'Retrospective Criticism, 395 



variety of Pontk brdssicoe, and P. Metm but a variety of P. rapae, should 

 ever be read collaterally with Mr. Rennie's article (Vol. II. p. 224.), which 

 excited them. No figure of P. Napae'ae occurs in that article (Vol. II. 

 p. 227.), which Mr. Bree regrets (Vol. III. p. 245.). — J^. D. 



Insect Monstrosities. — In recording (Vol. IV. p. 476.) the case of an 

 E'later murinus found with one of its antennae three-branched, the fact of 

 a Chlanius vestitus having been found with a supernumerary appendage 

 to the fourth joint of one of its tarsi, as figured and described in Vol. II. 

 p. 302., is cited as a somewhat parallel instance. Two still more remarkable 

 cases of insect monstrosity, as mentioned by Mr. Dale (Vol. IV. p. 21.), 

 should have been added to this citation. — J,D. 



London Fogs. (p. 304.) — To J. M.'s useful remarks on this subject, it 

 may be well to register the following supplementary ones : they are 

 transcribed from an ably written, and very recently published, pamphlet, 

 entitled, " Z)r. Weatherhead's Account of the Beulah Saline Spa^at Norwoody 

 Surrey. ^^ The site of the Beulah Saline Spa is the village of Norwood, 

 seven miles south of London, which stands on one of those elevations 

 known as the Norwood Hills. Dr. Weatherhead remarks : — " From trigo- 

 nometrical observation, it has been computed that the height of these hills 

 is about 390 ft. above the level of the sea at low water. By accurate ob- 

 servation of the height of the fog, relatively with the higher edifices whose 

 elevation is known, it has been ascertained that the fogs of London never 

 rise more than from 200 to 240 ft. above the same level." [The level of 

 the sea, not that of Norwood, as appears by the second sentence following.] 

 " In some instances, the line of demarcation between the pure air and the 

 fog is distinctly defined ; on other occasions, the latter dissipates itself so 

 gradually into the superincumbent atmosphere as to show no line of 

 separation. Thus placed above the fogs of the plain, and removed from 

 the smoky and contaminated atmosphere of the metropolis, the air [of 

 Norwood, and the neighbourhood of the Beulah Saline Spa, is meant] has 

 long been celebrated for its pure and invigorating qualities." — J. D. 



Anchor Frosts. — Since reading the articles by J. M., p. 91., and T. G., 

 p. 303., on this phenomenon, I have met with the following, which I beg 

 to hand to you : — " It is a curious particular in the natural history of the 

 Thames, that it always freezes just at the bottom : this habit is often found 

 to prevail among rivers in Germany, particularly in the northern parts ; 

 but is asserted by the writer of the article Ice, in the Encyclojjcsdia 

 BritannicUy never to be met with in the more temperate of the European 

 climates. The fact is assuredly otherwise : the congelation of the river 

 Thames uniformly commences in the lowest places. The mass then formed 

 rises, on a rude calculation, to about the middle of the water, where it pre- 

 sents, as on the streams of Germany, a resemblance to the partial consoli- 

 dation of nuclei or small hail. A second mass then forms at the bottom j 

 the central mass rises to the surface, and the new bottom, or ground ice, 

 takes its place, and gradually mounts to the superior fabric, with which it 

 speedily assimilates. Dr. Plott accounts for this circumstance, by suppos- 

 ing that the water of the Thames is more abundantly impregnated with salt 

 than that of other English rivers ; and that, as salt naturally sinks to the 

 bottom, and as naturally inclines to a state of congelation, the formation 

 of ice consequently takes place first at the greatest depth." {Faulkner's 

 Chelsea, p. 19, 20.^ 



Anchor Frosts, (p. 91.) — On the subject of anchor frosts, it is merely a 

 long and severe one, wherein large masses of ice are frozen to the stones 

 and gravel at the bottom of rapid streams ; not, as your intelligent corre- 

 spondent J. M. states, by crystals of ice floating down the streams, and accu- 

 mulating amongst the stones ; but simply by the stones acquiring a degree 

 of cold far below the freezing point, and the water in contact with them 



