OUR GUARDIANS ON THE DEEP 



673 



at Los Angeles during a violent storm, 

 when the needle shifted more than ii 

 degrees in a few hours. All these phe- 

 nomena are carefully studied in the hope 

 of discovering ultimately the secrets of 

 magnetism. 



A magnetic map of the United States 

 is made showing the lines of equal mag- 

 netic declination and those of equal dip. 

 For instance, the map for 1905 shows 

 that 5 degrees east declination runs 

 almost due north and south, while 10 

 degrees east crosses the Canadian bound- 

 ary near the Lake of the Woods, Min- 

 nesota, and swings around to the south- 

 west, 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 sur- 

 veyor 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, mark- 

 ing 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 loca- 

 tion, water-works planning, and the like. 

 The maximum error allowed in this work 

 is one-fifth of an inch to the mile. In re- 

 cent years a large number of long cir- 

 cuits 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 lev- 

 eling of over 4,000 miles of line. 



Precise leveling is now done with a 

 spirit-level attached to a telescope de- 

 signed and made by the Survey itself. 

 By the use of a nickel-iron alloy changes 

 due to temperature conditions are prac- 

 tically eliminated. One of the officers of 

 the Survey declared that he could take 

 one of these levels, put it into the hands 



of the youngest engineer in the Survey, 

 and get better results than the best engi- 

 neer the American Society of Civil Engi- 

 neers 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 opera- 

 tions (see page 675). 



Some leveling is done by the measure- 

 ment of vertical angles, although this sys- 

 tem 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). 



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 

 curve. 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. 



