August, '13] BUTTRICK: TIDES AND MOSQUITOES 353 



vey was obtained. These are published annually and predict for the 

 year the time and heights of the tides for certain important harbors 

 on the coasts of the United States. By simple calculations, the 

 predictions can be extended to almost any point on the coast. 



These predictions are obtained by methods of very great complexity 

 which it is not necessary to discuss here. They are highly accurate 

 as to time and reasonably so as to height. Their inaccuracies are 

 due largely to meterological causes which can be predicted only 

 approximately and for a short period in advance. The predictions 

 themselves are based on variation of the astronomical phenomena 

 which cause the tides. 



The height of the maximum high tide at a given station for each 

 day of the mosquito season may be plotted on cross section paper and 

 a curve drawn connecting these points. If desired, a second curve 

 may be plotted showing the height of the minimum high tides, for 

 the two tides which occur daily seldom rise to the same height. If 

 both high tides are plotted it will be seen that more regular curves 

 are obtained by crossing the curves on dates when both tides rise to 

 the same height which occurs about every fortnight. On the accom- 

 panying chart (figure 6) compare the 1912 and '13 prediction curves, 

 where both tides are plotted with those for 1910 and '11, where only 

 the maximums are given. 



Such curves show a variation of nearly half the height of the highest 

 tides, and also that there are more or less definite periods of extreme 

 high tides followed by periods of low high tides. By plotting the 

 phases of the moon on the same sheet, it may be seen that the periods 

 of extreme high tides fall under the new and the full moon. These 

 are called spring tides. The periods of low high tides fall under the 

 moon's first and third quarter and are called neap tides. The period 

 of highest high water is generally at the new moon and is called the 

 perigee tide. 



It should be understood that a curve of this kind does not show the 

 daily fluctuation of the water level, only the predicted daily maxi- 

 mums. Curves showing the predicted daily rise and fall may easily 

 be constructed but are of less value, since the discrepancy between 

 the predicted and the actual height for a given day may be quite 

 great, but when distributed over several days is reduced. 



After plotting the high tide curves the next point is to determine at 

 what height of tide a given marsh is flooded. There are three ways of 

 obtaining this. First; by setting a tide gage at some convenient point 

 on a stream or in the marsh. The records of this gage will establish 

 a flood line below which the general surface of the marsh is not cov- 



