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Page:Popular Science Monthly Volume 46.djvu/380

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366
THE POPULAR SCIENCE MONTHLY.

I am further informed, to preserve the honor of the pupils; but to me it has always seemed like an object lesson in crime.

Both by precept and example must justice be taught in our schools, and its observance strictly enforced, before we may expect to see fair play in the game of life.

 

THE BAROMETRIC MEASUREMENT OF HEIGHTS.
By J. ELLARD GORE.

THERE are several methods of measuring the heights of mountains and other elevated portions of the earth's surface above the sea level. Of these maybe mentioned the following: (1) by actual leveling with an engineer's spirit level and graduated staff; (2) by trigonometrical calculation based on the measurement of the angles of elevation observed at the extremities of a carefully measured base line; (3) by observing the temperature of the boiling point of water; and (4) by reading a barometer at the sea level, and again at the top of the mountain or elevation the height of which is to be determined.

The first of these methods is certainly the most accurate, but it involves a considerable amount of labor, and for very high mountains is sometimes impracticable. The second method is sufficiently accurate if carefully carried out and a nearly level plain is available for the measurement of a base line. The third method is not accurate enough to give reliable results. The fourth is the simplest and most expeditious of all. It is especially useful for finding the difference of level between two points at considerable distances apart, and would be sufficiently accurate if certain difficulties could be successfully surmounted. A consideration of this method and the difficulties to be overcome before its accuracy can be relied upon may prove of interest to the general reader.

The principle of the barometric method is as follows: The barometer measures the weight of the atmosphere. The column of mercury in an ordinary mercurial barometer is equal in weight to a column of air of the same diameter and of a height equal to that of the earth's atmosphere. The densest portion of the atmosphere is that close to the earth's surface, and its density diminishes as we ascend. At the top of a mountain, therefore, the pressure of the atmosphere will balance a shorter column of mercury, and hence the mercury descends in the tube. From the difference in height of the mercury at the level of the sea and on the top of the mountain it is possible to calculate the height we have ascended, as will be shown further on.