January 2, 1896] 



NATURE 



199 



for a convcniciu lent;;th lo diviile into a thousand parts, did not 

 take up the already existing yard, which is nearly the same as 

 the metre. Yard ~ looo = (metre -^ looo) - -004 inch. 

 The amount of confusion that would have been saved is almost 

 incalculable. 



The metric system usually carries with it the Centigrade scale 

 on the thermometer, and here the whole English-speaking world 

 should give no uncertain sound. I am not contending for any 

 scale in chemistry or geology, but in meteorology it would be 

 •difficult to find a worse scale than the Centigrade. The plea 

 that we must have just 100° between the freezing and boiling 

 points does not hold ; any convenient number of degrees would 

 •do. The Centigrade degree (i°"8 F.) is just twice too large for 

 •ordinary studies. The worst difficulty, however, is in the use 

 of the Centigrade scale below freezing. Any one who has had 

 to study figures, half of which have minus signs before them, 

 knows the amount of labour involved. To average a column of 

 thirty figures, half of which are minus, takes nearly double time 

 that figures all on one side would take, and the liability to error 

 is more than twice as great I have found scores of errors in 

 foreign publications where the Centigrade scale was employed, 

 all due to this most inconvenient minus sign. If any one ever 

 gets a " bee in his bonnet " on this subject, and desires to make 

 the change on general principles, it is very much to be hoped 

 that he will write down a column of thirty figures half below 

 32° F. , then convert them to the Centigrade scale, and try to 

 average them. I am sure no English meteorologist who has 

 ■ever used the Centigrade scale, will ever desire to touch it. It 

 is much easier and safer to convert the Centigrade scale to the 

 Fahrenheit before any studies are made. As a matter of fact, Euro- 

 jiean temperatures are a sealed book to active workers in meteor- 

 ology on account of this unfortunate scale. 



England and the United States may congratulate themselves, 

 however, in the fact that European meteorology is almost flat. 

 There are no low area or high area systems moving with 

 regularity at any definite speed or in any definite direction, such 

 as are experienced in the United States three or four times in a 

 month. The rains in Europe are mostly sporadic, and do not 

 accompany any well-defined low areas. 



If any change is to be made, cannot the meteorological world 

 come together upon a thermometer having its zero at - 40° both 

 lahrenheit and Centigrade. This would be a most convenient 

 >cale, and would eliminate nearly all minus signs. The con- 

 \ ersions from the old to the new would be very convenient by 

 dimply adding 40° to each. The conversion from the new 

 I'ahrenheit to the new Centigrade would also be extremely con- 

 \ enient by dividing by i '8 instead of the present very puzzling 

 ->stem, and vice versa. Centigrade degrees -i- i "8 would give the 

 Fahrenheit value. This would obviate the difficulty of the 

 minus sign, but there would still remain the fact that the 

 Fahrenheit degree is by far better than the Centigrade. 



Washington, D.C., December 18, 1895. H- -^- Hazen. 



Apparatus for determining the Specific Gravity of 

 Minute Fragments of Minerals. 



This is a contrivance designed to facilitate the measurement 

 •of the specific gravities of minute fragments of minerals by 

 means of a diffiision column of methylene iodide, as described 

 by Prof. Sollas in Nature, vol. xliii. p. 404, and vol. xlix. 

 p. 211. There is no new principle involved in the construction 

 of the instrument. It is merely the adaptation to this particular 

 purpose of a well-known device for getting rid of the error due 

 lo parallax, such as is employed in Prof. Joly's spring balance 

 for determining specific gravities, and the best form of tangent 

 galvanometer. The index, which in this case is a fine wire, or 

 filjre of any kind, is reflected in a mirror placed behind it, the 

 coincidence of the fibre and its reflection showing that the eye 

 of the observer is exactly on a level with the index. 



In the drawing, a (Fig. l) is a test tube containing the 

 methylene iodide column in which the fragments of mineral 

 imder observation are immersed, together with two or more 

 beads or fragments of mineral whose specific gravity is known. 

 This is held by a clip, />, in a slot cut through the upper end of 

 a thin piece of wood, some 12 inches high, fixed vertically to a 

 firm base, so as to bring the test tube and its contents to a con- 

 venient height above the table. A slip of mirror, d, carrying a 

 scale, e, divided in millimetres or any even graduations, is 

 fixed vertically at the side of the slot, and the fibre is carried by 

 a slider, < , which is shown separately in Fig. 2. This may be 

 made of a strip of thin sheet copper, and has a " window" cut 



NO. 1366, VOL. 53] 



through it at /;, so that the contents of the test tube may be 

 .seen. 



The readings are taken by bringing the fibre successively on a 

 level with the approximate centre of gravity of each fragment, 



and noting the division of the scale cut by it, taking care that it 

 coincides with its reflection in the mirror at each reading. The 

 instrument might be improved by making the scale movable 

 through a short distance vertically, so that one of the larger 

 divisions of the scale might always be brought on a level with 



the uppermost of the beads or fragments u.sed as indicators. The 

 whole thing may be made at a cost of less than a shilling. It 

 has been tried in the geological laboratory of the Royal College 

 of Science for Ireland with satisfactory results. 



T. D. La Touche. 



Cactaceae in the Galapagos Islands. 



Mr. Hemsi.ev is mistaken in .stating, as he does on p, 623 ot 

 Nature for October 24, 1895, that Dr. G. Baur was attached to 

 the U.S. Fish Commission Steamer .-///^a/;wj-. Dr. Baur had no 

 connection with the Albatross Expedition of 1891, the object of 

 which was deep-sea dredging, and only included an incidental 

 visit of a few days to the Galapagos. Nor did Dr. Baur write the 

 general sketch of that expedition, or have anything to do with 

 the photographs which accompany it, as one might infer from 

 a subsequent note by Mr. Hemsley on the Cactaceje of the 

 Galapagos (Nature, November 14, 1895, p. 31). 



As regards the Cactaceje of those islands, I collected branches of 

 Oputitia and of Cereiis from Chatham and from Charles Islands ; 

 what became of those pieces I do not know, as they do not seem 

 to have reached the hands of Dr. J. N. Rose, who described 

 the plants collected at the Galapagos by the Albatross 

 Expedition. 



Mr. Hemsley will find that both Dr. Wolf and myself state that 

 Cereus grows to 20 feet in height. Dr. Wolf also called 

 attention to some striking differences he had noticed in the 

 specimens of Optiiitia and of Cereus he found on the different 

 island.s. 



In stating his position, botanically speaking, regarding Dr. 

 Baur's theory of the origin of the Galapagos, Mr. Hemsley 

 would surely not give us to understand that Opuutia and Cereus 

 are limited to Chili on the west coast of South America, as his 

 second note (p. 31) seems to imply. Alexander Agassiz. 



Museum of Comparative Zoology, Cambridge, Mass., 

 December 2, 1895. 



[Mr. Hemsley admitted, in Nature of November 28, 1895, 

 that he was in error in thinking Dr. B^ur was attached to the 

 U.S. Fish Commission Steamer Albatross. — Ed. Nature.] 



