-V, 



Sect l\.] 



TIDES. 



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44. The tide observations made at any place, when th 



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times and heights of high water (and of low water) have 

 been deduced in the way directed in Articles 2, 3^ 4. 5, 

 6, may be entered in a table of wliich the form will be 

 given, and must then be sent to the Hydrographer''^ Office 

 in the Admiralty. 



45. It is to be remarked that^ though there is generally 



an A.M. and a p.im< tide, there is one day in every half- 

 lunation on which there is only one tide. 



(Because the interval of the two tidvo is, on the 

 average, about 12h. 24m. ; so that if there be a tide at 



llh. 50m. A.M., there will be no other tide till 12h. lAm. 

 P.M., that is Oh. 14m. a.m. of the next day.) 



46. Self'iegistering tide-machines are used in several 



places, and may be constructed at no great expense. 

 (They are made by Mr. Newman, of Regent Street, for 

 about 30/. : they are constructed so as to work with a tube 

 and float, as described in Art. 3.) 



These machines give the whole course of rise and fall 

 of the tide j and record several successive tides on the 

 same paper. 



47. The wind often produces a considerable effect upon 



the tides, especially upon the height, and should be noted, 



although it is difficult to give any general rule for the 

 effect. 



48. The surface of the sea rises and Mis as the 

 barometer falls and rises ; namely, about 1 inch for every 

 h ii^ch of mercury. This may be applied ;is a correction 

 when very exact observations are made. 



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