CHAP. IX.] TIDES 221 



the datum should be stated in the title of the chart, together 

 with the amount to which they fall below the datum. 



" Range " is the difference between the height of high and 

 that of the low water which immediately follows it, without 

 any reference to the datum. 



The " semimenstrual inequality of heights " is the difference 

 between the heights of spring and neap tides above mean 

 water-level. 



The " diurnal inequality of heights " is, in irregular tides, 

 the difference between the height of high-water of each suc- 

 cessive tide. 



The " age of the tide " is the interval between the time of 

 new or full moon and the time of the next spring-tide, and 

 varies from one and a half to three days. 



The " lunitidal interval " is the time that elapses each day, 

 between the transit of the moon over the meridian and high- 

 water following. 



The " establishment " may be also defined as the lunitidal in- 

 terval when the time of the moon's meridian passage is 0'\ 0'". 

 or 12^\ 00"". This is called the " vulgar establishment." 



The " mean establishment " is the mean of all the lunitidal 

 intervals in a semilunation, and may differ considerably from 

 the vulgar estabhshment. The latter is the high-water full 

 and change given in the charts. 



The " semimenstrual inequality of time " is the difference 

 between the greatest and smallest lunitidal interval. 



The " diurnal inequality of time " is, in irregular tides, the 

 difference between the lunitidal intervals of each successive 

 tide. 



The time and height of the tide is ever changing, caused ^^"?® ?^ 

 by the relative positions of the sun and moon, and these in Tides, 

 more or less regular variations are further affected by winds 

 and by the height of the barometer. The difference in level 

 due to the latter may be taken rouglily as a foot for every 

 inch of the barometer above or below the mean barometer, 

 high barometer causing lower tides. 



The time of the moon's transit over the meridian gives us a Observa- 

 rough measurement of the relative position of sun and moon ferred to 

 in right ascension, and it is therefore to this meridian passage ^°^'f 

 of the moon that we refer all calculations of the tides. 



