THE TIDES. (APP. /?.) 213 



that the angular velocities 7 <r -f- 5 and 7 cr & 

 are so nearly equal, that observations through 

 several years must be combined to distinguish the 

 two corresponding elliptic diurnal tides. Thus 

 the whole number of constituents to be determined 

 by one year's observation is twenty. The forty 

 constants specifying these twenty constituents are 

 probably each determinable, with considerable 

 accuracy, from the data afforded in the course of a 

 year by a good self-registering tide-gauge, or from 

 accurate personal observations taken at equal short 

 intervals of time, hourly for instance. Each lunar 

 declinational tide varies from a minimum to a 

 maximum, and back to a minimum, every nineteen 

 years or thereabouts (the period of revolution of the 

 line of nodes of the moon's orbit). Observations 

 continued for nineteen years will give the amount 

 of this variation with considerable accuracy, and 

 from it the proportion of the effect due to the 

 moon will be distinguished from that due to the sun. 

 It is possible that thus a somewhat accurate 

 evaluation of the moon's mass may be arrived at. 



4. The methods of reduction hitherto adopted, 1 

 after the example set by Laplace and Lubbock, 

 have consisted chiefly, or altogether, in averaging 



1 See Directions for Reducing Tidal Observations ; by Staff- Com- 

 mander Burdwood, London, 1865, published by the Admiralty ; 

 also Professor Haughton on the " Solar and Lunar Diurnal Tides 

 on the Coast of Ireland," Transactions of the Royal Irish Academy 

 f^r April, 1854. 



