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the scale and the hundredths of an inch on the vernier, as indicated by a division on the vernier coinciding with a division on the scale.
13. The aneroid barometer is shown in Fig. 5. These instruments are made in various sizes, from the size of a large watch up to an 8-inch or 10-inch face. They consist
of a cylindrical box of metal, with a top of thin, elastic, corrugated metal. The air is removed from the box. When the atmospheric pressure increases, the top is pressed inwards, and when it diminishes, the top is pressed outwards by its own elasticity, aided by a spring beneath. These movements of the cover are transmitted and multiplied by a combination of delicate levers, which act on an index hand and cause it to move either to the right or left over a graduated scale. These barometers are self-correcting (compensated) for variations in temperature. They are very portable, and are so delicate (when carefully made) that they show a difference in the atmospheric pressure when transferred from the upper part of a room to the floor. The aneroid barometer should be handled with care, as it is easily injured. A good aneroid barometer costing from $20 to $30, is of great value to the navigator as a "weather glass” if carefully observed, but its readings are not so accurate as those of a good mercurial barometer.
14. Adjustment of a Ship’s Barometers. – The Chief of the United States Weather Bureau, Washington, D. C., has issued the following announcement to shipmasters: “For the convenience of vessel masters passing the St. Mary's Falls ship canal, and for the better conduct of its own work, the Weather Bureau has constructed an office building on the canal grounds near the locks. All masters are cordially invited to this office at any time, day or night. Special attention will be given to setting barometers, and all masters are requested to avail themselves of the services of the official in charge of this station.”
15. Atmospheric Pressure on the Great Lakes. In making a trip from Duluth, Minn., to Buffalo, N. Y., the average atmospheric pressure, supposing, of course, that normal weather conditions prevail throughout the entire passage, should be approximately as follows: Starting from Duluth, in October, the barometer at that place should read about 29.34 inches; at Marquette, Mich., it should read about 29.35 inches. When passing through the locks at the "Soo," bound down, a slight increase in pressure (about two-hundredths of an inch) is noticeable, and likewise when going up the same amount of decrease is observed. Below the locks the barometer should read about 29.36 inches; at Alpena, 29.38 inches; at Port Huron, 29.42 inches; at Detroit, 29.44 inches; at Cleveland, 29.44 inches; and at Buffalo, 29.42 inches. On Lake Michigan the barometer should read about 29.42 inches under normal atmospheric conditions.
STORMS AND THE HANDLING OF VESSELS
16. The best available general information relating to the occurrence of storms and the handling of vessels in storms on the Great Lakes is undoubtedly that given by the Hydrographic Office in its publication “Notice to Mariners for the Great Lakes," of September 1, 1896. The most important of these notes are reproduced here and should be carefully studied and remembered.
17. Importance of Predicting an Approaching Storm. – The storms on the Great Lakes, because of their frequency during the season of navigation and the damage they inflict on the commerce of the lakes, especially on the lighter craft, demand serious attention.
A review of the circumstances attending the mishaps to vessels during storms has shown that in many cases all loss could have been avoided, or the severity of the damage largely diminished, by a close observance of the storm warnings issued by the Weather Bureaus of the United States and of the Dominion of Canada, coupled with a knowledge on the part of the master of the constitution of storms, and of the nature and behavior of the gales that attend them. The custom on the lakes of loading steamers until they can barely pass over the sills, and the unwieldy size of the tows carried by the towboats, put both classes of vessels at the mercy of the wind and waves. the first case, when overtaken by a storm, the heavily laden steamer, too deep in the water for proper handling, has simply to depend for safety on her staunchness; in the second, the only possible recourse is to cut adrift the tow, if it has not already gone to pieces. Under such circumstances, any means of predicting a coming storm, of foretelling the shifts of the wind, and the probability of a certain shore furnishing a continuous lee for anchorage has a double value. Many lake vessels are equipped with aneroid barometers and thermometers, and it requires but a brief period of
observation with these two instruments to enable the mariner to keep himself informed at all times of the approach of any extensive atmospheric disturbance likely to endanger his vessel.
18. The Barometric Changes Attending a Storm. Upon the weather map published for each day by the United States Weather Bureau, there will, in general, appear one or more approximately circular areas marked “Low," while others of rather more irregular outline are marked "High," the two alternating, one with the other. The first implies that the reading of the barometer within the area indicated is below the average; the second, that it is above it. If we examine a number of these charts from day to day in succession, we will note that the position of the center of each of these areas on any particular day is considerably to the eastward of that of the day previous; i. e., there is a rapid easterly motion of both lows and highs, continuing, in the case of the former, until they either fill up or disappear over the Atlantic. The presence of these lows exercises a very direct influence over the weather of the region throughout which they prevail. Ordinarily, during the season of lake navigation, they are felt merely as periods of warm rainy weather; in exceptional cases, however, the barometer falls far below the average, under which circumstances they are attended by the rough weather and heavy gales that constitute a storm.
As these lows approach from the westward, barometers in advance of them are gradually depressed below the mean, generally at a sufficient interval before the appearance of the worst features of the storm to allow the mariner to take proper ineasures for encountering it. It is on the barometer, therefore, that he must mainly rely for notice of the storm's approach.
19. Mean Barometric Pressures. – In order to use the barometer intelligently as a means of forecasting the weather for any region, it is necessary to have a knowledge of the mean, or average, barometric pressure prevailing over the region under ordinary weather conditions. All of the Great Lakes are at a considerable elevation, in consequence of which the barometric readings are considerably less than the readings at sea level. The pressures marked on the daily weather maps of the United States Weather Bureau have all been reduced to sea level by the addition of a certain correction. The elevation of each lake above the sea and the mean actual reading of the barometer under ordinary conditions are given in the following table:
It is for this reason that the barometric readings for the several lake stations given on the daily weather map are so much greater than those recorded by marine observers on the lakes. Another point in this connection is that the words "stormy," "change," etc. on the dial of the aneroid
' (see Fig. 5) are applicable, if applicable at all, only at sea level. They are therefore worse than useless when the aneroid happens to be used in the lake regions, or, indeed, at any point differing much from sea level.
As long as the barometer continues to read within a few hundredths of the average pressure, no decided change in the weather may be anticipated. It is, however, when the pressure starts to rise or fall that the indications of the instrument become of value, as betokening approaching changes, a rise being apt to be followed by an improvement in the general weather conditions; a fall by the reverse. Should the fall continue and be accompanied by other