March 27, 1879] 



NATURE 



481 



on the one hand, and Scrope and Lyell on the other, 

 Etna supplied so crucial a test. Nor can we regard the 

 few notes of Mr. Rutley on the microscopic characters of 

 several specimens brought to him by the author, excellent 

 as they are in themselves, as affording anything like an 

 adequate discussion of the nature of the Etnean lavas. 



There are not, indeed, wanting indications in the work 

 before us that the author has scarcely succeeded in so far 

 mastering the scientific questions connected with his sub- 

 ject as to qualify himself for giving an}'thing like authorita- 

 tive opinions concerning them. Thus on page iii we find 

 him speaking of a crater as " composed of a prehistoric 

 grey labradorite, and of doleritic lava." Again, so far as 

 can be gathered from the work before us, the hypotheses 

 of elevation craters and eruption craters are of about 

 equal value. We are informed simply that "the opinion 

 of geologists is divided as to the manner in which a 

 volcano is formed ; " and then follows'a statement of the 

 two rival hypotheses. Surely after the convincing reason- 

 ing of Scrope, and the patient observations of Lyell on 

 Etna itself, as detailed in the celebrated memoir read 

 before the Royal Society in 1858, it is strange to find such 

 language used upon the subject, more especially when 

 we recollect that no attempt was ever made by Lyell's 

 opponents to discredit his observations or to reply to his 

 deductions. We should almost as soon expect to read in 

 a modem work on astronomy that the opinion of astrono- 

 mers is divided as to whether the earth moves round the 

 sun or the sun round the earth. 



We find so much to praise in this little book, especially 

 in the clear resjime of the history of the mountain and its 

 eruptions, and the illustrations so carefully selected and 

 reduced from those of larger works which are not easily 

 accessible to general readers, that we regret we cannot 

 express more unqualified approbation of that portion of 

 the book which calls for especial notice in the pages of 

 this journal. We can only hope that in a second edition 

 the author may find an opportunity, which he will not 

 neglect, of considerably lengthening and very greatly 

 strengthening this scientific portion of his work ; and in 

 order to do so, without at the same time impairing its 

 popular character, we can scarcely suggest a better ex- 

 ample for him to follow than the work of Prof. Phillips, 

 to which we have alluded at the commencement of this 

 article. 



LETTERS TO THE EDITOR 

 [ The Editor does not hold himself responsible for opinions extrressed 

 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. 

 [ The Editor urgently requests correspondents to keep their letters as 

 short as possible. 7he pressure on his space is so great that it 

 is impossible otherwise to ensure the appearance even of com' 

 munications containing interesting and novel facts.] 



The Trans-Neptunian Planet 



The explanation given by Prof. Peters (Jstr. Nach., 2,240) 

 i the observations made at Washington in 1850 of this sup- 

 })osed planet is put beyond doubt by the examination of Mr. 

 Ferguson's observing-book. It is due, however, to Mr. Fer^son 

 to =ay that his record is full and complete, and that his changes 

 in the reductions were honestly made. The record is in pencil, 

 and no figures were erased or rubbed out. They are crossed out, 

 and the assumed figure is put by the side of the original one, 

 while at the bottom of the page is a note with pen and ink, and 



in Mr. Ferguson's handwriting, stating the changes that were 

 made. Prof. Peters's ingenious discovery of the truth was made 

 without knowledge of the observing-book. 



Such criticisms are instructive, showing how unsafe it is to 

 build theories before we are sure of the facts. They may also 

 be a means of avoiding a waste of labour. It is kno^vn to me 

 that at least two American astronomers, armed with powerful 

 telescopes, have been searching quite recently for a trans- 

 Neptunian planet. These searches have been caused by the fact 

 that Prof. Newcomb's tables of Uranus and Neptime already 

 begin to differ from observation. In this connection the 

 note of Mr. Dunkin on the errors of Leverrier's tables of 

 SatiUTi is interesting. But are we to infer from these errors of 

 the planetary tables the existence of a trans-Neptunian planet? 

 It is possible that such a planet may exist, but the probability 

 is, I think, that the differences are caused by errors in the 

 theories of these planets. My observations of the satellites of 

 Saturn are not yet discussed, but they indicate that Bessel's mass 

 of Saturn is nearly correct. Now Lererrier has diminished this 

 mass by about i^th, and it seems probable that this diminution 

 was caused by some error in his theories of Jupiter and Saturn. 



A few years ago the remark was frequently made that the 

 labours of astronomers on the solar system were finished, and 

 that henceforth they could tiuni their whole attention to sidereal 

 astronomy. To-day we have the lunar theory in a very dis- 

 couraging condition, and the theories of Mercury, Jupiter, 

 Saturn, Uranus, and Neptune, all in need of revision ; unless, 

 indeed, Leverrier's theories of the last two planets shall stand 

 the test of observation. But after all, such a condition of 

 things is only the natural result of long and accurate series of 

 observations which make evident the small inequalities in the 

 motions, and bring to light the errors of theory. 



Washington, March 7 Asaph Hall 



Rats and Water-Casks 



Mr. Nicols says, in Nature, vol. xix. p. 433 : — 

 " A ship's carpenter told me that, in the old days, before the use 

 of iron tanks on board ship became general, the rats used to attack 

 the water-casks, cutting the stave so thin that they could suck the 

 water through the wood without actually making a hole in it. If 

 any one conld substantiate this it would have an important 

 bearing on the question under consideration." 



Capt. Wickham, when First Lieutenant on board H.M.S. 

 Beagle, told me that when he was a midshipman it was his duty, 

 on one of the king's ships to see that certain vessels on deck 

 were always kept full of water, in order to prevent the rats 

 gnawing holes through the water casks, and that through such 

 holes nearly all the water in a cask would leak away. 



Charles Darwin 



Tides at Chepstow 



I observe two letters in Nature lately upon this sub- 

 ject. Many years ago I took some pains to ascertain the 

 greatest known rise of tide at Chepstow, for I doubted the 

 accuracy of the common statement that it was seventy feet and 

 upwards. At the time I made the inquiry the large railway bridge 

 at Chepstow to carry the South Wales Railway across the River 

 Wye was being constructed. I was acquainted with Mr. Oak- 

 den, one of the engineers on the work, and he, with great care, 

 took levels of the marks which had been made from time to 

 time recording the very high tides, some of them going back 

 many years. He found the highest of them to be some decimal 

 (of which I have no record) above fifty feet above ordnance 

 datum. I think this may be relied upon. It is corroborated in 

 a paper by the present Astronomer- Royal, on "Tides and 

 Waves," in the "Encyclopaedia Metropolitana," vol. v. p. 242, 

 paragraph 7, first edition. He says: "Thus, at the entrance 

 of the Bristol Channel the whole rise at spring-tides is about 

 eighteen feet, at Swansea about thirty feet, and at Chepstow 

 about fifty feet." W. B. Cleg ram 



Saul Lodge, Gloucestershire, March 18 



Migration of Birds 



Prof. Newton in his article on Migration of Birds (Nature, 

 vol. xix. p. 433) has omitted one, and a ver>' important limit to 

 the height at which birds of passage can perform their journeys. 

 This is temperature. The follow ing table of Daniell's will show 



