824 



THE NATIONAL GEOGRAPHIC MAGAZINE 



must have been passing over it for many 

 years. Notwithstanding close surveys of 

 the vicinity, it had escaped detection until 

 with increasing draft vessels began to 

 touch it. The steamer Pilgrim was dam- 

 aged in 1884 by striking a rock in the 

 East River lying 350 yards off shore. 

 This rock had but 13 feet over it, sur- 

 rounded by depths of 30 feet; it was 

 sharp and of small extent and was not 

 found in the first surveys. 



SWEEPING THE BOTTOM OE THE SEA 



Dragging for dangers has long been 

 resorted to for the investigation of iso- 

 lated spots. A valuable and successful 

 means has been employed recently of 

 making sure that an area is free from 

 shoals or rocks having less than a certain 

 depth. This is done by dragging through 

 the water a wire from 500 to 1,400 feet 

 long and suspended at the required depth 

 with suitable buoys and weights and kept 

 taut by the angle of pull. If, for instance, 

 the wire is set at a depth of 30 feet, it 

 will indicate the presence of any obstruc- 

 tion of less depth by catching on it and 

 upsetting the buoys, and such spots are at 

 once marked and investigated. Consider- 

 able work has been done with such drags 

 in the last few years on the Great Lakes 

 and on the Atlantic and Gulf coasts of 

 the United States. 



This is of course a somewhat tedious 

 process and gives no information as to 

 depths greater than that for which the 

 wire is set, but the experience already had 

 indicates its great value. It will prob- 

 ably be found desirable in time thus to 

 drag all water areas important to naviga- 

 tion where the depth is near the draft of 

 vessels and the irregular nature of the 

 bottom gives indication of dangers. In 

 extensive dragging operations near Key 

 West and in Jericho Bay, Maine, a 

 number of shoals have been picked up 

 which were not found in the original 

 surveys. 



A remarkable instance of the value of 

 the drag was the recent discovery of a 

 rock in Blue Hill Bay, on the coast of 

 Maine. This rock has but 7 feet of 

 water over it and is only 6 feet in diam- 



eter at the top. It is surrounded by 

 depths of 78 feet, and has a small base 

 from which it rises abruptly. The orig- 

 inal survey gave no indication of a dan- 

 ger here, and its existence was not sus- 

 pected until it was discovered by the wire 

 drag catching on it. 



ROCKAWAY BEACH, N. Y., GROWS AT THE 

 RATE OF 8 INCHES A DAY 



The making of the survey and the 

 printing of the chart do not complete the 

 problem of the chart-maker. Both na- 

 ture and man are constantly changing the 

 facts represented on the charts. Break- 

 waters and jetties are built, and channels 

 and harbors are dredged, and cities and 

 towns grow up. 



A comparison of an early chart of 

 New York Harbor, made in 1737, with 

 the present conditions illustrates well the 

 effect of the growth of a community and 

 port on the problem of chart construc- 

 tion. (See page 825.) 



Another interesting example is afforded 

 by Galveston Harbor. (See page 826.) 

 Before the adoption, in 1874, of the pro- 

 ject for jetties at Galveston Entrance, 

 there was a natural depth of 12 feet on 

 the outer bar; the effect of the jetties 

 and of dredging has been to give a chan- 

 nel depth of about 28 feet. 



Great natural agencies are also con- 

 stantly at work effecting changes in fea- 

 tures shown on the charts. The action 

 of currents and waves is continually cut- 

 ting away or building the shore, particu- 

 larly on sandy coasts exposed to storms. 

 When surveyed in 1849, Fishing Point 

 on the east coast of Maryland, was but a 

 bend in the shore line. By 1887 it had 

 built out about two miles in a southerly 

 direction, and in 1908 about one mile 

 farther, curving to the westward. Al- 

 together in 60 years this tongue of land 

 has grown out over three miles. (See 

 page 828.) 



Between the surveys of 1835 and 1908 

 Rockaway Beach, near New York, has 

 grown to the westward 3.3 statute miles, 

 pushing Rockaway Inlet before it. (See 

 page 830.) This is at an average rate of 

 238 feet each year, or nearly 8 inches 



