458 HYDROGRAPHICAL SURVEYING [chap. xx. 



1 square inch, weighs 0'445 pound, it follows that the depth is 

 obtained by the multiplication of that factor by the pressure 

 in pounds per square inch as indicated by the gauge. The 

 variation in pressure, provided the weight at the submerged 

 end of the indiarubber tubing has not moved its position, is 

 therefore a measure of the rise and fall of tide. 



Observations with this apparatus have been made success- 

 fully in depths of 35 fathoms, and the results, when compared 

 with observations of an ordinary tide-gauge on the beach in the 

 immediate vicinity, were found to agree very closely; an occa- 

 sional difference of 2 or 3 inches might be noted, but it seldom 

 exceeded 1 inch, or even less. 



PLOTTING. 



Use of Reference may be made to the device of introducing one 



Arbitrary qj, ^^ore arbitrary plotting points to facilitate the accurato 



Points. plotting of a series of secondary trigonometrical points. It 



sometimes happens that such points are connected with each 



other through primary trigonometrical stations which are too 



far distant to come within the limits of the plotting sheet. 



An occasion for the use of this method may arise in the 

 Home Surveys, which are usuall}'- based on the Ordnance 

 Triangulation. 



In Fig. 110, A, B, C, D, E, F are secondary stations not" 

 necessarily intervisible, lying in one general direction and in 

 the relative position shown. The connection between any 

 two of these stations can be found bj' the triangulation scheme 

 provided. 



Making a rough plot on a small scale, the most convenient 

 position for the arbitrary plotting points X and Y are deter- 

 mined, and the angles A F Y, F A Y, A F X, F A X, are taken 

 off to the nearest degree by station pointer. 



Using the angles thus taken off, and the calculated distance 

 A F, the triangles A F Y, A F X are solved. A and F being 

 placed on the paper with regard to scale and bearing, X and Y 

 are then plotted from A and F. 



0, occupying a central position, may be plotted from A, X, 

 and Y, which involves the solution of triangles A X C, X Y C, 



