DR HOPE ON THE MAXIMUM DENSITY OF SEA- WATER. 249 



retardation which occurs when the temperature is in the immediate vicinity of 

 that of the maximum density ; Avhich might, at first sight, suggest the startling 

 idea that water at that temperature becomes reluctant to part with its heat. 



I must here take the liberty of remai-king, that the experiments in my me- 

 moir in 1804, not only indicate the fact of the retardation, but also afford an ex- 

 planation of it. The detail of experiment 2d in that memoir, shews, that a ther- 

 mometer situated near the bottom of a cylindrical vessel eight and a half inches 

 deep and fom- and a half in diameter, filled with water of temperature 49°, and 

 immersed in cold water, fell from this degree to the 40th in thirty-eight minutes, 

 which, on an average, is four minutes and one-thu-d for each degree, but required 

 twenty minutes to descend to the 39th degree, or one degree more, after which 

 it fell three degrees, at the average rate of nine minutes for each degree. 



The first experiment recorded in the memoir, likewise indicates that a period 

 of retardation takes place in the acquisition of temperature by a thermometer, 

 the bulb of which reaches very nearly the bottom of a vessel containing water, 

 at temperature 39^°, which is the period when the anomaly in the relation of 

 water to heat terminates, and the water becomes obedient to the usual law. 



In the experiment alluded to, the same jar and quantity of water were em- 

 ployed as in the preceding experiment. 



The temperature of the water was 32°, and it was exposed to an atmosphere 

 of temperature 60° — 62°. The thermometer, situated within half an inch of the 

 bottom, rose from 32° to 38°, that is, gained 6° in fifty minutes ; it then remained 

 for twenty minutes with scarcely any perceptible rise. It afterwards ascended 

 for 6° uninterruptedly, at the average rate of 1° in thu-teen mihutes. We cannot 

 wonder that the occurrence of a remm'a or retardation in the rate of cooling or 

 heating of water, at a temperature coincident with that at which the strange al- 

 teration of water in relation to the effects of heat and cold upon its volume takes 

 place, should create surprise. At first sight, it seems to indicate that water, at 

 the moment when it deserts the general law of expansion by heat and contrac- 

 tion by cold, does, at the same instant, acquire a degree of reluctance either to 

 receive or part with heat. 



This, indeed, would be a very extraordinary circumstance, but there is no 

 occasion to have recourse to such a supposition. The remora, which appeared so 

 surprising to Mr Erman, admits of an easy explanation. It obviously proceeds 

 from the change in the direction of the currents in the water, accompanying the 

 change of law. 



When a vessel of water at 50° is exposed to a cold medium, the cooling wa- 

 ter descends to the bottom in consequence of its contraction, and a thermometer 

 placed near the bottom of the fluid will exhibit an uninterrupted fall of tempera- 

 tiu-e. As soon, however, as the water attains the 39^° it begins to expand, and 

 the cmrents in the water as it farther cools now ascend to the top, while the 



VOL. XIV. PART. I. I i 



