230 HYDROGRAPHICAL SURVEYING [chap. ix. 



This will of course be, as all attempts at arriving at anything 

 with insufficient data, only an approximation, but will probably 

 be near enough for the purposes we want. If we intend great 

 accuracy, we shall make arrangements to have the tide care- 

 fully observed throughout the day, whenever sounding is 

 going on, and take every precaution not to be reduced to these 

 straits. 

 Table of When tides are found to be regular, a table of reduction 

 Reduction may be formed from the observations during one or more 



under '^ . ... 



Certain Complete lunations, by tabulating the tides according to the 



Circum- i[i^q of moon's upper transit. Such a table may be very useful 



when the scale of the chart is small ; and sounding can be carried 



on under these circumstances, viz., regular tides, and scale of 



chart small, when no direct observations can be got. 



In many places external circumstances control our wishes. 

 For instance, it was found on the East Coast of Africa that if 

 men were landed to make a regular series of day and night 

 observations on the tides, fever generally ensued, and conse- 

 quently the record was restricted to day tides. Again, the 

 tide-pole may have to be so placed on a shelving shore, or 

 among reefs, that a boat would have to be used to go out at 

 high water to read it, and this may not be convenient. Often, 

 for considerable tracts among reefs, observations may be im- 

 possible, and a table deduced, as above suggested, from former 

 observations may be then used. 

 Graphic 1^1 forming such a table, it is best to project the tidal curves 

 ^J^®f*^°° as shown in Fig. 62, but by the hourly observations. It will 

 Move- then be seen whether the tides are regular, and days of similar 

 ment. time of upper transit of the moon can be compared, to see 

 whether a table will give us the reduction near enough for 

 practical purposes. 



Surveying ships are now supplied with abstract forms for 

 the projection of high and low waters in such a manner that 

 the regularity or otherwise of a tide can at once be seen. 



They provide for the record of the lunitidal interval, the 

 moon's meridian passage : the declination of the sun and moon, 

 apogee and perigee, and the mean time of the high water 

 following the superior transit, and of the highest tide in the 

 twenty-four hours. 



A horizontal line in the upper part of the diagram is divided 



