258 rori'LAR LECTURES AND ADDRESSES. 



highest position while the moon is rising to maxi- 

 mum altitude, and fall again after the moon crosses 

 the meridian till she sets ; and it will rise and fall 

 again through the same range from moonset to 

 moonrise. If the earth were perfectly rigid, and if 

 the locality is in latitude 45, the rise and fall 

 would be half a millimetre on each side of the 

 mean level, or a little short of half a millimetre 

 if the place is within 10 north or south of latitude 

 45. If the air were so absolutely quiescent during 

 the observations as to give no varying differential 

 pressure on the two water-surfaces to the amount 

 of , J millimetre of water or T4 Vu of mercury, the 

 observation would be satisfactorily practicable, as 

 it would not be difficult by aid of a microscope to 

 observe the rise and fall of the water in the smaller 

 cistern to -,-J^- of a millimetre ; but no such 

 quiescence of the atmosphere could be expected at 

 any time ; and it is probable that the variations of 

 the water-level due to difference of the barometric 

 pressure at the two ends would, in all ordinary 

 weather, quite overpower the small effect of the 

 lunar tide-generating motive. If, however, the two 



