18 



KNOWLEDGE. 



[January 1, 1897. 



be noticed that even now a tbrush will often have a note 

 closely like aery of the tawny owl, or the evening cry of 

 the partridge, or the prominent note of yet another species. 

 Especially does it produce brief but loud whistles re- 

 sembling those of the nuthatch. If persons who are 

 blessed with a thrush in the garden will listen attentively 

 to its song, they will perceive that while the first three 

 months of the year are passing away their bird is extending 

 its powers of variation and mimicry, so that the songs 

 heard at the end of this period will be hardly comparable, 

 as performances, with the primitive utterances of early 

 January. 



The same feature of extended variation, but without 

 mimicry, may be noticed in the blackbird, whicli sings in 

 January short and simple strains, with a tendency to repeat 

 them ; so that its songs then resemble not only those of a 

 young blackbird commencing vocalization in October, but 

 also those of its ally the missel-thrush. In fact, when 

 both species begin to sing, their utterances are approxi- 

 mately equal in barbaric simplicity as well as similar in 

 tone ; but, while the blackbii-d progresses week by week 

 towards the attainment of the long musical strains which 

 form a sort of bass to the chorus of May, the missel-thrush 

 seems to remain content with a repetition of two or three 

 notes — very full, indeed, but hardly exceeding the interval 

 of a third, and repeated many times with hardly a varia- 

 tion at all. This bird is supposed to sing especially in 

 stormy weather, and it sometimes does so ; but it sings 

 most continuously in fine warm weather. In February it 

 should be in full song. 



The rattling alarm of the blackbird is particularly 

 noticeable in the first two months of the year, durmg 

 which the bird betrays an increasing tendency to utter this 

 remarkable strain on the slightest alarm, or when there 

 is no alarm at all ; and then the cry — like the single 

 clicking note so often repeated at evening — seems to be 

 employed merely to make a fuss. 



The starling is, perhaps, the most persistent of all our 

 singers, for the only time when it ceases to pour forth from 

 our roofs its curious medley of original and bcrrowed notes 

 is when the young leave the nest, from which period till 

 August it is absent ; yet the noisy choruses of its assemblies 

 in the fields prove that it sings while abroad as much as 

 or more than at home. In the winter it often concludes 

 its song with harsh squealing notes. These are uttered 

 with increasing frequency as the season advances, and are 

 often addressed to the mate. They are worth noticing. 



At morning and evening in December and January the 

 chaffinch gives its cry, jiiik or tirik, sometimes with con- 

 siderable frequency; but it never produces a continuous 

 cry like the alarm of the blackbird or the long cry of the 

 nuthatch. The bird has, nevertheless, a tendency to repeat 

 this note rapidly, and the extent to which this prevails 

 should be observed. 



In February the bird will begin to sing. Then also the 

 greenfinch will begin to repeat its call-note for a song, both 

 when on the wing and when perched; and towards the end 

 of the month, or in March, the peculiar guttural song-note 

 will be noticed. In these two months the flocks of linnets 

 wiU sometimes be heard singing in a sort of chorus. 



In February we shall notice the song of the marsh 

 titmouse. This has been described as resembling the 

 words if he, but the best mode of suggesting the sound is 

 by striking two notes alternately in the interval of a third, 

 high on the piano — say, at an octave above G over the 

 treble stave — and repeating them at the rate of about three 

 couples per second. That gives a good idea of tlie song, 

 which is obviously as simple as the common call of the 

 cuckoo, and is repeated by the whole species, with wearying 



monotony. At the same period the pied wagtail will be 

 heard singing a jumbled kind of song consisting of hardly 

 any sounds except call-notes. 



In March the first warbler will arrive with cheery song. 

 Besides the M[t-chaif', whence the bird is named, there is 

 a cricket-hke chirp, which is given less often when the 

 bird has first arrived than later in the season, when also it 

 is sometimes varied in extent of repetition. 



Last mentioned and least necessary of our list, the 

 house sparrow will be observed to immediately commence 

 its seemingly noisy combats. The purpose of these assem- 

 blies, which often consist of several males and one female, 

 may be diflicult to determine ; but the cries employed at 

 them appear to exhaust the vocabulary of the species. It 

 will be noticed that the call-note of the male, chissick, 

 though uttered hardly at all at the beginning of the year, 

 occurs with increasiug frequency as the season advances. 



THE DECAY OF CLOUDS. 



By Dr. J. G. McPherson, F.R.S.E., late Mathematical 

 Examiner in the Universitij of St. Andrew's. 



UNTIL Mr. John Aitken proved by experiment that 

 cloud - particles are formed by the adhesion of 

 water vapour to the dust-particles invisibly floating 

 in the atmosphere, little was known about the 

 real nature of clouds. The lowest stage in the 

 formation of clouds is in the once little-understood pheno- 

 menon of haze. The cleai-est air has some haze, and as 

 the moisture increases the thickness of the air increases. 

 In this case condensation takes place on dust-particles, 

 even when the air is comparatively dry, before the tempera- 

 ture comes down to the dew-point. The sultry haze, the 

 suflbcating fogs, the drizzling mists, and the thin rain, as 

 well as the great thunder rain and pelting hail, and the 

 feathery snow, are now all known to be diflerent stages of 

 the formation of the vapour in the air on the minute dust- 

 particles at difl^erent grades of heat and cold. The forma- 

 tion of clouds is now distinctly understood. 



But the attention of the meteorologist has not been so 

 much directed to the decay of clouds. Now the processor 

 decay in clouds takes place in various ways. A careful 

 observer may discern the reverse process of the formation 

 of clouds. In May (1896) my attention was particularly 

 drawn to this in Strathmore, in Scotland. In the middle 

 of that exceptionally sultry month I was arrested by a 

 remarkable phenomenon. It was the hottest May for 

 seventy-two years, and the dryest for many years. The 

 whole parched earth was thirsting for rain. All the morning 

 my eyes were turned to the clouds in the hope that the 

 much-desired shower should fall. Till ten o'clock the sun 

 was not seen, and there was no blue in the sky. Nor was 

 there any haze or fog. The sky was full of clouds of 

 varied thickness and form. But, suddenly, the sun shone 

 through a thinner portion of a cloud, and to the north the 

 sky began to open. In a quarter of an hour there was more 

 blue to be seen than clouds. At the same time, near the 

 horizon a haze was forming, gradually getting denser as 

 time wore on. In an hour the whole clouds were gone, 

 and the moisture returned to its thin air form. This was 

 a pointed and rapid illustration of the decay from cloud 

 form to haze, and then to the blue vapoury sky. It was an 

 instance of the reverse process. As the sun cleared through, 

 the temperature in the cloud-land rose, and evaporation 

 took place on the surface of the cloud-particles, until by 

 an untraceable but still a gradual process through fog the 

 haze was formed. Even then the heat was too great for a 

 definite haze, and the water vapour returned to the air. 



