FARQUH ARSON ON FORMATION OF GROUND ICE. 351 



These quotations will suffice to show the general nature of the phenom- 

 ena observed. In addition to the above, tvi^o rather irregular effects of 

 the ground ice were observed. 



At times during the winter, the water as delivered by the hydrants is 

 very muddy. This occurs when there has been no rise or freshet, the river 

 being generally low, and at its clearest stage, and is the result of the 

 lower projecting parts of the spongy masses, when laden with sand and 

 mud coming in contact with the rough line of the conduit in the river 

 bottom, and there depositing their muddy freight. Again, when the 

 floating ice is very abundant in the river, the ferry boat meeting it, has 

 her hull so incrusted with this spongy ice, which becomes compacted or 

 balled by impact, that her draught of water is increased by several feet, 

 and to reach the usual landing-place on the Illinois side, she has to run 

 off and scrape herself where the bottom is hard, as the whale is said to 

 rid itself of barnacles and other obstructive parasites. 



The phenomenon of the formation of ground ice or bottom ice, or 

 anchor ice, always in running water, has been observed in many parts of 

 the world, and was for many years quite a puzzle, or at least a perplex- 

 ing problem in physics. By general consent it is now explained in ac- 

 cordance with the well known properties of water, and the varying 

 changes of density it undergoes in passing from the fluid to the solid 

 state. 



Fresh water attains it maximum density at 4 'centigrade, or39.'3oF., 

 above or below this, as it is warmer or colder, it bf^comes lighter ; water 

 at 4*^ centigrade being 1000, ice has a specific gravity of 920. Again, if 

 you expose water in a metalic vessel, a common iron pot, to a freez- 

 ing temperature, say to our winter air when the thermometer is below 

 20« F., ice to a certain thickness will form on the surface, and 

 along the sides and bottom of the containing vessel, but, a very long ex- 

 posure, or a very reduced temperature is necessary to freeze the mass of 

 water solid ; if however, by any means, the water be so stirred as to be 

 thoroughly mixed, the whole mass will become solid ice in a compara- 

 tively short time. This is also observed in the familiar process of mak- 

 ing ice-cream or water ices, when the crust of ice forming at the outside 

 must be constantly scraped off, and the whole mixed by motion, in order 

 to freeze the mass. 



The formation of ground-ice in our great natural freezer or ice-ma- 

 chine, will be better understood after a consideration of the nature of 

 the Rock Island Rapids, the description of which is made much clearer 

 by an inspection of the accompanying cartoon, giving a sketch or plan 

 of the rapids, for which I am indebted to the courtesy of Col. Flagler, 

 commandant of the U. S. Arsenal, and to the kindness of our associate, 

 Mr. W. Otto Gronen, who has made a perfectly accurate copy from the 

 official surveys as made by the U. S. Government. 



