494 



NATURE 



{Oct. 7, 1875 



extended by the enthusiastic local collector. The number 

 of species described as undoubtedly British is one hundred 

 and twenty, including the slugs, which, "though generally 

 regarded as shell- less, have the shell placed beneath the 

 mantle." 



A Manual of the MoUusca. By S. P. Woodward. Third 



Edition. (London : Lockwood and Co., 1875.) 

 In noticing a third edition of the late Mr. S. P Wood- 

 ward's well-known " Manual of the MoUusca," our object 

 is only to indicate wherein it differs from its predecessors. 

 The body of the work is unaltered ; whilst the new editor, 

 Mr. Ralph Tate, in order to bring the work up to the 

 present state of our knowledge, has added an appendix, 

 containing the description of those recent and fossil genera 

 which, either from more recent discovery or oversight, are 

 not to be found in it. This appendix, with its separate 

 index, occupies eighty- five pages, and is illustrated with 

 twenty-seven woodcuts, including drawings of Clydonites 

 costatus, Cochloceras Jischeri (Hauer), Eucychis goniatus 

 (Desl), Niicleospira ventricosa (Hall), &c. Its separate 

 existence we do not object to, on account of the expen- 

 sive typography of a work of the kind ; nevertheless, the 

 outlay involved in an incorporation of the two indexes 

 into a single whole would have been fully made up for by 

 the extra faciUty of reference afforded, and the diminu- 

 tion in the chance of any additional remarks on previously 

 described genus being overlooked. In the preface to the 

 second edition, which is retained in that under notice, it 

 is remarked that " the chapter on Tunicata has been 

 omitted, since they aie more nearly allied to the Polozoa 

 than to the MoUusca proper, and since the MoUuscoidan 

 group would have made the work inconveniently bulky." 

 Such being the case, we cannot help asking why the 

 Brachiopoda are not also removed. I^ it not because 

 they have shells, whUst the Ascidians are deficient in in- 

 destructible parts ; not, by the way, that Ascidians are 

 MoUuscoidan now-a-days. Additional remarks will be 

 found on the nature of Belcmnites ; that Crioceras must 

 merge into Ancyiocei'as is shown to be certain ; the genera 

 Vermetus and Siliquaria are placed in a family by them- 

 selves, at the same time that their differences from the 

 mimetic SerpilidcE are explained. Severe! of the fami- 

 lies are re-arranged, at the same time that the newly added 

 genera are introduced. The work with the appendix is 

 as accurate a representation of the state of conchology in 

 1871 as was the first edition on its publication. We put 

 it thus because we can find no difference between this 

 third edition and the second, which has latterly been 

 been bound up with Mr. Tate's appendix in exactly the 

 same form as it appears in the newly produced work. 



LETTERS TO THE EDITOR 



[The Editor does not hold himself responsible for opinions expressed 

 by his correspondents. Neither can he undertake to return, 

 or to correspond with the writers of, rejected manuscripts. 

 No notice is taken of anonymous communications. "X 



Oceanic Circulation 



I HAVE just read Dr. Carpenter's letter in Nature (vol. xii. 

 p. 454) in reference to my paper on the Challenger'' s crucial lest 

 of the wind and gravitation theories of oceanic circulation, read 

 before the British Association, and am somewhat astonished at 

 the nature of the objections which he advances. 



"The doctrine," says Dr. Carpenter, " to which he (Mr. CroU) 

 applied his test, was not mine, but a creation of his own. For 

 his whole argument was based on the assumption that the ocean 

 is in a state of static equilibrium ; whereas the theory I advocate 

 is, that the ocean never is and never can be in a state of equi- 

 librium, so long as one part of it is subjected to polar cold and 

 another to equatorial heat, but that it is in a state of constant 

 endeavour to recover the equilibrium which is as constantly being 

 disturbed." 



Those who were present at the meeting and heard my paper 

 read, or who have since seen it in the September number of the 



Philosophical Magazine, will no doubt feel surprised that the fol- 

 lowing paragraph should have escaped Dr. Carpenter's notice : — 

 " It will not do as an objection to assert that according to the 

 gravitation theory the ocean never attains to a condition of static 

 equilibrium. This is perfectly true, as I have shown on a former 

 occasion ;* but then it is the equator that is kept below and the 

 poles above the level of equilibrium; consequently the disturbance 

 of equilibrium between the equatorial and polar columns would 

 actually tend to make the difference of level between the equator 

 and the Atlantic greater than 3^ feet, and not less, as the objec- 

 tion would imply." 



If Dr. Carpenter will refer to my examination of the mechanics 

 of the gravitation theory in the Philosophical Magazine for October 

 1871, "Climate and Time," chaps, ix., x., alluded to in the 

 above paragraph, he will find page after page devoted to prove 

 that a constant disturbance both of Icz'el and of static equilibrium 

 is a necessary condition to circulation by gravity. Physicists 

 may differ from me in regard to whether or not the present differ- 

 ence of temperature between the ocean in equatorial and polar 

 regions is sufficient to produce circulation, but I do not expect 

 that anyone familiar with mechanics, xoho has been at the trouble 

 to read what I have zuritten on the subject, will do so materially 

 in regard to the way in which difference of temperature is con- 

 ceived to produce motion. 



It is singular that Dr. Carpenter should not have observed 

 that his objection strengthens my argument instead of weakening 

 it. For if it be true that the equatorial column, though in a 

 state of constant upward motion, never attains to the height 

 required to balance the polar column, then it must follow as a 

 necessary consequence that the rise from the equator to latitude 

 38° in North Atlantic must be greater than I have estimated it 

 to be ; and, therefore, so much the more impossible is it that 

 there can be any surface flow from the equator to the pole due 

 to gravity. 



The next objection is as follows : — " The only objection raised 

 by Mr. CroU which has even a show of validity is based on the 

 supposed ' viscosity ' of water, which he asserts to be sufficient 

 to prevent' the disturbance of thermal equilibrium from exerting 

 the effect which the gravitation theory attributes to it." 



What possible connection can "viscosity " have with the 

 crucial test argument ? Suppose water to be a perfect fluid and 

 absolutely frictionless : this would not in any way enable it io plow 

 tip-hill. 



The crucial test argument brings the question at issue, in so 

 far as the North Atlantic is concerned, within very narrow limits. 

 The point at issue is now simply this : Does it follow, or does it 

 not, from the tempa-attire- soundings given in Dr. Carpenter' s 

 own scciiott, that the North Atlantic at lat. 38° is above the level 

 of the equator? If he or anyone else will prove that it does 

 not, I shall at once abandon the crucial test argument and 

 acknowledge my mistake ; but if they fail to do this, I submit 

 that they ought at least in all fairness to admit that in so far as 

 the North Atlantic is concerned, the gravitation theory is unte- 

 nable. 



The Atlantic column is lengthened by heat no less than eight 

 feet above what it would otherwise be were the water of the 

 uniform temperature of 32° F., whereas the equatorial column is 

 lengthened only four feet six inches. The expansion of the 

 Atlantic column below the level of the bottom of the equatorial 

 not being, of course, taken into account. How then is it pos- 

 sible that the equatorial column can be above the level of the 

 Atlantic column ? And if not, let it be explained how a surface- 

 flow from the equator pole-wards, resulting from gravity, is to 

 be obtained. James Croll 



Edinburgh, Sept. 29 



Dehiscence of Collomia grandiflora 



The following account of some observations of mine on the 

 dehiscence of Collo7?iia grandiflorarazy possibly prove interesting 

 to some of your botanical readers. I can find no allusion to the 

 singular mode in which the capsules as well as the seeds of this 

 plant become liberated. The fruit is a three-celled capsule, and 

 is almost wholly included within the tube of the cal>x. When 

 quite ripe it is of a pale straw colour, and becomes cartilaginous 

 and highly polished, as does also the internal surface of the calyx 

 tube. The latter is ribbed with fifteen prominent lines disposed 

 in threes, each set pertaining respectively to the five sepals, and 

 extending into their free portions. These ridges may possibly 

 help to give direction to the capsule during its exit. Dehiscence 

 ■* Phil. Mag., Oct. 1871 ; "Climate and Time ; " chap. ix. 



