70 



SEC. 3. MEASUREMENT. 



The apparatus for the determinations of the mean height of the sea is 

 explained by the following remarks : 



It is the question how to define the mean height of the water for a certain 

 period. 



This height would be the arithmetical mean between high and low water, 

 supposing a regular form of the tide-curves. In fact, the real observed tide- 

 curves differ very much from this theoretically-defined regular form (curve of 

 sines). To show this the tide-curves of Cuxhaven, Southampton, and Ipswich 

 will suffice as examples. The irregularities in these curves are evident and 

 easily to be seen in figure 2, where the curve of sines is drawn for the purpose. 



MEAN 

 MITT 



SOUTHAMPTON. f 6 IPSWICH. 



Fig. 2. 



If h be the difference between high and low water, the mean height is 

 for Cuxhaven 0*527^, for Southampton 0'567/J, and for Ipswich 0'47lA 

 instead of 5 h, the amount of the mean height, supposing a regular form of 

 the curve. The mean height of the sea is one of the most important result* 

 of the tide observations. It is the only datum by which to define the invaria- 

 bility, or the measure of the variation, of height of the continent and islands. 



The line of mean height acgi must be in such a position that the areas 

 abc and gfi (figure 2) together equal the area edge. This is to be found by a 

 tedious calculation of areas of the tide-curves drawn on the cylinder L. 



The apparatus here described gives the requisite data for an easy deter- 

 mination of the mean height by means of two rollers ( JJ) (one of them con- 



