§ 441, 442. THE SPECIFIC GRAVITY OF THE SEA, ETd. 



219 



at the poles, of course, the freezing point of sea water. It will be 

 very readily understood that in this statement there is nothing- 

 repugnant to hydrostatical laws, the compressibility of water in- 

 suring an increase of density in descending within much wider 

 limits of temperature than here contemplated." 



44:1. The temperature of 39°.o was assigned to this floor prob- 

 Thermai dilatation ^^^7 ^^i^cr the suppositiou that sca water follows 

 of the water. fy^g]^ ^^ ^^^ j^^^g of thermal dilatation. ISTot so: 



while fresh water attains its maximum density at 39°.5, average 

 sea water does not arrive at its degree of maximum density until 

 it passes its freezing point (27°. 2) and reaches the temperature of 

 25°. 6. In the winter of 1858 a very elaborate series of observa- 

 tions was conducted at the National Observatory, by Professor 

 Hubbard, upon the thermal dilatation of sea water, and with the 

 following results, 60° being the standard temperature : 



Thermal Dilatation of Sea Water* 



442. The dilatation of the glass tube is included in this table. 

 Experiments on the ^o determine thc freezing point of average sea wa- 

 freezing point. ^^^^ j ^y\q^ a glass jar 18 inches high, and 8 inches 



imum density of sea water was at the same temperature as that of fresh, but it is 

 some 12° or 14° lower. 



* This agrees more nearly with Despretz (p. 245) than with Dr. Marcet. The 

 latter states that sea water decreases in weight to the freezing point until actually 

 congealed. In four experiments Dr. Marcet cooled sea water down to between 18° 

 and 19" Fahr., and found that it decreased in bulk till it reached 22", after which 

 it expanded a little, and continued to do so till the fluid was reduced to between 19'' 

 and 18°, when it suddenly expanded, and became ice with a temperature of 28°. 

 It should always be recollected that a saturated solution of common salt does not 

 become solid, or converted into ice, at a less temperature than 4" Fahr. ; and, there- 

 fore, if the sea should be, as is sometimes supposed, more saline at great depths, 

 and as it appears to be in the MediteiTanean from the experiments of Dr. Wollaston, 

 ice could not be formed there at the same temperature as it could nearer the surface. 

 —iVide M. de la Beche, Manual Geology, p. 22.) 



