300 Of the Propagation of Heat 



on the ice was at the temperature of 190 deg., 542 grains 

 only of ice were melted in 30 minutes ; whereas in the 

 next experiment (No. 42), when the water was at 41°, or 

 149 degrees colder, 573 grains were melted in the same 

 time. 



Finding that covering' up the jar with a thick and warm 

 covering of cotton caused more ice to be melted by the 

 hot water, I was curious to see what effects would be pro- 

 duced by keeping the jar plunged quite up to its brim in a 

 mixture of snow and water, instead of merely surround- 

 ing that part of it which was occupied by the cake of ice 

 by this cold mixture. 



I was likewise desirous of finding out — and, if pos- 

 sible, at the same time — whether water at a temperature 

 something above that at which that Fluid ceases to be con- 

 densed with cold would not melt more ice in any given 

 time than an equal quantity of that Fluid, either colder 

 or much hotter. The result of the 43d experiment had 

 shewn me, — what indeed a very simple computation 

 would have pointed out, — namely, that, when the tem- 

 perature of the water is but a few degrees above the point 

 of freezing, if its quantity or depth is not very consider- 

 able, it will soon be so much cooled as very sensibly to 

 retard the process of melting the ice ; and with respect 

 to hot water, the increased quantity of ice which was 

 melted by it when the jar was covered up with a warm 

 covering convinced me that the real cause which pre- 

 vented the hot water from melting as much ice as the 

 cold water in my experiments was the embarrassments 

 in the process of melting the ice, which were occasioned 

 by the descending currents formed in the hot water on 

 its being cooled by the. air at its surface, and by the sides 

 of the jar. 



