88 Dr. G. J. Stoney on the Cause 



optical phenomenon with which we are now dealing will not 

 occur. But if the opposite conditions prevail (which they do 

 on rare occasions), if the vapour had been evenly distributed, 

 and if the precipitation took place slowly, then will the crys- 

 tals in any one neighbourhood be little ice-crystals of nearly 

 the same form and size, and from one neighbourhood to 

 another they will differ chiefly in number and size, owing to 

 the process having gone on longer or taken place somewhat 

 faster, or through a greater depth, in some neighbourhoods 

 than others. This will give rise to the patched appearance of 

 the clouds which prevails when this phenomenon presents 

 itself. It also causes the tiny crystals, of which the cloud 

 consists, to grow larger in some places than others. 



Captain Scoresby, in his ' Account of the Arctic Regions,' 

 gives the best description of snow-crystals formed at low tem- 

 peratures with which I am acquainted. From his observations 

 it appears — (a) that when formed at temperatures several 

 degrees below the freezing-point, the crystals, whether simple 

 or compound, are nearly all of symmetrical forms ; (b) that 

 thin tabular crystals are extremely numerous, consisting either 

 of simple transverse slices of the fundamental hexagon, or, 

 more frequently, of aggregations of these attached edgewise 

 and lying in one plane ; and (c) that, according as atmo- 

 spheric conditions vary, one form of crystal or another largely 

 preponderates. A fuller account of these most significant 

 observations is given in the Appendix to this paper. 



Let us then consider the crystals in any one neighbourhood 

 in the sky, where the conditions that prevail are such as to 

 produce lamellar crystals of nearly the same thickness. The 

 tabular plates are subsiding through the atmosphere — in fact 

 falling towards the earth. And although their descent is very 

 slow, owing to their minute size, the resistance of the air will 

 act upon them as it does upon a falling feather ; it will cause 

 them, if disturbed, to oscillate before they settle into that 

 horizontal position which flat plates finally assume when fall- 

 ing through quiescent air. We shall presently consider what 

 the conditions must be, in order that the crystals may be liable 

 to be now and then disturbed from the horizontal position. 

 If this occasionally happens, the crystals will keep fluttering, 

 and at any one moment some of them will be turned so as to 

 reflect a ray from the sun to the eye of the observer from the 

 fiat surface of the crystal which is next him. Now, if the 

 conditions are such as to produce crystals which are plates 

 with parallel faces, and as ihey are also transparent, part only 

 of the sun's ray that reaches the front face of the crystal will 

 be reflected from it : the rest will enter the crystal, and, falling 



