THE TIDES. 177 



out from the complicated curve traced by the tide- 

 gauge the simplest harmonic elements. A simple 

 harmonic motion may be imagined as that of a 

 body which moves simply up and down in a 

 straight line, keeping level with the end of a clock 

 hand, moving uniformly round. The exceedingly 

 complicated motion that we have in the tides is 

 analysed into a scries of simple harmonic motions 

 in different periods and with different amplitudes 

 or ranges ; and these simple harmonic constituents 

 added together give the complicated tides. 



All the work hitherto done has been accom- 

 plished by sheer calculation ; but calculation of 

 so methodical a kind that a machine ought to 

 be found to do it. The Tidal Harmonic Analyser 

 consists of an application of Professor James 

 Thomson's disk-globe-and-cylinder integrator to 

 the evaluation of the integrals required for the 

 harmonic analysis. The principle of the machine 

 and the essential details are fully described and 

 explained in papers communicated by Professor 

 James Thomson and the author to the Royal 

 Society, in 1876 and 1878, and published in the 

 VOL. in. N 



