448 



NATURE 



[February i, 191 2 



and Holborn, nnd the silcnre due to the absence of vehlcle», 

 alt came to mind on reading Mr. Harding's letter. It took 

 me on that occasion more than four hours to fierform a 

 journey of about two miles, and progression was only 

 made possible by encasing my boots in the folds of a 

 woollen scarf " which 1 was wearing at the time, which 

 I took off and cut into two portions for the purpose. 

 There was no viaduct at that time, and Holborn Hill 

 interposed serious difficulties. 



The explanation of the phenomenon is no doubt that 

 given in "The Observer's Handbook" quoted Jjy Mr. 

 Harding, viz. the sudden freezing of supercooled water 

 drops on shock. In connection with this explanation there 

 naturally arises the question as to the particular conditions 

 which admit of supercooling without actual conversion into 

 hail. Clearly these conditions are but rarely complied 

 with. The actual date could no doubt be found by hunting 

 through newspaper files, but there must be many Londoners 

 now living who can remember the occasion. 



January 26. R. Meldola. 



The Radiating Power of Air. 



It has been assumed in investigations of atmospheric 

 radiation that the values of the radiating power obtained 

 in laboratory experiments are comparable with the values 

 obtained from meteorological observations, and agreement 

 between values obtained by the two methods has been 

 quoted as evidence of the accuracy of the determinations. 

 In an investigation of the problem from the meteorological 

 side, I discovered that the quantities used to represent the 

 radiating power were different in the two cases, and the 

 distinction is important. 



In the meteorological method, if 6 is the temperature of 

 the air at time t during the night, values of o, tf, are found 

 to satisfy approximately the equation 



= - a(fl - ej 



(I) 



and ope is taken to represent the radiating power of the 

 air, where c is specific heat, 0239, ^^^ P '^ density. 



If the radiation from a horizontal layer of air i cm. 

 thick is f(B) per unit area from each face, the absorption 

 by it will be 2/ffl') per unit volume if its surroundings are 

 at temperature 0'. In that case 



oc'^=-2[f(e)-/(d')] 



at 



= -2 (fl-e') •^- higher powers of (fl-e') . . (2) 



and by compari<;on with (i) it is seen that 



api' 



-?r, ».=•■■ 



Now, in laboratory experiments on the radiation of air, 

 the quantity measured is the excess of the radiation per 

 unit area from one face of a column or layer of hot air 

 over the corresponding radiation from a column or layer of 

 cold air, and this quantity, reduced to 1° C. difference of 

 temperature for a layer t cm. thick, is denoted by h, and 



is used to represent the radiating power. Clearly '*= ^"1 



and consequently apc = 2h, and not h, as hitherto assumed. 

 If in the laboratory experiments the radiation emitted by 

 the layer in a direction perpendicular to its face is com- 

 pared with that emitted normally by a black surface, the 



value of h will be onlv i l/ or iapc, since the ratio of the 



off 

 total radiation to the normal radiation is t for the black 

 surface but 2ir for a ihin layer of air. 



The confusion arose from the fact that h and ape were 

 taken to represent the rate at which air is losing heat by 

 radiation to surroundings 1° C. colder, but while in the 

 case of ape the radiation in all directions was taken into 

 account implicitly, in the case of h the necessary adjust- 

 ment was not made. E. Gold. 



4 Hurst Close, Hampstead Garden Suburb, N.W. 



NO. 2205, VOL. 88] 



Microtcope Stands. 



A UCNGTHKKBD experience in the use of the microscope 

 impels me to ask you to allow me to take exception to 

 one of the statem<-nts m-idc- by the writer of the article 

 on microscope stands which appears in Nature of 

 January 11. Referring to the circular rotating and 

 centring stage of the better class of Continental stands, 

 the writer says, " the use of which for anything but 

 petrology it is difTicult to guess." 



After working for upwards of thirty years with an 

 Knglish stand, and, es|M-cially during the latter part of 

 that time, constantly feeling the desirability of a rotating 

 stage, I decided three years ago upon the purchase of a 

 new stand, and the circular rotating stage was the feature 

 that led me to decide upon one of Continental manuf.i' 

 ture, after carefully considering the merits of two . 

 English manufacture. If well made, the rotafii i> 



of great utility. If one wishes to examine, .'i: ily 



to draw, say, one of a number of -scattered U,,.. ....... or 



Echinoid plutei, it is a great convenience to be able to 

 bring its sagittal plane into a vertical position in the field 

 of view, and, as I know from much irritating experiem - 

 this is seldom possible on a fixed rectangular stage pi 

 vided with mechanical adjustments, or even a sliding bar. 



What is really needed to make the rotating stage of th> 

 Continental microscope much more efficient is a removable 

 sliding bar, upon which it would be possible to support a 

 3XI3 inch slip, so that a series of sections mounted upon 

 it might be examined carefully with the microscope in an 

 inclined position. The rotating stage of a high-class stand 

 by one of the foremost English makers, now before me, 

 provided with such a bar, which slides in a groove cut 

 the stage ; but its utility for the purpose indicated above- 

 is nullified by the projecting heads of two screws which 

 hold together parts of the mechanical adjustments, and th<^ 

 whole instrument is little more than an ornament on 11 

 work-table. 



I have never found any use for the excentric rotating 

 movement below the Abbe condenser, and especially for 

 the cylinder diaphragm, which, I suppose, is a sop thrown 

 by Continental makers to those teachers who, in my 

 student days, derided the use of any form of substage 

 illuminator. In my opinion the expense incurred in the 

 manufacture of these redundances might with great 

 advantage to workers like myself be devoted to t' 

 improvement of the stage on the lines I have indicated. 



H. C. Chadwick. 



The Biological Station, Port Erin, January 26. 



Meteor-showers. 



The following meteor-showers become due in February. 

 The epochs are arranged according to the times of the 

 principal maxima : — 



Epoch February 4, 3h. 30m. (G.M.T.), fifth order of 

 magnitude. Principal maximum, February 3, 8h. - S5m. ; 

 secondary maxima, February 3, 3h. 40m. and 2oh. 20m. 



Epoch February 3, qh. 30m., nineteenth order of 

 magnitude. Principal maximum, February 4; 2ih. 1511 

 secondary maxima, F'ebruary 4, iih. 25m., and February 

 6h. 50m. 



Epoch February 9, 4h., twenty-first order of magnitude. 

 Principal maximum, February 10, 8h. 40m. ; secondary 

 maxima, February 10, ih. 30m., and February n. 

 8h. 25m. 



Epoch February 13. iih., fifth order of magnitude. 

 Principal maximum, February 12, i3h. 45m. ; secondary 

 maxima, February 11, 22h. 30m., and February 13. 

 loh. 45m. 



Epoch February 14, iih. 30m., thirty-third order of 

 magnitude. Principal maximum, February 15, 22h. 45m.; 

 secondary maxima, February 14, iih. 35m., February- 15, 

 i5h. 30m.," and February 16, 7h. 



Epoch February 16, 8h., approximately tenth order of 

 magnitude. Principal maximum, February 17, oh. 30m. ; 

 secondary maximum, February 17, i5h. 40m. 



Epoch February iq, ih., approximately thirteenth order 

 of magnitude. Principal maximum, February 18,. 

 5h. 40m. ; secondary maxima, February iSh., 3h. 40m., 

 and February 18, i8h. 55m. 



Epoch February 20, 7h., fifteenth order of magnitude. 



