1879.] Mr. E. Roberts. On a new Tide-Predicter. 199 



ponents sufficient to predict the Indian Ocean tides "with all the 

 accuracy necessary for practical pnr poses. 



The present machine is the ontcome of the recommendation. 



The instrument combines the following twenty tide-components : — 



The mean lunar semidiurnal ; 



The first and second overtides of the mean lunar semidiurnal ; 



Two elliptic lunar semidiurnal ; 



Two evectional lunar semidiurnal; 



One variational lunar semidiurnal ; 



The mean solar diurnal ; 



The mean solar semidiurnal ; 



The lunisolar semidiurnal ; 



The lunisolar diurnal ; 



The lunar diurnal ; 



The solar diurnal ; 



One lunisolar elliptic diurnal ; 



One lunar elliptic diurnal ; 



One compound (Helmholtz) lunisolar semidiurnal ; 

 One compound (Helmholtz) lunisolar quarter- diurnal ; 

 The solar annual ; and 

 The solar semiannual. 



Strictly speaking, there is no sensible astronomical tide-component of 

 twenty-four mean solar hours' period, but for the purposes of prediction 

 it is necessary to include such a term, a very regular and sensible result 

 of this period being obtained in the analysis, due probably to wind or 

 temperature. The same remark applies partially to the solar annual 

 and the solar semiannual, the theoretical tides of these periods being 

 very small ; the analysed results, however, are of considerable value, 

 being due probably to the effect of rainfall and the regularity of the 

 monsoons. These three components should, therefore, be regarded 

 more as meteorological than astronomical. 



The stipulation that the scale for heights should be one inch per foot 

 range for Bombay necessitated a recording barrel of some 18 inches 

 at least ; the actual length adopted for the instrument, however, is 

 22 inches. The delineation of the curves on such a large scale ren- 

 dered necessary some modification of the system of eccentric pulleys, 

 as fitted on the tide-predicter of the British Association. It was at 

 first contemplated to fit parallel slides only to the larger of the tidal 

 components ; the whole of them have, however, been so provided. 



The chief difficulty in the construction of the machine is the finding, 

 within reasonable limits, of proportions which shall represent with 

 sufficient accuracy the periods of the several components, in order that 

 the machine may be used for a considerable period of prediction — say, 

 for twelve months. Very great success has been attained in this 

 respect in the present instrument. For instance, the error of the 



p2 



