ADDRESS SECTION A, B.A. 1876. 257 



45 above or below the horizon. When this 

 greatest value is reached, the plummet is drawn 

 from its mean position through a space equal to 

 TeWoirw f tne length of the thread. No ordinary 

 plummet or spirit-level could give any perceptible 

 indication whatever of this effect ; and to measure 

 its amount it would be necessary to be able to ob- 

 serve angles as small as T 2inroin7Tro f tne radian, or 

 about o^o"- ^ P resen t no apparatus exists within 

 small compass by which it could be done. A sub- 

 merged water-pipe of considerable length, say 12 

 kilometres, with its two ends turned up and open, 

 might answer. Suppose, for example, the tube to 

 lie north and south, and its two ends to open into 

 two small cisterns, one of them, the southern for 

 example, of half a decimetre diameter (to 

 escape disturbance from capillary attraction), and 

 the other of two or three decimetres diameter (so 

 as to throw nearly the whole rise and fall into the 

 smaller cistern). For simplicity, suppose the time 

 of observation to be when the moon's declination 

 is zero. The water in the smaller or southern 

 cistern will rise from its lowest position to its 

 VOL. II. S 



