PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 383 



Dr. Vandervveyde compared it to the deposit of dew. The 

 bottom of the stream radiates its heat through the water, becom- 

 ing colder than the water, and at last cold enough to freeze the 

 •water in contact with it. 



Mr. Smith had observed the phenomenon only in rapidly flow- 

 ing streams, and liis own explanation would be that the ice crys- 

 tals, formed on the surface, are washed away by the current, 

 M'hile the rocks upon the bottom form centers of crystalization 

 to which the particles of ice can attach themselves. 



Dr. Yanderweyde said that might assist in the operation. 



Mr. Dibben explained the formation of ice at the bottom of 

 running water, under certain circumstances, in another way. 

 For instance, in New York bay, there is an incoming tide for six 

 hours, of salt water, of a temperature perhaps as low as 25 .deg,, 

 cooling the surface of the earth below ; then the tide turns, and 

 comparatively fresh water conies down over that very earth, and 

 is consequently frozen. It is wortliy of mention that ice, in a 

 still place, is more brittle than that where the water is in motion. 

 After the surface is covered the particles of water cannot move 

 freely, and the crystals will be differently formed. It is very 

 probable that the last formed crystals thaw first. 



Dr. Stevens related the disastrous effects resulting from 

 anchor or ground ice in Southern New York, where it is the 

 general opinion that anchor ice never forms where there is a 

 large amount of snow on the ground. The ground seems to be 

 colder than the stream. 



The subject of "Ice and Methods of Refrigeration," was 

 continued. 



The Association adjourned to meet on Wednesday evening 

 next at a quarter before eight o'clock. 



American Institute, Polytechnic Association, 



June 5, 1861. 

 Prof. Cyrus Mason in the chair. 



CAR-BRAKE. 



On motion of ifr. Yeeder, the committee appointed March 6th, 

 on a new car-brake, Avas discharged from the further considera- 

 tion of the subject. 



