August, '13] BUTTRICK: TIDES AND MOSQUITOES 355 



ered at high tide but above which it is. This method requires much 

 time and is expensive but is the most accurate in the long run. 



Second, to ascertain the general level of the marsh with reference 

 to mean low water; this being the datum plane from which predic- 

 tions are based. In order to do this it is necessary to use bench marks 

 or points of known elevation with reference to sea level, and work 

 from them. The United States Coast and Geodetic Survey, the 

 United States Geological Survey, and the Engineer Corps, United 

 States Army have each established bench marks along the coast 

 which are supposed to be reckoned from mean low water, which is 

 generally taken as mean sea level, but there may be some discrepancy 

 and, before starting work from a given bench, it should be known 

 that it is referred to the same datum plane as the predictions in the 

 tide tables. Cities, towns, and railroads frequently have established 

 bench marks which are conveniently located, but may be located 

 with reference to high water or an assumed datum. If the correction 

 factor is between them and the low water datum is known they may 

 be used; otherwise, unless a line connecting them with a bench re- 

 ferred to mean low water is run, they are of no value. 



By correspondence with the proper departments at Washington 

 or by appealing to the proper local government officers these points 

 can be determined. After establishing the bench mark a line of levels 

 must be run from it to the marsh and the general level of the latter 

 obtained within a tenth of a foot, as a tenth of a foot more or less in 

 the height of the tide may determine whether or not a marsh will 

 be covered sufficiently to flood the breeding pools. As entomolo- 

 gists are not generally also engineers it may be necessary to turn this 

 work over to an engineer. 



By adding a tenth or two of a foot to the marsh level it is possible 

 to predict at what height of the tide it will be flooded. This for con- 

 venience may be called the flood line. The height of the flood line is 

 not necessarily the height at which a marsh is entirely covered, simply 

 that at which the breeding pools are filled and the grass breeding 

 areas are flooded. In fact, at times the general level of the marsh 

 may be scarcely covered, as the flooding of the breeding places may 

 take place by seepage, leaving the higher parts uncovered. It may 

 be that the height of the flood line will vary slightly from time to 

 time according to the condition of the marsh which possibly acts 

 as a sponge and does not always stand at precisely the same level. 

 This matter would be a fruitful field for investigation. 



The third method is to visit the marsh at high tide on different days 

 and determine by observation at what height of tide they are flooded. 



