LECTURES. 117 



vidiials or families living on the spot. Others are extremely slow, 

 and go through so large a space of time, that the particular circum- 

 stances escape the memory of individuals and even of generations, and 

 can only be ascertained from history and written documents. These 

 recollections and traditions of the local population, as well as the re. 

 cords of local history, are always valuable and may he consulted- 

 But in most cases, especially if a particular application of ihe phe- 

 nomena is to be made, such a precision as to the facts, and such a 

 nicety of observation are requisite, as can only be obtained by a series 

 of mathematically accurate pictures, that is to say maps, of the 

 changed locality. 



If our forefathers for two or three centuries past had been as correct, 

 conscientious, and minute in the construction of special maps of all 

 parts of a country, of its rivers, coasts, ports, banks, &c., as we now 

 are, a complete collection of their maps would be invaluable. But 

 even as they are, incomplete, often unreliable, and for the most part 

 too general, they are for the history of those changes and all that de- 

 pends upon them of the highest importance; because they often are 

 the only documents which wo can consult, and from which we can 

 form a judgment. 



How desirable also in this respect a complete collection of former 

 maps would be has been observed in this country on various occasions. 

 Harbor commissions, coast survey officers, military engineers, archi- 

 tects, in constructing bridges or moles for the protection of ports, have 

 repeatedly felt this great and essential want. 



There is perhaps no other country in the world which has such 

 changeable coasts and rivers as the United States. The whole extent 

 of the shores of the Mexican Gulf, more than 1,500 miles in length, are 

 low, and consist of shifting materials, partly of sand and partly of coral 

 rocks. Changes on a great scale have occurred there every year as 

 long as the Gulf has been known to us. A mighty circular current, 

 accompanied by many side currents, moves in this large basin, and is 

 constantly at work, abrading and altering after its own manner the 

 configuration of its coasts. Heavy gales, and consequent inundations, 

 are frequent phenomena ; of some of which it is recorded that in the 

 short space of one or two days they have torn asunder islands, 

 filled ports, heaped up sand-banks, destroyed settlements, and thus 

 changed at once the whole physiognomy of a long coast-tract of some 

 hundreds of miles. 



Into the Mexican Gulf empties that mighty river the Mississippi, 

 the delta of which, one of the most interesting in the world, is a per- 

 fect labyrinth of natural changes. This delta has been explored, and 

 somewhat more accurately studied since the time of the French dis- 

 coverers, Iberville and Bienville, about a century and a half ago. 

 These Frenchmen gave their names to branches of the Mississippi 

 which now no longer exist. They built fortifications and beacons on 

 the then extreme spits of land, which are now situated far in the 

 interior. Tliey speak in their reports of sand-banks with deep sound- 

 ings upon them, which now have become inhabited islands. They 

 would in many parts scarcely recognise the old Mississippi delta in 

 the maps which we could now lay before them. No harbor can be 



