Page:Popular Science Monthly Volume 13.djvu/131

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ther to the deceptive and dangerous character of the defects, I have, in the course of my experiments, found a number of persons who were unable to distinguish between the primary colors at night, while their perception or sensation of color by daylight was apparently perfect. Again, I have found another anomaly which, until it has been more thoroughly investigated, and the real causes that produce it are understood, I shall designate as a form of color-blindness, although I am in doubt myself as to its dependence upon any of the principles that enter into that defect; this is an inability to distinguish between or to recognize the primary colors at certain distances, varying more or less in individuals. This was found to be the most difficult of all defects to detect in the various cases I have examined, amounting to some nine or ten, in the regular course of my business as optician during the past three years. I have found no two of them at all alike except in general results.

"I have kept records of various accidents that have occurred, both upon land and water, during the past few years, and I have gathered such information about some of them as I could get outside of official sources—often I was unable to get any of any value, but I am convinced beyond a doubt that a large proportion of them could have been traced to this defect for a correct solution as to the primary causes of the accident. The query has been made, that if these defects in their various forms are as numerous and of such a dangerous character as has been shown, how can we account for such a comparatively small number of accidents occurring which might be charged to them? I have attributed it to the high average intelligence and acquired cautiousness of engineers and pilots as a class. They have become so accustomed to be on the lookout for danger that their suspicions are easily aroused, which creates a sort of instinct that governs their actions, and they do not recognize but that their perceptions are correct."

Sewer-building.—The general principles of sewer-building are, says the Polytechnic Journal, that each day's influx should be promptly passed out by natural flow or flushing, and not allowed to deposit sediment. The alignment should be good, especially at the bottom; the descent should be uniform, and the interior surface smooth, so as to reduce friction and not to cause clogging; the walls should be absolutely impervious, and the suction such as will cause the most rapid possible flow, with a minimum of sewage. Rapid flow being essential, smooth interior walls should be provided; mortar projecting from the joints of a brick sewer markedly impedes the flow and arrests putrefiable matter. A flat-bottomed sewer is the worst form as regards the velocity of the flow; a circular bottom is better; an egged-shaped section, with the point downward, permits of a minimum current flushing and cleansing the bottom. In brick sewers the mortar, constantly moist, must sooner of later succumb to the disintegrating action of the matters passed through it, and the whole line gradually passes into the condition of a sieve, allowing the liquid portions of the sewage to pass through it and to saturate the subsoil, but retaining the solids. From the consequent saturation of the soil result contagious fevers. Hence vitrified clay pipes are now almost universally employed. The "slip" glazing applied to these pipes resists the severest chemical action of sewage-water. The "slip" glaze is produced by dipping the unbaked clay into a mixture of "slip-clay" or Albany earth and water, which, under a white heat continued from twelve to thirty hours, produces a vitreous and very durable silicious surface upon the wares.

Remarkable Land-Slides.—Bear-Tooth Mountain is one of the most prominent landmarks in Northern Montana; it is plainly visible from Helena, thirty miles distant. It presents, or rather used to present, the appearance of two great tusks rising hundreds of feet above the general contour of the mountains. One of these tusks, the smaller one, which was fully five hundred feet high, three hundred feet in circumference at the base, and one hundred and fifty feet at the top, was recently dislodged from its place and precipitated into the valley below. A few weeks since, according to the Helena Independent, a party of hunters chasing game several miles north of the Bear Tooth