Precise or first‐order leveling, as it is now called, was first undertaken by the United States Coast and Geodetic Survey in 1878 when field‐work was begun on the line of levels of high precision which was to follow the general trend of the Transcontinental Arc, an arc of triangulation extending across the United States from Chesapeake Bay to the Golden Gate and following approximately the 39th parallel. From this single‐line beginning, the control‐leveling of the Survey has been extended until the fundamental leveling net now includes more than 260,000 miles of first‐ and second‐order leveling. About two‐fifths of this is first‐order leveling, the remaining three‐fifths being second‐order or single‐line leveling.

The author has given an interesting and enlightening account of the difficulties met in keeping accurate levels on a site which is being used for construction operations. He and his assistants are to be congratulated on the patience and ingenuity they showed in carrying their job to a successful conclusion. From the nature of the work described in the paper, it is evident that the expense involved was by no means negligible. The author was fortunate that in this case the size and public importance of the White House project ensured that the necessary funds were provided, once the need for keeping accurate level records was recognized. On less important construction jobs, the cost such a task is often a stumbling block, even when the resulting records might be considered an essential part of good construction practice. Certainly the obstacles reported in the paper are not encouraging to anyone planning similar work with limited funds at his disposal.

No abstract

The investigation under discussion has resulted from a growing conviction on the part of some of us in the Section of Leveling of the Coast and Geodetic Survey that, in many cases at least, poor checks obtained in repeat leveling, when starting or tieing out new lines of levels, result from lack of stability of the bench‐marks instead of “sloppy leveling” as is often assumed. It seems only natural to be skeptical of the stability of a bench‐mark in the top of a concrete post extending 3‐½ or 4 feet into the ground and to believe that a bench‐mark in the foundation of a large building or a heavy bridge‐abutment will maintain its original elevation. However, we have gradually been forced to the conclusion that not all is stable that looks that way and in order to test, under controlled conditions as far as possible, the possibility that massive structures can undergo changes in elevation, we started wishing for a big construction job, conveniently located on which we could experiment. About that time the new War Department Building across the Potomac River was proposed and we began scheming to make this gigantic building produce a by‐product of possible interest to the surveying profession.

Going back, in our imagination, to the early stages of civilization on this earth, we may find a man having established his home somewhere in the flat country just back of a gently-sloping sand beach. He must have done this for reasons which seemed good to him, and if we pursue our inquiry further, we would suggest that they may have been about as follows:(1) The flat fertile country of the lowlands would make it easier for him to grow the food he needed than if he cultivated the steeply-sloping hillsides farther inland.(2) He would be near an additional source of food supply, to be taken from the , sea.(3) He would have more ready means of communication with his like-minded fellows by means of water transportation. Our remote ancestor, then, has established his home on the coast and it is not long before he has learned from experience that if he lands his dugout on the beach when the water's edge is far U. 8. Coast and otodetic Survey, Washington D. 0. This paper, illustrated by lantern slides, wai presented at the meeting of th% Section on November a, 1947.out from his door and does not pull his canoe well up on the beach, it will probably float away within a matter of relatively few hours. Conversely, if he lands when the water's edge is in close to his door he will, if he wishes to set sail again when the water has receded, have to drag his boat a long way down the beach before it can be floated.This state of affairs is forced on his attention and he eventually notices that there is a somewhat rhythmic quality to the behavior of the water's edge with relation to the tree line back of the beach.He talks the matter over with his family and perhaps with some of his neighbors. They agree that there is a tendency toward a repetitive cycle in the events concerning the behavior of the water's edge. Someone, perhaps one of his greatgrandchildren, eventually tumbles to the fact that this cycle of events comes fairly close to repeating itself every time the moon goes through its changes from new moon to half woon, full moon, half moon and back to new moon again. Once this idea has dawned on these people and they seek to prove that it is a fact, the study of the tides has begun.As it is not within the scope of this discussion to trace the history of tidal in- TRAH84CTIONS of The New York Academv of Sci-Editor: ROY WALDO MINER.