OUR GUARDIANS ON THE DEEP at Los Angeles during a violent storm, when the needle shifted more than 1 degrees in a few hours. All these phenomena are carefully studied in the hope of discovering ultimately the secrets of magnetism. runs A magnetic map of the United States is made showing the lines of equal magnetic declination and those of equal dip. For instance, the map for 1905 shows that 5 degrees east declination almost due north and south, while 10 degrees east crosses the Canadian boundary near the Lake of the Woods, Minnesota, and swings around to the southwest, reaching the Pacific Ocean across Lower California. The 70-degree line starts in in the neighborhood of the southern Delaware boundary, and sweeps through northern Virginia, southern Ohio, and middle Nebraska, reaching the Pacific through southern Washing ton. The magnetic map is always changing, but the information is carried with it which enables the mariner or the surveyor to make the necessary corrections at any given point. PRECISE LEVELING The United States Coast and Geodetic Survey has established precise levels in every section of the United States, marking the altitude at convenient points by means of bench-marks. These marks are the beginning points of all local surveys, such as are necessary in railroad location, water-works planning, and the like. The maximum error allowed in this work is one-fifth of an inch to the mile. In recent years a large number of long circuits have been made, and where these are closed the maximum error has been about one five-hundredths of an inch to the mile. This applies to the precise leveling of over 4,000 miles of line. Precise leveling is now done with a spirit-level attached to a telescope designed and made by the Survey itself. By the use of a nickel-iron alloy changes due to temperature conditions are practically eliminated. One of the officers of the Survey declared that he could take one of these levels, put it into the hands 673 of the youngest engineer in the Survey, and get better results than the best engineer the American Society of Civil Engineers could get with any other level. After inspecting the Coast and Geodetic level the challenge was not accepted. Many other countries have adopted this instrument in all their leveling operations (see page 675). Some leveling is done by the measurement of vertical angles, although this system is not regarded as accurate enough for the finest work of the Survey. THE TIDES In addition to its other work the Coast and Geodetic Survey keeps the records of the tides, and furnishes tables which show the fluctuations for every hour and every day for as much as a year ahead. The Survey has made a great mechanical prophet that can predict the recurrence of tides as accurately and as unerringly as an astronomer can predict the coming of a solar eclipse. Its brain of brass can, with the aid of one man, calculate the recurring tides as rapidly as a hundred brains of flesh and blood can do it (see page 676). curve. The machine is known as "United States Tide Predicting Machine No. 2." The operator turns a crank until it stops. He then copies the readings on a number of dials and removes from the machine a roll of paper on which is plotted a tidal This process is repeated until tide-tables have been prepared, showing the height to the nearest tenth of a foot of every high and every low tide at a given seaport for every day in the year. This is repeated for some seventy big seaports and in less complete form for 3,000 others. There are 37 factors in the making of tides, sometimes all of them entering into the process and sometimes fewer of them. We know that they are produced by the force of gravity between the earth, the sun, and the moon, and the tide predicting machine is simply a mechanism that brings these bodies into their proper relations for every hour and every day of the year, and automatically computes the effect of these relations on the tides and registers it on the dials. THE MOST PERFECT PRECISE LEVEL IN THE WORLD This level was designed and made by the Coast and Geodetic Survey and is recognized by engineers as being superior to every other known make. It can produce more rapid and efficient work for a less cost than any other type. It has been officially adopted not only by the government of the United States, but also by those of India, Mexico, Canada, and Egypt (see page 673). We say that it is simply a mechanism that does these things; but when it is remembered that the machine has 15,000 parts, and that 15 years of careful work were required in its construction, we understand that it is not very simple after all. The records made by this machine are used wherever men sail the seas, and as the question of a foot or two in depth. on a dangerous shoal may involve the lives and the property on a passing ship, while the commercial prosperity of a port may depend upon information about the tides, this great machine serves more than a theoretical purpose. Its tabulations do for the waters what the American Ephemeris and Nautical Almanac does for the stars in aiding navigators. THE WONDER OF THE TIDES The tides are among the most remarkable phenomena in the world of nature, and they give rise to many peculiar notions concerning the level of the oceans. The tide at Panama, for instance, rises much higher than the tide at Colon, and this leads many people to think that the Pacific has a higher level than the Atlantic. As a matter of fact, however, while the tidal fluctuations on the Atlantic side have a range about 2 feet and those on the Pacific side a range of about 20 feet, the level of the oceans is the same. The difference is caused by the narrowing shoreline of Panama Bay, which takes a broad expanse of water as it enters the bay and narrows it down as it sweeps into the V-shaped entrance. What it loses in breadth it must make up in depth, and hence the higher tides on the Pacific side of the Isthmus Panama. at The same phenomenon is witnessed in the Bay of Fundy, where at the head of |