenomena themselves; and it is the general opinion of those who have most carefully studied the subject that no single cause is competent to account for all that occur. The most evident fact about the cause of the recent shocks by which Charleston has been afflicted is, that there is nothing volcanic about it. Otherwise, our geologists incline to the belief that they are the concomitant of a line of weakness extending near the Atlantic coast from about Troy, New York, by Baltimore, Washington, and Richmond, to the Carolinas, and that the phenomena were immediately the result of a renewed faulting or displacement in the latter region. Tidal action may have had something to do with it; and notice has been taken of the fact that at the time of the severe shock at Charleston, the moon had been new, at perigee only about sixty hours, or a fair time for the accumulation of the effect, previous to its occurrence. The fact particularly illustrates Perrey's theory of tides of the fluid interior of the earth. How little, however, is really known about the causes of this or of any other earthquake is somewhat amusingly illustrated by a remark of Professor Dawson's the day after he had delivered his address as President of the British Association, and after the news had reached him of the disaster in Charleston. "The phenomena of the present earthquake convulsions in America and elsewhere, but particularly in America," he said, "are extremely puzzling, and completely upset some of the conclusions set forth in the address I read last evening." The Geological Survey has sent out a circular asking from observers as definite statements as they can obtain respecting the details of the phenomena. The questions have reference to the perceptible occurrence of the shock; its exact hour, minute, and second in standard time; the duration of the shocks; the accompanying noise, if any; the number, etc., of the shocks; the measure of intensity—whether very light, light, moderate, strong, or severe; the possibility of the existence of any other cause for what happened than an earthquake; and whatever other particulars of interest may have been noticed or learned by hearsay.

Mr. Darwin on Geologic Time.—Mr. G. H. Darwin, President of the Geological Section of the British Association, made a survey of the theories of geological time, including those of Mr. Croll and Sir William Thomson, and concluded from them that something has been acquired to our