On the Theory of Hail. 169 



Iu order to compare these deflexions with each other, each 

 deflexion was reduced to that which would he given by a fibre 

 of I centim. in length by multiplying the deflexions by the 

 length of the fibre used. Thus, corresponding to the twist of 

 one turn of the torsion-head in a fibre of 1 centim. long, we 

 have : — 



For silk fibre . . . 864"'0x 31 = 26800 ,/ 



• i r 16"-3x2-3 a _„ K 



.b or spider-hne . . . yr = 6T ,m b. 



From this we get the ratio of the torsional rigidity of the 

 spider-line to that of the silk fibre to be 1 : 710. 



The diameters of the fibres were microscopically measured, 

 and gave the following values : — 



Silk fibre 0*00091 centim. 



Spider-line 0*00028 „ 



If the elastic qualities of these fibres were the same, the 

 ratio of the torsional rigidity would have come out 



(28) 4 : (91) 4 or 1 : 112 ; 

 and hence the torsional rigidity of spider-line is less than one 

 sixth of that of silk fibre of the same thickness. 



The above result gives us only a relative value of the 

 rigidities between the two fibres. If we take the mean value 

 of the torsional rigidity of silk fibre to be 0*0012 C.G.S. on a 

 length of 1 centimetre (not per square centimetre), as found 

 by Mr. T. Gray*, the torsional rigidity of the spider-fibre of 



the above experiment will be ^ 1f = 0*000002 C.G.S. , the 



mode of reckoning being the same. 



Mr. Gray's silk fibre may have had a slightly higher rigidity, 

 as he states that it was boiled in water, while the fibre of the 

 experiment just described was taken from those boiled in 

 dilute potash water, as is the usual practice of preparing 

 " mawata," which is a very soft kind of silk. 



XX VI. On the Theory of Hail. 



By Charles Tomlixson, F.E.S., F.C.S., #c.t 



[Plate V.] 



EE.OM the time of Franklin even to the present day an 

 article in the orthodox meteorological creed requires us 

 to believe that the formation of hail is due to electrical action. 

 It is true that nonconformists have appeared from time to time, 



* Phil. Mag. Jan. 1887. 



t Communicated by the Author. 



