-1859] WRITINGS OF JOSEPH HENRY. 199 



exposed to the air and the thickness of the ice produced 

 measured each day. From what has been said it is evident 

 — first, that the vessel should be made of wood or some other 

 non-conducting substance, in order that the freezing may not 

 take place at the sides; and second, that the water should be 

 always of the same depth; for if there be two vessels of the 

 same diameter, one containing more water than the other, the 

 thickness of ice formed in the two will be different, unless the 

 fluid in both is at the temperature of thirty-two degrees at the 

 commencement of the exposure. If we would ascertain more 

 accurately the measure of effect, the ice must be broken and 

 its thickness measured or the amount weighed very carefully 

 every day, for if we suffer it to accumulate we shall have a 

 less result, since the first coat tends to screen the water, so 

 that with the same temperature the process goes on more 

 slowly. This method is very simple, and when properly 

 employed furnishes reliable data for determining the relative 

 intensity of different winters. By simply measuring the 

 thickness on a lake or pond from year to year we may ap- 

 proximately arrive at a similar result. But as we have said 

 the upper stratum screens the lower ones, and a knowledge 

 of this fact has been taken advantage of in some parts of 

 New England to increase the quantity of the ice for econom- 

 ical purposes. To this end water is suffered to flow over a 

 surface of ice already frozen, and thus by frequently repeat- 

 ing the operation a much greater aggregate thickness of ice 

 is produced. Ice made in this way is more porous however 

 and contains more air than that formed by ordinary freezing, 

 since all the air evolved from the strata after the first must 

 be retained by the next below. 



The more solid the ice, the longer it will resist thawing; 

 first, because it contains more water under a given external 

 surface, and second, because a portion of radiant heat is 

 always absorbed at any surface, whether it be external or 

 internal ; for example, if we expose a piece of ice containing 

 a bubble of air to a source of radiant heat, we shall find that 

 the bubble will gradually enlarge, thus proving an inter- 

 nal melting to be going on. In the preservation of ice for 



