38 



SCIENTIFIC NE\VS. 



Qan. 13, ig 



we consider that it is produced by the reflection of a light 

 thrown from some neighbouring house, since the ■wisp 

 generally appears far away from houses. Fireworks are 

 an extremely improbable agency. Not to speak of the 

 slowly progressive movements of the wisp sometimes 

 observed, it is in the highest degree unlikely that any 

 practical joker would convey a quantity of pyrotechnic 

 appliances by night into lonesome moorlands, woods, and 

 peat-bogs on the slender chance of being able to startle 

 some stray traveller. 



(To be continued.) 



THE TIME IT TAKES TO THINK. 



MEN in all ages have been familiar with the fact that 

 thought is habitually more active and more speedy 

 in the case of some persons than of others. Whether in 

 the exercise of the imaginative or the reasoning faculty, 

 or in the action of that guiding purpose of life which we 

 call the will, the general truth of this observation has 

 been always evident. As we come to a higher stage in 

 human development, with its corresponding increase of 

 intelligence, it is not remarkable that the mind should 

 seek to subject itself to a closer scrutiny, and to catalogue 

 more exactly the results of its introspection. This has hap- 

 pened with regard to the question before us. In parti- 

 cular has the period which intervenes between the 

 impact of impressions from without and the visible 

 response of the will in muscular action or other mode 

 of expression become of late years a subject of careful 

 investigation. Since the first quarter of the present 

 century, when the comparative speed of visual observa- 

 tion, was chronicled in the case of the astronomers Bessel 

 and Struve, the varying rate at vv'hich the mind responds 

 to the numerous impulses which reach it from the outer 

 world has been measured with some exactitude. Most 

 of our readers are familiar with the term " reaction " 

 period. They will recognise it as expressive of the 

 whole interval between the action of a stimulus, of 

 whatever kind, and the visible response of the individual 

 affected by it. They will also perceive in the period 

 of " reduced reaction" that interval shortened by the 

 space of time allotted by experimental precision to the 

 transit of purely sensory and motor impulses — the 

 period, namely, which changes the one into the other, 

 and represents in the mind the average time required 

 to think and to will. Discoveries not now very recent 

 have taught us that the ordinary duration of this period 

 is, in the case of simple impressions, about one-tenth of 

 a second, and that, where the impression is rendered 

 complex by affording the mind a choice of sources to 

 which it may be referred, the time required is increased 

 by about another tenth of a second. An equally in- 

 teresting side of the subject, however, is that which 

 exhibits the force of impressions as modifying the 

 rapidity with which they are accepted and, in familiar 

 phrase, digested by the mental centres before they are 

 visibly transformed into action. The rapidly energising 

 power of sudden fear, joy, hope, and desire is thus ex- 

 plained. So likewise the contrary effect of some of these 

 emotions may be explained as being due to a stunning 

 effect produced by the causes which give rise to them 

 upon the mental centres. It is also manifest that the 

 energy of mental reaction must largely depend upon the 

 reserve of force in those centres at the time of activity. 

 They must not be overtaxed by prior stimulation and 

 excessive use ; if their response is to be a vigorous one, 



they must be well nourished and have seasonable rest. 

 There is, therefore, even in these psychological minutiae, 

 a moral for the times — namely, that if we would preserve 

 our mental efficiency amid the changing pressure of cir- 

 cumstances, we must by occasional recreation withdraw 

 the mind from too continuous application. — The Lancet. 



THE GROWTH OF RAINDROPS. 



WHEN several rain gauges are set up in the same 

 locality, but at different heights, a curious 

 fact, says Mr. W. Mattieu Williams, in the Gentleman's 

 Magazine, usually presents itself. " The quantity of 

 rain that is falling on a given surface is shown to 

 diminish with the height. This, according to ordinary 

 notions of the supply of rain from the clouds, appears 

 very paradoxical. Some meteorologists even question 

 the accuracy of the rain-gauge record. Thus Professor 

 Cleveland Abbe attributes the difference to the action of 

 the stronger winds to which the rain-gauge is exposed 

 v>rhen set high up. These, he suggests, carry the drops 

 to one side, so that the higher gauge catches less than 

 the lower one. I do not see how that accounts for the 

 observed facts, but they are easily explained if we reflect 

 a little on the ordinary physical conditions of rainfall. I 

 say the ' ordinary ' conditions, not the exceptional condi- 

 tions. One of these ordinary conditions is that the air 

 through which the drops of rain fall is fully saturated or 

 even supersaturated with aqueous vapour ; and another is 

 that the temperature above is lower than that below, and 

 therefore the drops of rain coming from above are cooler 

 than the air through which they are falling. This being 

 the case, each drop acts as a condenser to the vapour 

 through which it is passing, and thus grows in size as it 

 descends. This increase of the size of the drops has 

 been well observed, and is not at all covered by Professor 

 Abbe's explanation. The following is an experience of 

 my own. I started on a ' soft day ' to ascend Ben Nevis. 

 Rain was falling at Fort William. At about halfway up 

 the mountain there was a mixture of rain and sleet. 

 Gradually the proportion of snowflakes increased, and 

 finally, before reaching the summit, dry snow was falling. 

 I have passed through the same series on other occasions. 

 It would be the common experience of tourists, but for 

 the fact that we rarely start to climb a mountain in wet 

 weather. The characteristic ' nimbus ' or rain-cloud is a 

 cumulus or rounded cloud extending downwards in shape- 

 less mass, cloud above, mist below. The whole cloud is a 

 supersaturated stratum of atmosphere in the condition of 

 condensation and precipitation, the rounded upper sur- 

 face indicating the upper boundary of this condition. 

 Rain is produced throughout this cumulo-stratus cloud at 

 all elevations from its woolly summit down to its base, 

 which very commonly rests on the earth's surface. There 

 are occasions when raindrops diminish as they fall. 

 This must of necessity occur whenever the rain is formed 

 above a dry stratum. In such case the falling drops 

 must rapidly evaporate. The north side of the Romsdal 

 (Norway) is a magnificent wall of dark-coloured rock, 

 ranging at the lower part of the valley from 2,000 to 

 3,000 feet in height. Over this are poured a multitude of 

 cascades, some of them mere threads of water. On 

 a clear summer's day the continuous sunshine warms the 

 dark rock so effectively that some of these minor falls, 

 after breaking as they all do into snow-like spray, vanish 

 altogether by evaporation. I witnessed this on both my 

 visits to this valley on hot days of different summers." 



