214 POPULAR LECTURES AND ADDRESSES. 



the heights and times of high water and low water 

 in certain selected sets of groups. Laplace com- 

 menced in this way, as the only one for which 

 observations made before his time were available. 

 How strong the tendency is to pay attention chiefly 

 or exclusively to the times and heights of high and 

 low water, is indicated by the title printed at the top 

 of the sheets used by the Admiralty to receive the 

 automatic records of the tide-gauges ; for instance, 

 " Diagram, showing time of high and low water at 

 Ramsgate, traced by the tide-gauge." One of the 

 chief practical objects of tidal investigation is, of 

 course, to predict the time and height of high 

 water ; but this object is much more easily and 

 accurately attained by the harmonic reduction of 

 observations not confined to high or low water. 

 The best arrangement of observations is to make 

 them at equi-distant intervals of time, and to 

 observe simply the height of the water at the 

 moment of observation irrespectively of the time 

 of high or low water. This kind of observation 

 will even be less laborious and less wasteful of time 

 in practice than the system of waiting for high or 

 low water, and estimating by a troublesome inter- 

 polation the time of high water, from observations 

 made from ten minutes to ten minutes, for some time 

 preceding it and following it. The most complete 

 system of observation is, of course, that of the 

 self-registering tide-gauge which gives the height 

 of the water-level above a fixed mark every instant. 



