Physios and Chemistry. 149 



vail, which, if sufficiently slight, would not much interfere with 

 the regularity of the crystals, and might yet be sufficient to oc- 

 casion little draughts, which would blow them about when formed. 

 Or, if the colder layer is above, and if it is in a sufficient degree 

 colder, there need not be any previous relative motion of the two 

 layers; the inevitable convection-currents will suffice. Another, 

 and probably the most frequent, cause for little breezes in the 

 neighborhood of the cloudlets is, that when the cloudlets are 

 formed they immediately absorb the heat of the sun in a way 

 that the previously clear air had not done. If they absorb enough 

 they will rise like feeble balloons, and slight return currents will 

 travel downward round their margins, throwing all crystals in 

 that situation into disorder. 



I do not include among the causes which may agitate the crys- 

 tals another cause which must produce excessively slight currents 

 of air, namely, that arising from the subsidence of the cloudlets 

 owing to their weight. The crystals will fall faster where in cloud 

 masses than in the intervening portions' where the cloud is thinner. 

 But the subsidence itself is so slow, that any relative motions to 

 which differences in the rate of subsidence can give rise are prob- 

 ably too feeble to produce an appreciable effect. Of course, in 

 general, more than one of the above courses will concur; and it 

 is the resultaut of the effects which they would have separately 

 produced that will be felt by the crystals. 



If the precipitation had taken place so very evenly over the sky 

 that there were no cloudlets formed, but only one uniform veil of 

 haze, then the currents which would flutter the crystals may be 

 so entirely absent that the little plates of crystals can fixedly 

 assume the horizontal position which is natural to them. In this 

 event the cloud will exhibit no iridescence, but, but instead of it, 

 a vertical circle through the sun will present itself. This on some 

 rare occasions is a feature of the phenomenon of parhelia. 



It thus appears that the occasional iridescence of cirrus clouds 

 is satisfactorily accounted for by the concurrence of, conditions, each 

 of which is known to have a real existence in Nature. . . . — Phil. 

 Mag., July, 1887. 



2. Note on some Experiments on the Viscosity of Ice' by J. F. 

 Main (Abstract). — The paper contains an account of some experi- 

 ments on the continuous extension of bars of ice subjected to ten- 

 sion, made during the last winter in the Engadine. To eliminate 

 the influence of regelation, the experiments have been carried on 

 at such low temperatures as to preclude the possibility of any effect 

 being produced by this cause. The highest temperatures during 

 the experiments were —2-6° C. in Experiment I; — 1'0° C. in 

 Experiment II; and —0-5° C. in Experiment III. These maxi- 

 mum temperatures only obtained for a very short time on one or 

 two days. 



The bars were tested in a compound level testing machine with 

 accurate knife edges, the load being a known weight of shot. 

 The whole apparatus was enclosed in a double wood box. A 

 delicate thermometer graduated to tenths of degrees, attached 



