Retrospective Criticism, 303 



vesicles, or air-bags, like bubbles they rise to the highest part of the 

 surface, which we uniformly find the latter do in a glass of ale or other 

 liquor. I find that the animalcula first figured (fig. 52. a) in my notice 

 respecting the luminousness of the sea, in your Vol. IV. p. 285., is en- 

 graved in the tab. v. of Oken's Zoology from Slabber's Medusa marina, 

 tab. viii., fig. 4. and 5. It is named by Oken, Slabben'«, and the habitat 

 given is the North Sea. — Samuel Woodward. Norwich, Jan. 19. 1832. 



Anchor Frosts. — Sir, J. M., in his remarks on this subject (p. 91.), 

 appears to me to have fallen into several errors in his endeavour to account 

 for anchor frosts (they are called bottom frosts in Yorkshire). For, ad- 

 mitting that main springs are of the temperature stated (40*^), I am by 

 no means prepared to believe that they keep that temperature long, or 

 that the water issuing from them does not mingle intimately and imme- 

 diately with the water of the river into which it flows ; especially in the 

 situations where anchor frosts are most common, and which are rough 

 and rapid streams. From J. M.'s statement, it would appear that globules 

 of water of different temperatures mix together without the one impart- 

 ing its excess of caloric to the other, which is contrary to the expe- 

 rience of every one j it is true that in still places there will be different 

 temperatures in the same body of water, but it is not owing to the main 

 springs of which J. M. speaks, but to the peculiar way in which it is 

 affected by cold. It is well known that water increases in density down 

 to 40", at which temperature in begins to expand, and this increase of 

 volume continues to take place until it reaches the freezing point, so that 

 in severe frosts there will be strata of different temperatures from 32^ to 40°. 

 Again, he says that " the crystals of ice are intercepted by the interstices 

 of the stones, and then become heaped together in thick beds j " but, if 

 my observations are correct, these depositions always begin first round the 

 large stones, which are not likely to stop small spicula any more than are 

 the water-gates of mills, where, he says, the accumulations also take 

 place. 



Anchor frosts are most common in the rapid streams occurring below 

 deeps in rivers ; and T have seen a wear on the river Wharfe (Yorkshire), 

 which had a wall of ice 4 ft. high, formed upon it in a single night, by a 

 sharp north wind. In my opinion a sufficient reason for these frosts is to be 

 found in the fact, that water, when kept from agitation, may be cooled 

 down below the freezing point without being congealed ; but if the vessel 

 in which it is kept be shaken, a portion of it is converted into a porous 

 spongy ice, like that formed by anchor frosts, and the temperature imme- 

 diately rises to 32°, In the deeps of rivers the same cooling below the 

 freezing point takes place without congelation, but as soon as this water 

 reaches the stream below, the agitation immediately converts a portion of 

 it into ice, which collects round the large stones at the bottom, in the 

 same way that crystallisation connnences in a solution of salt or sugar, 

 around a piece of thread, or other substance, which may be suspended in 

 it. If a severe frost is followed by a bright day, thousands of these detached 

 pieces of spongy ice may be seen rising from stones which have served as 

 nuclei for them ; which proves that the detention of them at the bottom 

 is not merely mechanical, but that precipitation (if I may be allowed to 

 call it so) takes place in the first instance, the stone serving as a nucleus, 

 and that this adhesion is destroyed by the action of the sun's rays. 



I have never seen any attempt to explain the phenomenon of bottom 

 frosts before this of J. M.'s, and I am not philosopher enough to speak 

 positively on the subject ; but the above is the way in which I have 

 always endeavoured to account for it. Perhaps some of your scientific 

 readers may be able to give much better reasons for it than have been 

 offered either by J. M. or myself. I am, Sir, yours, &c. — T. G. Clitheroe, 

 Lancashire, Jan. 17. 1832. 



