July, 1901.] 



KNOWLEDGE 



157 



gi-o\vth of spots (^first winter after minimum, to winter 

 ending iu maximum yeai'). a', b', c', d', e', the same in 

 thi-ee wiutei-s following maximum. The one set are all 

 abuve average, the other all below. 



Alex. B. INLvcDhwam.. 



70 

 (w 



60 

 55 

 50 

 45 

 40 

 35 

 30 

 25 

 20 

 15 

 10 



a h r d e 



'44 '57 'tiS '79 '90 



-'48 -'60 -'70 -'83 -'93 



56-2 61-5 5(i-3 61 G 620 



a' b' c d' e' 



'49 '61 '71 '84 '94 



-'51 -'63 -'73 -'86 -'96 



45 7 42 47-3 490 43 



DETERMINATION OF NOON. 



TO THE EDITORS OF KNOWLEDGE. 



SiKS, — We live in a rather isolated disti'ict and, as we 

 have no sun-dial, find it difficult to get accurate 

 time. Is there any simple means, by the shadow of a 

 pole or building, by which wc could tell exactly when 

 it is noon .' Any suggestion you cau give us in this 

 matter will be thankfully received. Wm. D.wies. 



Concord. Dominica, 

 British West Indies, 

 April 24th, 1901. 



[The exact detenniuation of time requires the use of 

 insti-uments, but the Approximate time of noon may be 

 ascertained by watching the coincidence of the shadow 

 of a vertical rod with a meridian line drawa on level 

 ground from the foot of the rod. The meridian line is 

 usuallv formed by drawing a portion of a circle through 

 the end of the shatlow some hours before noon, with the 

 foot of the rod a.s centre, then marking the point on 

 the circle where the tip of the shadow reaches it in the 

 afternoon, and joining the middle of the arc thus formed 

 with the foot of the rod. Several circles should be 

 drawn, and the mean of the positions should be atlopted. 

 Noon deteiTttined in this manner will be local apparent 

 noon, and correction must be piade for the question of 

 time in order to find the local mean solar time which 

 should be shown by a wat«h. Several suggestions for 

 observing the .sun were made by Mr. Maunder in Know 

 LEDOE for June. 1900. p. 133. Ens.] 



CLOUDS ON MARS. 



TO THE EDITORS OF KNOWLEDGE. 



Sirs, — Your correspondent, Mr. E. Lloyd .Jones, has, 

 in the April and .June issues of tliis magazine, stated 

 some objections to my previous connuunication (Feb- 

 ruary). I fear I sUitcd the question rallier bluntly 

 when 1 said, " The atmosphere of Mais is thin, and 

 consequently free from clouds." What I meant at the 

 timo of writing wiis that the clouds on Mars are so 

 rarely visible that they ai-e scai-cely worth recording. 

 Mr. Jones will lind that this is the view of almost all 

 our modern astronomers. As I stated before, my views 

 ou the subject have hu-gely been gained from Mr. 

 I'en-ival Lowell's hook on " Mars, " and should Mr. 

 .Jones desire further infoi-mation I would refer him to 

 this work. T. K. Waking. 



Liverpool. 



THE ICE AGE. 



TO THE EDITORS OF KNOWLEDGE. 



Siiis, — Some years ago you inserted letters of mine 

 on the subject of the Ice Age. I hope the following 

 additional remarks may also iind a place in your 

 columns. 



The laLe Prof. Tyndall stated that what was requiied 

 in order to account for the Ice Age was a better dis- 

 tilling apparatus than at present. But this remark 

 cannot be adopted without qualifications; for with a 

 better distilling apparatus the aqueous vapour might 

 be deposited in the form of rain, not snow. And, in 

 fact, the Ice Age seems ia have been succeeded by a 

 Rain Age, for 1 do not think that the melting of the 

 snows will account for the very large amount of water 

 which seems to have been then deposited on the land. 

 The Ice Age and the Rain Age seem Xo have resulted 

 from increased distillation under difi^erent circumstances. 



Air at a high temperature may contain much 

 moisture, but it can hardly be deposited as snow. Air 

 at a very low temperature contains very little moisture, 

 and can hardly produce a snow-cap of any considerable 

 depth. A heavy snow-fall only occurs when the tem- 

 perature is little below freezing-point, but to produce 

 such a heavy snow-fall two further conditions ai'e 

 requisite : first, that the fall of temperature should Ije 

 rapid, and secondly that the air should be moist. The 

 latter condition in these regions requires that the wind 

 should be from the W. or S.W. ; or at least that the 

 conflict between the wind and one from the opposite 

 quarter slioidd take place over this (lountry before the 

 temperature h;is sunk too low. 



We then require a rapid fall of tiuiiperature a( a time 

 when the mean temperature is near freezing-point and 

 the air is moist. If the rapid fall takes place before 

 this period of the year there will be a rain-fall not a 

 snow-fall. If later the snow-fall will not be so lica.vy. 



The fall of temperatui-e dependent on the seasons 

 does not vary with the eccentricity of the earth's orbit. 

 The most favourable condition for tlic formation of a 

 snow-cap at any particular place will, therefore, bo that 

 the suns distance should increase most rapidly at the 

 time of the year when the mean temperature is nearly 

 at freezing-point. I mean, of course, before the winter. 

 For when a similar state of things occurs in spring a 

 rapid increase in the distance of the sun would be 

 balanced by a like increase in the length of the day and 

 of the sun's meridian altitude. Suppose then that we 

 take a country in which tiie mean temperature sinks 



