462 



NATURE 



[iMarc/i 17, 1SS7 



LETTERS TO THE EDITOR 

 [The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake to 

 return, or to correspond 7vith the writers of, rejected manu- 

 scripts. No notice is taken of anonymous communications. 



The Earthquake 



■\VrrH refeience to the earthquake which occurred on the 

 morning of February 23 last, it may be of interest to inform you 

 that two of the magnetic registers of the Royal Observatory, 

 Greenwich, entirely confirm the fact shown by the Kew hori- 

 zontal-force register (Nature, March 3, p. 421), of the shoclv 

 having been sensible in England. The particulars are as 

 follows : — 



At 5h. 3Sm., Greenwich civil time, the declination and hori- 

 zontal-force magnets were suddenly thrown into vibration by 

 some cause not magnetic, the extent of vibration being in the case 

 of declination 20' of arc, and in the case of horizontal force '004 

 of the whole horizontal force. Other smaller vibrations will be 

 observed, on the annexed co]iy of the Royal Observatory photo- 

 4« 5" 6" 7" 



Copy of the photographic registers of the declination and horizontal-force 

 magnets, as recorded at the Royal Observatory, Greenwich, 1887, 

 February 23. 



graph, as occurring in declination at about 6h. om., and in hori- 

 zontal force at about Sh. 45™., 7h. 40m., and 7h. 50m. 

 respectively. No motions of this character were shown in 

 the vertical- force magnetic register, the two earth-current 

 registers, or in any of the meteorological registers. 



It may be mentioned that the declination magnet is a bar 

 2 feet long, suspended by a single thread about 6 feet long, and 

 stands in the magnetic meridian, and that the horizontal-force 

 magnet, also 2 feet long, has a bifilar suspension, the threads, 

 about 74 feet long, being twisted horizontally to cause the 

 magnet to stand at right angles to the magnetic meridian. The 

 time of vibration of the declination magnet is 24 seconds, and 

 that of the horizontal-force magnet 21 seconds. The magnetic 

 declination at Greenwich at the present time is about 17° 53' 

 west. W. H. M. Christie 



Royal Observatory, Greenwich, March 10 



The Engineer on the Dimensions of Physical Quantities 

 In a brief book-notice [ante, p. 387) I commented on the 

 grave error of measuring potential energy in terms of horse-power, 

 comparing it with the allied absurdity of measuring distance in 

 terms of speed. 1 also cited the following pasiage : — 



"dividing 3,942,400 foot-pounds per minute by 33,000 foot- 

 pounds, we get 1 19 '4 horse-power" ; 

 and I put beside it the allied absurdity : — 



" dividing 500/. a year by 50/., we get 10/. a year." 

 I thought it superfluous to point out the nature of the mistake, 

 but 1 judged rashly. For the Engineer (in a leader, of March 4, 

 1887) has made a somewhat e.xcited attack on this and other of 

 my statements : — remarking 



"we are in doubt whether 'P. G. T.' really has any idea 

 what (sic) the expression means." 



To this charge I plead guilty. For if I were myself to divide 

 3,942,400 foot-pounds per minute by 33,000 foot-pounds, the 

 result would contain the unit of time alone ; and could certainly 

 not express horse power. It might be angular velocity perhaps. 

 It is tj-ue that if I were to divide 3,942,400 foot-pounds per 

 minute liy the mere number 33,000, 1 should probably obtain 

 the result 1 19 '4 horse power. But the Engineer will ascribe 



all this to the pedantry of the "professor," for its article goes 

 e\en farther in absurdity than does the passage quoted above. 

 It leaves out the " per minute" and says the author " is strictly 

 correct (sic) when he says that 3,942,400 foot-pounds are to be 

 divided by 33,000 foot-pounds to get the horse-power " ! Alas 

 for Fourier, and Dimensions of physical quantities ! ! 



I wonder what the Engineer would assign as the result of 

 dividing 10 eggs per minute by 2 eggs. Would it, or would it 

 not, be 5 eggs per minute? P. G. T. 



Tabasheer 



Mk. W. T. Thiselton Dyer's ingenious contribution on 

 Tabasheer in Nature (p. 396) will doubtless be interesting in 

 connection with the subject of the nature and mode of distribu- 

 tion of silica in vegetable bodies, in which it is so often 

 contained. 



Brenster, in 1819, says [Edinburgh Philosophical f oicrnal , n. 

 I, p. 147) : — " It is found in the joints of the female (?) bamboo, 

 sometimes in a fluid state like milk, sometimes with the con- 

 sistency of honey, but generally in the form of a hard concretion. 

 Some specimens of it are transparent, and resemble vei-y much 

 small fragments of the artificial pastes made in imitation of 

 opal ; others are exactly like chalk, while a third kind is of an 

 intermediate character, and has a slight degree of translucency. 



" In the year 1804, Messrs. Humboldt and Blonpland brought 

 with them from America some specimens of tabasheer, called 

 guaduiis butter by the Creoles, taken from the bamboos which 

 grow to the west of Pinchincha in the Cordilleras of the Andes 

 (Humboldt's 'Personal Narrative,' vol. i. Introd. p. xii. note). 

 These specimens were analysed in 1805, by Messrs. Fourcroy 

 and Vauquelin (MJmo'res de I'Institut, torn. vi. p. 382), who 

 found them to be difierent from the tabasheers of Asia. Instead 

 of being wholly composed of silex, they contained only 70 per 

 cent, of -this earth, and 30 per cent, of potash, lime, and 

 water. " 



Cohn speaks of two kinds of tabasheer, viz. crude and 

 calicified. The former consists of roundly-angular pieces of 

 unequal sizes, possessing all degrees between transparency and 

 opaqueness, and passing from brownish, reddish, yellowish, or 

 dark gray to black in colour ; the latter is opal-like, milky, or 

 pale in colour, not unlike a lump of sugar. Tabasheer can be 

 cut into pieces very easily, and shows, in polarised light, only 

 extremely feeble double reflection. 



Brewster, moreover, by studying the optical properties of 

 tabasheer, fonned one of the semi-transparent specimens, which 

 he obtained from Nagpore and Hyderabad, into a prism, and 

 found to his "great surprise that the refractive power of taba- 

 sheer was not only lower than water, but so much lower, as to 

 be almost intermediate between water and gases." The results 

 he obtained are as follows ; — 



Index of 

 refraction 



Air ... I-OOOO-I- 



Tabasheer from Hyderabad, yellowish by reflected 



light I'III5 



Tabasheer from Nagpore ... \ (i'l454 



,, ,, ,, harder ' bluish by re- ) I '1503 



,, ,, ,, ... ( fleeted light ) i'i535 



>, ,, ,, very hard J f 1-1825 



Water I '3358 



As to the chemical constituents of this substance, Poleck's 

 recent analysis (Bot. Ccnlralbl. Band xxix., 1887, p. 95) shows 

 that it contains 99 6 per cent, of pure silicic acid and only o'4 

 percent, of other mineral matters, as natrium, sulphuric acid, 

 &c., but neither potash nor phosphoric acid has been detected. 

 The crude specimens contain 58 per cent, of water ; the calicified 

 specimens, on the other hand, are free from water. 



We may perhaps dwell shortly upon the ha'nlat of tabasheer. 

 Mr. Dyer has fully shoun its occurrence in India ; now let us 

 consider whether it occurs further in the eastern pans of Asia, 

 as in China and Japan, where the growth of bamboos is still in 

 full vigour. In China, tabasheer is known as Tien-chii-hwang 

 or Chii-hwang, that is, the "yellow (substance) of bamboo" ; 

 or sometimes called Chti-kaou, or the " cerate of l)amboo." 

 " Pun-tsaou-kang-muh " says : " They are produced inside the stem 

 of banboos, and look somewhat like yellow earth ; they may 

 be often found attached in masses to the inside of the bamboo 

 cane." 



From the well-known old Japanese encyclopaedia, "Wakan 

 Sansai Dsiiye," the following descriptions may be quoted : 

 " After bamboos have been cut down in March or April, and 



