12 TIDES. 



or one low and the next low tide is twelve hours and twenty-six 

 minutes. Hence, as there are two such periods in a day, and only 

 twenty-four hours in that period, high tide occurs fifty-two minutes 

 later each successive day. In a month's time this very nearly co- 

 incides with the period of a lunar month, and in fact we find that 

 the highest tides occur at such a time as answers to the new and 

 full moon, and the lowest at the change of first and third quarter. 

 In a table of tides, therefore, the tide, if marked for new moon at 

 a certain place, will answer for every new moon at that place through- 

 out the year and following seasons. Thus in a book of sailing di- 

 rections, a table of high and low water, or rather of spring and neap 

 tides, gives the height of water or time of occurrence of the high- 

 est spring tide (when the moon is exactly at new and full) at any 

 time of any year ; and also the height of water and occurrence of 

 any neap tide (when the moon is at exact first or third quarter) at 

 any time of any year. 



The interval between is easily determined by taking the time of 

 highest or lowest water, and for each day subtracting fifty-two min- 

 utes from said time. Thus by means of these tables navigators can 

 always tell high or low water at any day or any time of the day for 

 any year, at any place mentioned in these tables ; or they can by 

 marking the spring and neap tides make a table of their own, that, 

 if their reckoning is correct, will answer equally well for other par- 

 ties as for themselves. But a few words of explanation as to the reason 

 of high and low water. I have said that both sun and moon have 

 their influence in the tide of our larger bodies of water throughout 

 the globe. The influence of the moon is about three times that of 

 the sun. The moon attracts both land and water ; one can easily see 

 that the movable particles of the latter will respond to this mag- 

 netic force much more noticeably than the almost immovable 

 particles of the former. The rising of the crest of water, directly 

 under the centre of influence of the moon at fufl or new, causes 

 of course a depression at the sides, being greatest at the centre of 

 this non-magnetic attraction ; thus the centre of attraction will be 

 distant from the centre of depression by 90°. To restore the equilib- 



