Page:Popular Science Monthly Volume 39.djvu/288

276 which have been followed out, and others which are still incomplete. Of the cases, twelve experienced between the fourth and the twenty-sixth day of inoculation a mild attack of yellow fever; twenty-four experienced no symptoms within twenty-five days, but contracted a mild attack before the end of three years; twelve exhibited no symptoms of the disease within three years; three, who had no symptoms within twenty-five days, contracted well-marked yellow fever within three years; and one, who had a mild attack in consequence of inoculation, contracted a severe attack later on which proved fatal. Thus only eight per cent of those who had been inoculated contracted the disease in a well-marked form, and the mortality was less than two per cent. Of sixty-five monks who came to Havana and lived there under similar conditions, thirty-three were inoculated and thirty-two were not. Only two of those who were inoculated had well-marked attacks, and these were not fatal; while eleven of those who had not been inoculated were severely attacked, and five died. But inoculations performed in cold weather do not seem to be wholly trustworthy and need to be repeated in the spring; also it appears that a person who has been three years in the city without having the disease has become acclimatized, and is not likely to be attacked afterward.

Facts about the Aurora Borealis.—The present condition of the investigation of the phenomena of auroras is thus described by Mr. G. S. Griffiths, F. G. S., F. R. S., in an address on the objects of antarctic exploration. The nature of auroras, the author says, is very obscure; but recently a distinct advance has been made toward discovering some of the laws which regulate them. "Thanks to the labors of Dr. Sophus Tromholt, who has spent a year within the Arctic Circle studying them, we now know that their movements are not as eccentric as they have hitherto appeared to be. He tells us that the aurora borealis, with its crown of many lights, encircles the pole obliquely, and that it has its lower edge suspended above the earth at a height of from fifty to one hundred miles, the mean of eighteen trigonometrical measurements, taken with a baseline of fifty miles, being seventy-five miles. The aurora forms a ring round the pole, which changes its latitude four times a year. At the equinoxes it attains its greatest distance from the pole, and at midsummer and midwinter it approaches it most closely; and it has a zone of maximum intensity which is placed obliquely between the parallels of 60 and 70 north. The length of its meridional excursion varies from year to year, decreasing and increasing through tolerably regular periods, and reaching a maximum about every eleven years, when, also, its appearance simultaneously attains to its greatest brilliancy. Again, it has its regular yearly and daily movements and periods. At the winter solstice it reaches its maximum annual intensity, and it has its daily maximum. .. . Whether or not there is any connection between auroral exhibitions and the weather is a disputed point. Tromholt believes that such a relationship is probable. He says that ' however clear the sky, it always became overcast immediately after a vivid exhibition, and it generally cleared again as quickly.' Payer declares that brilliant auroras are generally succeeded by bad weather, but that those which had a low altitude and little mobility appeared to precede calms. Ross remarks of a particular display that ' it was followed by a fall of snow, as usual.' Scoresby appears to have formed the opinion that there is a relationship indicated by his experience. It is, therefore, allowable to regard the ultimate establishment of some connection between these two phenomena as a possible contingency. If, then, we look at the eleven-year cycle of auroral intensity from the meteorological point of view, it assumes a new interest, for these periods may coincide with the cycles of wet and dry seasons which some meteorologists have deduced from the records of our Australian climate, and the culmination of the one might be related to some equivalent change in the others. For, if a solitary auroral display be followed by a lowered sky, surely a period of continuous auroras might give rise to a period of continuous cloudy weather, with rain and snow. Fritz considers that he has established thi3 eleven-year cycle upon the strength of auroral records extending from 1583 to 1874, and his deductions have been verified by others."