Mr. A. Tylor on Tides and Waves. 215 



being inversely as the square of the distance, the force at E will 

 be to that at W as 31 2 to 30 2 , or as 961 to 900 (that is, one 

 fifteenth greater) . This calculation ought to agree with obser- 

 vation at ports where the variation in height each alternate tide 

 is eliminated from other disturbances, and where there are no 

 exceptional circumstances, if this is a correct explanation of the 

 difference between the height of the diurnal and semidiurnal 

 tides (which I term the near-side and far-side tides). 



The luni-solar attraction-rays in passing through the earth 

 may encounter changes from fluid to solid substances having 

 surfaces not at right angles to the incident rays ; and the rays 

 would not then follow straight lines, although I have for con- 

 venience represented them as straight in fig. 4, PI. IV. 



Those rays passing through the higher parallels of latitude 

 far from the centre might affect the tides apparently in an irre- 

 gular manner. These changes of direction might explain why, 

 in order to predict with great accuracy the height and time of 

 the tide at some stations, Sir W. Thomson and Mr. E. Roberts 

 have been obliged to employ twenty-seven fictitious stars instead 

 of only the number to express the effects of the moon and sun's 

 various positions. 



The different currents that occur, causing different establish- 

 ments at ports near each other, seem to indicate movements of 

 masses of water apparently at different angles to each other. 

 These motions can be illustrated by an experiment in the injector. 

 In the water-pipe, at right angles to the body of the injector 

 (where steam is at 101 lbs. pressure), there is a partial vacuum, 

 say equal to 2 inches of water. I find that the motion of the 

 steam will increase its own pressure 1 lb. by friction against the 

 metal instrument. The steam travelling with great velocity de- 

 flects the water-current and bends it into its own direction, and 

 forces water into the steam-boiler, where the pressure is 100 lbs. 

 The water-pipe is all the time open to the atmosphere and to 

 the boiler two ways through the injector; but little steam 

 escapes through the open water-pipe. The barometer is another 

 instance. The column of mercury ought to lengthen if that 

 instrument registered the absolute weight of the atmosphere 

 alone, when the column of air is loaded with vapour. The mo- 

 tion of the vapour in the act of condensing, however, generates 

 currents and produces motion of particles in a direction across 

 the column. This reduces the pressure of the column on the 

 cistern of the barometer; and therefore the column shortens 

 for motion instead of lengthening for weight. Motion in 

 main water-pipes reduces pressure in branches where there is 

 no motion. 



Currents in motion in different directions, owing to different 

 temperatures or other causes, affect the tidal currents materially, 



