ON THE ACTION OF WAVES AND CURKENTS. 331 



These scales would stand in the sand edgeways to the current, and so 

 be made into permanent sand-gauges, which could be read periodically 

 without removing the glass or stopping the tide. For tank B the scales 

 were half the size of those for tank A. 



The resistance which a few such thin obstructions offered to the water 

 would be very small, but if the gauges were numerous the resistance 

 would be a serious matter, so that a more general method of taking a 

 final survey was necessary. 



The ease and simplicity with which the contour-line could be found 

 when the tides were not running, by adjusting the level of the still water 

 and observing its boundary on the sand, reduced the question of making 

 a contour survey to the providing of the means — 



1. Of adjusting the level of still water to any required height. 



2. Of rapidly and accurately determining the horizontal position of 

 points on the edge of the water. 



The tide-gauge, shown in the drawing on the top of the tank, which 

 would stand on the glass which gave a level surface, answered well to 

 determine the level of the water. 



For the purpose of surveying the contours a system of horizontal sur- 

 vey-lines were set out in the covering frames, consisting of black thread 

 stretched immediately beneath the glass in the frames. The lines are G" 

 apart ; those parallel with sides are called lines, and those at right angles 

 sections. The first section is 3 ' from the end of the tank ' and the 

 lines are so placed that one of them bisects the tank. 



These sui-vey-lines divide the entire surface of the fixed tray into 

 equal squares. They are, however, 11" f i-om the bottom and about 8 from 

 the sand ; besides, they are six inches apart, so that to make accurate use 

 of them for surveying the sand it was necessary to use some means of 

 projecting a point vertically up to the level of the glass and scale its dis- 

 tance from a line and a section. This is accomplished by a little instru- 

 ment, which may be called a projector, shown on the top of tank A. 



It has a foot which consists of two scales placed at right-angles, so 

 that the zero-lines on both, if produced, would meet in a point about 

 half an inch from the edge of the scale. About this point there is a hole 

 through the foot with cross-wires so placed that they intersect in the point 

 of intersection of the zero lines. Vertically above this is a horizontal 

 plate with a pin-hole, so that when placed on the horizontal glass any 

 point below seen through the pin-hole on the cross-wires is vertically be- 

 low the intersection of the zero-line of the scales ; and hence if these scales 

 are parallel to the lines and sections the distances of the point from these 

 are read at once on the scales. This method of surveying lends itself 

 readily to plotting on section paper. This may be done directly, the glass 

 cover of the tank serving for a table ; each point may bo plotted as it is 

 observed ; and in this way Mr. Bamford is now able to survey and plot 

 a complete contour-line in from fifteen to thirty minutes, and requires 

 only about five hours to make a complete survey plotting the charts. 



One very great desideratum has been a graphic recording tide-gauge. 

 So much depends on the manner of rise and fall of the tide that it does 

 not seem sufficient to know that it is produced by a simple harmonic 

 motion ; the curve should be recorded for each experiment at different 



' This somewhat awkward arrangement was necessary on account of the wood in 

 the frames. 



