232 HYDROGRAPHICAL SURVEYING [chap. ix. 



water be observed at the following times, reckoned fiom the 

 coniniencement : 



" The observations may be regarded as forming three groups 

 of three each, the member of each group being separated by 

 eiglit hours solar or lunar, Avhile one group is separated from the 

 next by eight hours lunar or solar. In the mean of the nine 

 results the lunar and solar semidiurnal and diurnal inequalities 

 are all four eliminated. 



" Nine is the smallest number of observations which can 

 form a complete series. If the solar day be divided into m 

 and the lunar into n equal parts, where m and n must both be 

 greater than 2, there will be m" observations in the series ; 

 and if either m or ii be a multiple of 3, or of a larger number, 

 the whole series may be divided into two or more series having 

 no observation in common, and each complete in itself. The 

 accuracy of the method can thus be tested, by comparing the 

 means obtained from the separate sub-series of which the whole 

 is made up. 



" Should the ship's stay not permit of the employment of 

 the above method, a very fair determination may be made in 

 less than a day, by taking the mean of n observations taken 

 at intervals of the nth. part of a lunar day, n being greater 

 than 2. Thus if w = 3, these observations require a total 

 interval of time amounting to only sixteen hours thirty-two 

 minutes. The theoretical error of this method is very small, 

 and the result thus obtained is decidedly to be preferred to the 

 mere mean of the heights at high and low water. 



" The mean level thus determined is subject to metero- 

 logical influences, and it would be desirable, should there be an 

 opportunity, to redetermine it at the same place at a different 

 time of year. Should a regular series of observations for a 

 fortnight be instituted, it would be superfluous to make an 

 independent determination of the mean sea-level by either of 

 tha above methods at the same time." 



Owing to diurnal inequality and other causes, principally 

 perturbations of atmospheric pressure and the effect of wind, 



