PRESIDENT’S ADDRESS. 57 
represents, ther, the compound effect of all the different 
component simple waves. If we could separate out one of 
these components, and record it ona tide gauge, it would 
mark a wavy curve on the paper such that each wave would 
be an exact replica of the one before it. But, unfortunately, 
we cannot do this; neither can we calculate what should be 
the dimensions of any particular component at any place. 
It is possible, however, if we are in the possession of the 
records from a tide-gauge extending over a sufficiently long 
period, to disentangle the various components. and so de- 
termine their actual values and phases at the place from 
which the records are taken. The work involved in the 
analysis is laborious, but not difficult, and cuce the com- 
ponents have been well determined, we are in the position of 
being able to predict what their resultant effect will be at 
any future tune. The method furnishes us with by far the 
most satisfactory way of obtaining tidal predictions for any 
port, and for such purposes it is coming into very general 
use where systematic predictions are issued. !n addition, 
however, the accumulation of results of such analyses at 
different places is gradually furnishing us with data that 
will in the future enable the progress of the tidal wave over 
the earth’s waters to be studied in a very much more 
thorough way than it can be at present. 
Systematic work of this kind is now carried out by the 
Uniied Scates Geodetic Survey, the Indian Gevernment, the 
French Hydrographic Service, the Director of the Gbserva 
tory at Batavia (amongst the islands of the Hast Indian 
Archipelago), and, less systematically, at a number of other 
places. The published results are already considerable. 
Once the tidal components of a port have been determined 
the tidal curve forsa whole year in advance may be very 
quickly obtained from a tide-predicting machine. This is 
a mechanical contrivance for compounding simple harmonic 
waves, the form in use by the Indian Government being that 
invented by Lord Kelvin. The United States have a dif- 
ferent type, devised by Ferrel. These are both very ex- 
pensive instruments, costing, I believe, well over £2000, so 
that the construction of such apparatus would not be justifi- 
abie unless it is to be used for predictions at a great number 
of ports. The two machines mentioned could easily do all 
the tidal prediction required for the whole world. At 
Adelaide the cheap and simple form of tide-predictor, in- 
vented by Captain Inglis, and previously described before 
this Association, is doing efficient work. 
