NATURAL PHILOSOPHY. 171 



forces caused by the irregularity both of the directions and sttength of the 

 winds. The losses already mentioned, occasioned by violent storms, extended 

 in part to these spring-tides. We were fortunate enough, however, to obtain 

 good quarter-hourly observations for as many as twenty-four of these semi- 

 diurnal spring-tides. The mean result from them is shown in the preceding 

 Table (III.), page 170. 



Here we have, again, thirty minutes after the time of the moon's meridian 

 passage as the time of high water at the period of lunar spring-tides; and 

 we have two hundred and fifty-four thousandths of a foot, equal to 3.408 

 inches, United States measure, as the height of the lunar spring-tidal wave 

 at its summit. 



In accordance with custom, in like cases, we indicate as the established 

 mean for the port of Chicago, 



i Foot, Oh. 30m. 



Although this indication may be but of small practical advantage to navi- 

 gators, yet it may serve as a memorandum of a physical phenomenon whose 

 existence has very generally, heretofore, been either denied or doubted. 



We think it probable that if the effects of unfavorable winds, and all other 

 extraneous forces which produce irregular oscillations in the elevation of the 

 lake smface, could be fully eliminated, a semi-diurnal lunar spring-tide would 

 be shown as great as one-third of a foot, or four inches, for the periods of 

 highest tides. 



The time of low water, and the relative times of duration of the flood and 

 ebb tides, are given only approximately. The extreme rise of the tide being 

 so little, the precise time of the change from ebb to flood, and hence the 

 duration of the flow of each, can only be accurately determined by numer- 

 ous observations at short intervals of time, say three to five minutes apart, 

 from about an hour before to an hour after the turn of the tide from ebb 

 to flood. 



In conclusion, we offer the foregoing observations as solving the problem 

 in question, and as proving the existence of a semi-diurnal lunar tidal-wave 

 on Lake Michigan, and consequently on the other great fresh-water lakes of 

 North America, whose coordinate of altitude at its summit is as much as .15 

 to .254 of a foot, or from 1.8 to 3.048 inches, U. S. measure. 



ON THE PHENOMENON OF WAVES ON THE SURFACE OF MERCURY. 



Nothing can be more interesting than the rippling of water under certain 

 circumstances. By the action of interference its surface is sometimes 

 shivered into the most beautiful mosaic, shifting and trembling as if with a 

 kind of visible music. When the tide advances over a sea-beach on a calm 

 and sunny day, and its tiny ripples enter at various points the clear, shallow 

 pools which the preceding tide had left behind, the little wavelets run and 

 climb and cross each other, and thus form a lovely chasing, which has its 

 counterpart in the lines of light converged by the ripples upon the sand 

 underneath. When waves are skilfully generated in a vessel of mercury, 

 and a strong light reflected from the surface of the metal is received upon a 

 screen, the most beautiful effects may be observed. The shape of the vessel 

 determines, in part, the character of the figures produced; in a circular dish 

 of mercury, for example, a disturbance at the centre propagates itself in 



