Dec. 30, [880] 



NATURE 



195 



the camera to any degree of altitude. These supplementary 

 boards are then > ivoted at the centre of part of a divided circle, 

 previously inlaid in the wood at the extremities of the liase line, 

 in such a manner that a line passing through the axis of the 

 lenses would cut the [ ivots. The cameras thus furnished can be 

 adjusted with ease to any vertical or horizontal angle. These 

 angular adjustments of the two instruments must always coincide, 

 with the slight exception that the horizontal ones must make 

 internal angles with the base included between them, or, in other 

 words, the lenses of both require to be directed to a point 

 opposite to the centre of the base line. 



The cameras also require their rapid exposing shutters to be 

 electrically connected, to ensure the pair of sensitive plates being 

 impressed at the same instant, and each dark slide employed to 

 have a fine w ire strained at its centre from top to bottom imme 

 diately in front of the prepared plate, and as close as possible to 

 it without touching. The transparent lines produced in the 

 developed negatives by these wires will constitute the zero of 

 distance of any pair, and during the operation of reading off 

 must be made to agree with similar ones on the scale of measure- 

 ments obtained as follows : — 



Upon a large cardboard rule a number of squares in fine black 

 lines, one inside the other, and each one slightly out of the 

 centre of its predecessor to the right hand, the out-ide square 

 being then divided w'vh. a line at a tenth part of its diameter to 

 the left of its centre. This line will indicate the zero of the 

 scale. After placing a distinguishing mark or number in the 

 corner of eery square for purposes of identification, the card- 

 board will be ready to be photographed and reduced at the same 

 time to the intended size of the cloud nega'ives. Two trans- 

 parent positives copied from this and observed when placed side 

 by side in a suitable stereoscope with the * dges representing the 

 left-hand one of the cardboard, together, will appear in that 

 instrument with the lines composing the zero only a few inches 

 away, and the squares as a succession of veriical planes com- 

 mencing some disiance from that and receding from the eye in 

 the order of greater to less, each one representing its own 

 distance in space. 



To find the value of these distances it will be necessary to 

 focus the two cameras upon some terrestrial objects whose dis- 

 tances can be measured by any of the known method^, and 

 negatives taken. The two resulting landscapes, when placed in 

 the stereosc 'pe, each superimposed face to face upon its respec- 

 tive scale, and the fine vertical lines of the whole made to 

 occupy one apparent distance, an operaiion offering but little 

 difficulty, every object or point of the landscape will be found to 

 stand out in the vertical plane suited to its own distance, the 

 relation between them being noted for the values found by 

 measurement of the one to be marked upon the other. As a 

 scale prepared thus would be of no value for any other angle at 

 w hich the cameras might be placed, it would be most convenient 

 to make use of two or three angles only, m ire being quite un- 

 necessary, and prepare a scale for each, or one « ith a reference 

 table of values for the respective angles would suffice. Again, 

 iii respect of altitudes. As the terrestrial measurements would 

 only be absolutely accurate for those of clouds in the zenith, or 

 of them, if it were possible, from the earth's centre in any 

 direction, the tables of reference would have to include calcu- 

 lated corrcciions for altitude, or the graduations could be valued 

 for the m St useful degrees by experimental means. 



It will be gathered from the above that the constancy of length 

 of the I ase line can be ascertained, and corrected if necessary, 

 by taking a couple of views of the same landscape for compai i- 

 son with the preceding pair ; slight lluctua'ions of length would 

 not how ever be of much consequence in dealing with the com- 

 paratively coarse meaurements of thick masses of cloud floating 

 in so short a distance as the few miles of atmosphere capable of 

 forming them consists. 



To ascertain the height of clouds photograph a pair of nega- 

 tives, and place these in the stereoscope with a pair of scale 

 plates agreeing with the angle at which they were taken, and 

 adjust as for the landscapes described above. The data required 

 may then be read off by noting the vertical plai.e each stratum 

 occupies. 



Prints of these negatives should afterwards be made for the 

 particulars of height, direction of motion of the re-pective layers, 

 point of compass, wind rate, state of barometer, thermometer, 

 and general lem-arks upon the weather, to be recorded upon them 

 for comparison or ciiculatioD. 



Meteorological [ob.-ervatories fitted with such an addition to 

 their pesent splendid collection of instruments would have their 



powers of dealing with the atmosphere ar.d weather changes 

 greatly reinforced. John Harmer 



Wick, near Arundel 



Correction of an Error in "Island Life" 



My friend Dr. Giinther has kindly called my attention to an 

 extraordinary error at ]"). 322-323 of my "Island Life," where 

 I state that the Loch Kdlin Charr (Salmo Killineiius) inhabits a 

 lake in Mayo County, Ireland ; instead of a small lak<? in 

 Inverness-shire, 2000 feet above the level of the sea, as given in 

 Dr. Giintber's original description in the Procc<:din^s of the 

 Zoological Society, 1865, p. 698. On referring to my MSS. notes 

 for this p.art of my work, I find that the habitat w as first correctly 

 given, but subsequently scored out and altered to the erroneous 

 Irish locality ! \Vhy this was done 1 cannot now discover; and 

 I can only regret that I should have fallen into so palpable an 

 error, and request such of the readers of Naturis'. as possess my 

 book to make the necessary alterations. 



Alfred R. Wallace 



Natural Science for Women 

 Will you allow me to supplement your kindly reference to 

 the instruction in physical science given to women in Bedford 

 College, London, V>y the statements that for the last two sessions 

 a class in hioloL'y has 1 een conducted there by Mr. Charles 

 Stewart of St. Thomas's Hospital Medical School. The course 

 of study is in every sense a practical one, with special reference 

 to th: Preliminary Scien'ific and First B.Sc. examinations at 

 the University of London, ami the best testimonial to the excel- 

 lence of the iastru(jti> n in these various subjects is furnished by 

 the remarkable success during the present year of the Bedford 

 College pupils at the University examinations, a success not less 

 marked in the .Science than in the Arts examinations. 



Alfred W. Bennett 



Movements of Leaves 



A year ago we had in our conservatory a healthy young plant 

 of Acacia iiioHissima. It bore no flowers, but consisted of a 

 simple axis adorned with the soft feathery leaves of its genus, 

 which closed up at night. Our gardener however thought it 

 would improve in appearance if it could be made to bear a few 

 branches; and with that view he cut it back. His end was 

 achieved : a new stem shot up from the section, and graceful 

 limbs were thrown out in turn^by it. But along with this a 

 strange result followed : the fresh leaves borne by the new stem 

 and by the branches no.v closed at night, while the old leaves 

 below the section ceased to do so. These lower leaves have 

 long since fallen off, tut the upper ones kej.t to their habit, and 

 at the preent time all fold up at dusk save a few of the very 

 oldest, which only partially shut, or, in one case, do not shut at 

 all. When our plant w.as cut back it stood three feet high ; now 

 it stands stven: which shows that the vig. ur of the plant as a 

 M hcle in no wise diminished by the operation. 



Chislehurst, December 23 M. L. Rouse 



ON DUST, FOGS, AND CLOUDS' 



DUST, fogs, and clouds seem to have but little connec- 

 tion with each other, and we might think they could 

 be better treated of under two separate and distinct heads. 

 Yet I think we shall presently see that they are more 

 closely related than might at first sight appear, and that 

 dust is the germ of which fogs and clouds are the 

 developed phenomena. 



This was illustrated by an experiment In which steam 

 was mixed with air in two large glass receivers ; the one 

 receiver was filled with common air, the other with air 

 which had been carefully passed through a cotton-wool 

 filter and all dust removed from it. In the unfiltered air 

 the steam gave the usual and well-known cloudy form ot 

 condensation, while in the filtered air no cloudiness what- 

 ever appeared. The air remained supersaturated and 

 perfectly transparent. 



The difference in the behaviour of the steam in these 

 two cases was explained by corresponding phenomena, 



' Abstract c f a paper read to the Roy.il Society of Edinburgh, December 

 20, by Mr. Juhn Aitken. tiiinishea 10 Nat uuli by the C./uncil of the 



