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SURVEYING UNDER DIFFICULTIES IN ALASKA This picture gives an idea of some of the difficulties encountered by the surveyor. To obtain the necessary elevation the instrument had to be

taken to the top or a snow-clad mountain peak, which was so small that it hardly afforded room for the instrument and the two observers

OUR GUIRDIANS ON THE DEEP

673

at Los Angeles during a violent storm, of the youngest engineer in the Survey, when the needle shifted more than I } and get better results than the best engidegrees in a few hours. All these phe neer the Imerican Society of Civil Enginomena are carefully studied in the hope neers could get with any other level. of discovering ultimately the secrets of After inspecting the Coast and Geodetic magnetism.

level the challenge was not accepted. I magnetic map of the United States Vany other countries have adopted this is made showing the lines of equal mag

instrument in all their leveling operanetic declination and those of equal dip. tions (see page 675). For instance, the map for 1905 shows Some leveling is done by the measurethat 5 degrees east declination runs ment of vertical angles, although this sysalmost due north and south, while 10 tem is not regarded as accurate enough degrees east crosses the Canadian bound for the finest work of the Survey. ary near the Lake of the Woods, Min

THE TIDES nesota, and swings around to the southwest, reaching the Pacific Ocean across In addition to its other work the Coast Lower California. The 70-degree line and Geodetic Survey keeps the records starts in in the neighborhood of the of the tides, and furnishes tables which southern Delaware boundary, and sweeps show the fluctuations for every hour and through northern Virginia, Southern every day for as much as a year ahead. Ohio, and middle Nebraska, reaching The Survey has made a great mechanical the Pacific through southern Washing- propliet that can predict the recurrence ton.

of tides as accurately and as unerringly The magnetic map is always changing, as an astronomer can predict the coming but the information is carried with it of a solar eclipse. Its brain of brass can, which enables the mariner or the sur

with the aid of one man, calculate the veyor to make the necessary corrections recurring tides as rapidly as a hundred at any given point.

brains of flesh and blood can do it (see

page 676). PRECISE LEVELING

The machine is known as “United The United States Coast and Geodetic States Tide Predicting Machine No. 2.' Survey has established precise levels in The operator turns a crank until it stops. every section of the United States, mark Ile then copies the readings on a number ing the altitude at convenient points by

of dials and removes from the machine means of bench-marks. These marks are

a roll of paper on which is plotted a tidal the beginning points of all local surveys,

This process is repeated until

tide-tables have been prepared, slowing such as are necessary in railroad loca

the height to the nearest tenth of a foot tion, water-works planning, and the like.

of every high and every low tide at a The maximum error allowed in this work

given seaport for every day in the year. is one-fifth of an inch to the mile. In re

This is repeated for some seventy big cent years a large number of long cir

seaports and in less complete form for cuits have been made, and where these

3,000 others. are closed the maximum error has been

There are 37 factors in the making of about one five-hundredthis of an inch to tides, sometimes all of them entering into the mile. This applies to the precise lev the process and sometimes fewer of eling of over 4,000 miles of line.

them. We know that they are produced Precise leveling is now done with a by the force of gravity between the spirit-level attached to a telescope de earth, the sun, and the moon, and the signed and made by the Survey itself. tide - predicting machine is simply a By the use of a nickel-iron alloy changes mechanism that brings these bodies into due to temperature conditions are prac their proper relations for every hour and tically eliminated. One of the officers of every day of the year, and automatically the Survey declared that he could take computes the effect of these relations on one of these levels, put it into the hands the tides and registers it on the dials.

curve.

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This photograph was taken by a camera specially designed for photo topographic work (see page 669). The rear frame of the inner camera box is supplied with noches 10 mark the horizon and the principal lines. All these notches print on the edges of the negatives, giving ready means for checking distortions in the prints, and at the same time showing the scale in conjunction with the cross-lines. In this picture, reading from left to right, two triangulation stations are shown, then a camera point (marked No. 4), then the height (in meters) of four peaks, cte. From such photographsil is possible to construct an accurate map of the region within the liolelor vision.

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TITE JOST PERFECT PRECISE LETEI, IV Till TORID 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).

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We say that it is simply a mechanism and they give rise to many peculiar nothat does these things; but when it is re tions concerning the level of the oceans. membered that the machine has 15,000 The tide at l'anama, for instance, rises parts, and that 15 years of careful work much higher than the tide at Colon, and were required in its construction, we

this leads many people to think that the understand that it is not very simple l'acific has a higher level than the Atafter all.

lantic. Is a matter of fact, however, The records made by this machine are

While the tidal fluctuations on the Itlanused wherever men sail the seas, and as

tic side have a range about 2 feet and the question of a foot or two in deptlı

those the Pacific sice a range of about a dangerous shoal may involve the

20 feet, the level of the oceans is the lives and the property on a passing ship, while the commercial prosperity of a port

The difference is caused by the

narrowing shoreline of Panama Bay, may depend upon information about the tides, this great machine serves more

which takes a broad expanse of water as than a theoretical purpose. Its tabula

it enters the bay and narrow's it down as tions do for the waters what the mer

it

sleeps into the l-shaped entrance. ican Ephemeris and Nautical Almanac

Ilhat it loses in breadth it must make up does for the stars in aiding navigators.

in depth, and hence the higher tiles on

the Pacific side of the Isthumus TIN WONDER OF THE TIDES

lanama. The tides are among the most remark The same phenomenon is witnessed in able phenomena in the world of nature, the Bar of Finely, where at the head of

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MACUNT TILAT PROPITESIES This machine--the only one of its kind in the world—will predict the tides at any seaport as much as 100 years in advance. It is used in the compilation of the de tables issued ead year by the Coast and Geodetic Survey. A simple turn of the handle will record on the dials ile time and height of the water at high or low tide for each day in the year at any selected port, and at the same time trace a tie curve which forms a permanent record of its predic

The machine-an improvement of Lord Kelvin's tide-curve machine—was designed and constructed by the Coast and Geodetic Survey and is a triumplı of applied science.

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