444 



NATURE 



[March i r, 1880 



Turning now to Dundee, observations there show that the 

 barometer fell very fast till about 7 P.M., after which it remained 

 nearly stationary for about two hours; at 7.15 the Tay Bridge 

 was blown down ; about 7.45 the actual trough of the cyclone 

 passed over the town, and about 9.30 p.m. the barometer began 

 to rise. The wind, which had been strong all day, rose to a strong 

 gale with violent gusts and squalls at 5 p.m., and lasted till S.30, 

 when the weather began to moderate. 



Thus it would appear that in this storm at Dundee, as is often 

 the case, the worst weather occurred just before the barometer 

 ceased to fall, and during the two hours it remained nearly 

 stationary previously to rising rapidly. The Tay Bridge was 

 blown down by an ordinary gust during this time, and not in 

 any squall during the time of the actual passage of the cyclone's 

 trough. 



On the whole it may be said that though the storm which 

 d stroyed the Tay Bridge was in many ways of the most ordinary 

 character, it was exceptionally squally and gusty, doubtless 

 owing to the unusually rapid rate of its motion. 



One word in conclusion, as to the destructive effect of wind. 

 A gust strikes with a blow, which can no more be calculated 

 from the velocity of the wind, than the blow of a sledge-hammer 

 can be estimated by a pressure in tons, or by the energy of so 

 much momentum. But observation also shows that in squalls 

 and gusts there is a great deal of local compression of the air ; 

 fluid pressure must then come into play, and in this we have, 

 probably, the explanation of the remarkable lifting power of 

 wind, which has been so often described in great storms. Un- 

 fortunately in our present state of knowledge, this lifting force is 

 as incapable of numerical estimation as the lateral blow of the 

 gusts. 



My acknowledgments are due to Dr. Copeland for his courtesy 

 in furni-hing me with copies of the meteorological records of the 

 Dunecht Observatory, situated about fifty-six miles north-north- 

 east of Dundee. Ralph Abercromby 



7, Royal Terrace, Folkestone 



A Lecture Experiment on Ice-Ctystals 



Though different processes are at command on the lecturer's 

 fable to illustrate the artificial formation of ice, none of them 

 may lie said to yield the very forms of ice-crystals that are 

 observed in sno\ -figures or in hoar-frost. I have hit upon a 

 method for producing them in an equally simple and satisfactory 

 way. If a glass tube (with a length of one or two decim. and 

 four or five millim. wide) has by means of the blowpipe one of 

 its ends reduced to a diameter of two millim., some fibrous 

 matter, as loose cotton-wool or gun-cotton, &c, must be intro- 

 duced into this part, in such a way that many single fibres protrude 

 out of the tube. They form the lower part when the tube is 

 now fixed in a vertical position, and some sulphuric ether 

 ped in through the upper end in sufficient quantity to keep 

 the fibrous substance moist, but not enough to run over. 

 An active evaporation favoured by a comparatively large 

 surface and the radiation from a multitude of points, sets in 

 immediately, and within a minute ice-cry&tals, as a deposit from 

 the atmospheric moisture, are seen growing in all directions on 

 the fibres, imitating exactly the snow-figiues. If very small 

 quantities of ether are now continually supplied, a group of 

 crystals and needles, sometimes to a length of two centim., 

 is readily obtained, affording, when projected on a screen, a very 

 elegant experiment which is rapidly going on and is succe^ful 

 even at a surrounding temperature of 13° C. 



The way by which these crystals are here obtained may eluci- 

 date the question on the formation of the ice-crystals observed 

 by the Duke of Argyll and recently discussed in NATURE. I 

 think that the ligneous substance, from its rotten condition, pre- 

 sents an innumerable quantity of very thin fibres, cooling after 

 sunset rapidly by radiation, and their surfaces, getting to a tem- 

 perature beneath the freezing-point, cause the vapour of water, 

 with which the surrounding atmosphere becomes now surcharged, 

 to be slowly deposited in the crystallised form exactly as in the 

 above experiment. The crystals, ending in needles and sharp 

 points, continue to cool by radiation, and therefore increase 

 at their extremities, till their length is sufficient to have gravity 

 exerting its influence in curling them round the bark. 



The Hague, Holland, March L. Bleekroi>e 



Cloud Classification 



Those who have long taken an interest in the subject of the 

 classification of the clouds, will heartily congratulate themselves 



that this study is again resuming a fair share of the attention of 

 meteorologists, and is likely to be more fully discussed than it 

 has been for many years. 



Luke Howard's Classification was, as far as he knew at the 

 time, a first attempt to introduce order into fields of observation, 

 then almost untried by scientific men. No one could suppose 

 that it would at once exhaust the whole subject and be incapable 

 of either extension or modification by later observers who possess 

 the advantage of a much more mature stage of the science of 

 meteorology. Still I may be allowed, without prejudging the 

 result of the present discussion, to suggest one or two practical 

 cautions to those who may be taking the subject in hand. 



Firstly — Luke Howard's nomenclature of clouds has, since his 

 time, been passed from hand to hand by a great number of 

 observers, many of whom have apparently never taken much 

 trouble to ascertain what he really intended to define by certain 

 names, or what were the principles on which the classification 

 was based. Therefore, before too readily finding fault either 

 with the names or the original application of them, it might be 

 well to give somewhat thoughtful study to the very carefully 

 worded descriptions and definitions in Luke Howard's own work 

 on the subject. 1 



Secondly — Clouds are by their very nature liable to frequent 

 changes from one class to another, during which they must pass, 

 more or less rapidly, through intermediate forms. If an 

 attempt is made to classify all these temporary and intermediate 

 varieties, the science will become rather unusually complicated. 

 Were the same principle carried out in other branches of obser- 

 vation we should, for example, have to classify the tadpole as an 

 important separate variety of the batrachians. 



In conclusion I may remind the observer of the advice of 

 Goethe in his remarks about Howard's nomenclature 8 — which 

 advice is as applicable now as when it was fir.^t written — "Not 

 to allow himself to be led astray by the occurrence of certain 

 indistinct appearances, but to practice himself in referring the 

 same to the main rules (or classes) under which they come." 



Walthamstow, March Eliot Howard 



Diatoms in the London Clay 

 Since you were good enough to allow me to announce in 

 Naturf, the discovery of diatoms in the London clay, I have 

 been able to trace the band in which they occur throughout the 

 whole extent of the London clay in East Kent — and at one spot 

 in Mid-Kent. In continuing the inquiry, with vol. iv. of the 

 Memoirs of the Geological Survey for my guide, I have found that 

 sections that were visible when that valuable work was published 

 are now overgrown or have been removed. Under these circum- 

 stances will you allow me to ask your readers for information as 

 to places where tolerably fresh sections of the lower part of the 

 London clay can be seen, especially at or near the northern and 

 the western outcrop of the formation ? 



As regards the eastern part of Kent, the investigation i-. 

 complete, and therefore no correspondence need take place 

 respecting sections in that district. 



Also I should like to have information as to any wells in 

 course of being sunk in any part of the London basin. 



1 may observe that I have invariably found these fossil diatoms 

 only in clay of a uniform dark slate colour, that dries out dan. 

 grey, and has a tendency to lamination. 



On splitting open a fresh piece of clay, the diatoms, if present, 

 are easily seen with the help of a pocket lens, as shining specks, 

 and if plentiful their metallic lustre is evident to the unassisted 

 eye. W. II. Shrubsole 



Meteor 



Yesterday evening, when observing the zodiacal light, i;i 

 order to get its limits among the stars, I remarked a fire-ball in 

 the same direction, which may have been perceived also in 

 England, where it was seen, perhaps, near the zenith. I give 

 you the elements of my observation to be published in your 

 journal. 



Mean local time 7h. 20m. evening, March 3. 



Direction of the apparent path from the width of the arc 

 M - £., Ceti towards the width of the arc y Ceti - a Ptscium. 



The beginning was very small, but towards the end the 

 brightness increased very quickly, and the phenomenon ended 



' F.ssriy on the Modifications of Ouds, by Luke Howard, F.R.S. 1 bird 

 edition, with plates. (Churchill, 1S65.) 



2 Quoted in preface to third edition of the essay, &c. 



