352 



DAVENPORT ACADEMY OF XATUKAL SCIENCES. 



1 



:{ 



Legend.— 0, Davt-nport Bridge; 1, Lower Chain; 2, Moliiie Chain; 3, Duck Creek Chain; 

 4, Winuebajjo Chain; a. surface of water; b, river bottom; liorizontal scale, 1-142,560; ver- 

 tical scale, 1-228. 



From this it appears that the volume of tlie Mississippi is precipitated 

 down an inclined plane, whose fall in 14 miles from LeClaire to Daven- 

 port is 25.74 feet, or at the rate of 1.84 feet per mile.* Not only is there 

 an inclined plane, but the channel is an exceedingly tortuous one, turn- 

 ing in almost all directions, and the bottoui again, so far from being 

 smooth enough to facilitate the descentof the falling water, is roughened 

 to the highest degree, by means of boulders and other detached masses 

 of rock, of crevices in the layer of rock forming the bottom, and of nu- 

 merous excavations and pot holes. Thus is formed the machine for the 

 mixing and churning process, and a very effectual one it is. 



At the head of the rapids the water is cooled by radiation and the 

 contact of the cold air below the freezing point, but long before the three 

 hours necessary for the passage of the rapids, indeed, in all probability 

 soon after the descent is begun, the whole mass of the water has by the 

 mixing process been reduced to the same temperature throughout, and 

 being thus on the point of freezing, needs but the slack water afforded 

 by the eddy of a boulder, or a pot hole to freeze instantly into a spongy 

 mass, including in its embrace all the small stones, sand, mud or other 

 sediment in the pot hole or eddy ; the mass thus formed becoming, even 

 with its included freight, lighter than a corresponding bulk of the sur- 

 rounding water, it must rise to the surface, where the action of the 

 waves and of the wind smooth off the upper surface, which is soon ren- 

 dered solid by radiation and contact with the colder air. 



That the soft ice forms under the lee, as it were of the stones, as well 

 as in the holes and hollows, the presence of the gravel and sand would 

 indicate ; for each stone or other obstruction to the current has on its 

 lower side a small delta of sand, gravel and mud, and it is just here that 

 the ground-ice forms, bearing off this sediment in its embrace. 



The Bibliography of this subject, at least of the books accessible to 

 me, is very meagre indeed, consisting of an article in the Smithsonian 

 Report of 18G6, being a translation from the "• Annales de Chimie et de 



* HaU'H Ceolojjical Survey of [owa. Vol. 1, p. 7. 



