March 30. 1899] 



NA TURE 



517 



conclusions of a practical kind. One of the most interesting 

 points which had to be worked out was the condition that a 

 machine could be ridden without holding the handles. Mr. 

 Whipple finds that there are four critical velocities connected 

 with the stability of the motion, which he calls Vj, Vj, V.), and 

 V4. For velocities greater than \\ the motion is unstable, but 

 may be rendered stable by a rider who turns the first wheel 

 towards the side on which he is falling, or moves his body away 

 from that side. The force he has to exert in the former oper- 

 ation is comparatively great, whereas the distance he has to 

 move his body in the latter case is small. For velocities be- 

 tween V, and V.J the motion is stable, even when the rider does 

 not move his body and makes no use of the handles. For 

 S'elocities less than V., the motion without hands is unstable, 

 but between Vj and V3 it is stable for a rider who moves his 

 body through a very small distance in the same direction as the 

 fall is carrying him. This distance is about 1/20 of the distance 

 he is moved by the swaying of the machine. For velocities 

 between V.j and V^ the motion is stable for a rider who keeps 

 the motion of the handles as small as possible. For velocities 

 below V4 a rider who combines the two methods, using both 

 his weight and his hands, may be successful. The balance for 

 such low velocities is not automatic, but is a feat which requires 

 conscious attention. Mr. Whipple, considering a typical 

 machine, obtains the following values in miles per hour : 

 \\ = 12-2, V, = 10-4, V3 = 8'5, V4 = 7-4. He considers that 

 practically V., is the most important factor in determining the 

 ease of riding, but unfortunately its calculation for any given 

 machine is not easy. In connection with the effects of spinning 

 friction, it is pointed out that a well-inflated tyre is conducive to 

 stability. Mr. Whipple's paper appears in the Quarterly foiirnal 

 of Pure and Applied Mathematics for March. 



A NEW method of photographing in natural colours is reported 

 by Sciiiiic to have been discovered by Prof. R. W. Wood, of 

 the University of Wisconsin. The colours are said to be ob- 

 tained by diffraction ; and, though at present the production of 

 ihe first finished picture is somewhat tedious, duplicates can be 

 printed as easily as ordinary photographs are made. The 

 pictures are on glass, and are not only colourless, but almost in- 

 visible when viewed in ordinary lights ; but when placed in a 

 viewing apparatus, consisting of a convex lens on a light frame, 

 show the colours of nature with great brilliancy. The principle 

 is that the picture and the lens form spectra which overlap, and 

 the eye placed in the overlapping portion sees the different 

 portions of the picture in colour depending on the distance be- 

 tween the grating lines at that place. Prof. Wood says the 

 finished picture is a transparent film of gelatine with very fine 

 lines on it, about 2000 to the inch on the average. The colours 

 depend sflely on the spacing between the lines, and are pure 

 spectrum colours, or mixtures of such, the necessity of coloured 

 screens or pigments, used in all other processes except that of 

 Lippmann, having been overcome. The pictures can be pro- 

 jected on a screen by employing a suitable lantern, or can be 

 viewed individually with a very simple piece of apparatus con- 

 sisting of a lens and perforated screen mounted on a frame. It 

 is difficult to form an opinion upon the method or results from 

 the information so far available, and we hope that further details 

 will soon be published. 



It is well known that aneroids are not to be depended upon 

 for the determination of altitudes. When an aneroid and a 

 mercurial barometer are subjected to a diminishing pressure, 

 brought about either by increase of altitude or experimentally 

 by means of an air-pump, it is found to indicate a lower reading 

 than that shown by the mercurial barometer. The lower the 

 pressure, and the greater the length of time the diminution of 

 pressure is experienced, the greater is the loss in any individual 

 NO. 1535, VOL. 59] 



aneroid. As the extent of the loss of an aneroid subjected to 

 diminished pressure depends upon the length of time during 

 which the instrument is exposed to this pressure, eviden'ly a 

 way to remedy the defect is to obtain an instrument which can 

 be put in action when required to make a determination of 

 pressure, and put out of gear or thrown out of action when not 

 wanted for use. An aneroid which fulfils these conditions has 

 been invented by Colonel H. Watkin, C. B., and is manufactured 

 by Mr. James J. Hicks. The instrument has precisely the 

 same appearance as an ordinary aneroid, and the only addition 

 is a fly-nut at the back, by means of which the vacuum-box can 

 be put in action when a reading is required. This aneroid has 

 been reported upon favourably by Mr. Whymper and other 

 travellers, and it is certainly an advance upon the ordinary in- 

 strument. When the instrument is inaction a line on the ring, 

 to which the fly-nut is fastened, coincides with an arrow upon 

 the case. This, we think, admits of improvement, for the 

 coincidence cannot be very accurately determined. In taking 

 readings before and after putting the instrument out of .action, 

 we found a slight ditiference on each occasion, which may perhaps 

 be due to the difficulty in bringing the ring back to the same 

 point. 



From the Journal of the Franklin Institute we learn that 

 the Elliott Cresson gold medal has been awarded to Mr. 

 Clemens Herschel, and John Scott legacy premiums and 

 medals to Messrs. Frederick N. Connet and Walter W. Jackson 

 for their joint invention of the Venturi meter, an apparatus 

 designed for measuring the flow of liquids in pipes of any 

 desired dimensions up to 60 inches or more in diameter. The 

 \'enturi proper, invented by Mr. Herschel, consists essentially 

 of a tube containing a constriction through which the water has 

 to flow, and by measuring the difference of pressure between 

 the wider and narrower parts, the rate of flow of the liquid can 

 be calculated by well-known formuki;. The two last-named 

 inventors have devised the elaborate registermg apparatus 

 driven by clockwork, whereby the indications of the piezometer 

 are made to give a record of the total quantity of water flowing 

 through the pipe by a species of mechanical integration or 

 " quadrature." 



The fifth edition of the Naturalist' s Directory for 1899, pub- 

 lished by Mr. L. Upcott Gill, is to hand. It contains the names, 

 addresses, and specialities of several thousand field naturalists, 

 as well as curators of museums and professors and lecturers on 

 natural science. The zoologists alone occupy seventy-two pages, 

 and a rough statistical tabulation of a few pages, selected at 

 random, shows that ornithology and lepidoptera head the list, 

 the numbers of specialists in each of these being more than 

 double of that in any other department ; ornithology and 

 oology when added together have the majority. Mollusca, 

 malacology and conchology, when combined, come next in 

 point of numbers, entomology (in general) next, and coleoptera 

 next again. Although the other orders of insects and the 

 other branches of zoology are mostly represented, their devotees 

 fall greatly behind those of the afore-mentioned subjects. 

 Microscopy occupies twelve pages, and shows a large pre- 

 ponderance of specialists in pond life and vegetable physiology. 

 Diatoms and foraminifera come next, but a long way behind, in 

 popularity ; and after these, bacteria, marine zoology, biology, 

 photo-micrography, and micro-entomology divide the favours 

 about equally. In botany, which occupies fifteen pages, phane- 

 rogams, as might be expected, have an overwhelming majority ; 

 while among cryptogams, mosses and hepaticLie appear to be 

 most popular. Geology and palaeontology extend over thirteen 

 pages. In addition the volume contains a trade directory, a list 

 of societies, field clubs and museums, from the Royal Society 

 and British Museum downwards, and a list of books of the year 



