ClIAP. 1. 



DESCRIPTIVE GEOGRAPHY. 



15 



FORMATION OF ANCIIOR-ICE. 



SMOKY ATMOSPHERE, 



DARK DAYS. 



does not vary much from 45°, and this is 

 about the uniform temperature of the 

 cartli at some distance below the surface. 

 Wliile then the mass of tlie waters of the 

 lake is at 40°, and ice is forming at the 

 (op, the earth, beneatia the water, is at the 

 temperature of 4.5°, or .5° warmer than the 

 water. Heat will, therefore, be constantly 

 imparted to the water from beneath, when 

 tlie temperature of the water is less than 

 4.5°. The only effect of this communica- 

 tion of heat to the water from beneath, 

 during the earlier and colder parts of the 

 winter, is to retard the cooling of the lake 

 and the formation of ice upon its surface. 

 But after the cold abates in the cud of 

 winter and beginning of spring, so that 

 the lower parts of the ice are not affected 

 by the frosts from above, the heat, which 

 is communicated from below, acts upon 

 the under surface of the ice, and, in con- 

 junction with the sun's rays, which pass 

 through the transparent surface and are 

 intercepted by the more opaque parts 

 below,* dissolves the softer portions, 

 rendering it porous and loose like wet 

 snow, while the upper surface of the ice, 

 hardened by occasional frosts, continues 

 comparatively more compact and firm. In 

 this state of things, it often happens that, 

 by a strong wind, a rent is made in the 

 ice. The waters of the lake are immedi- 

 ately put in motion, the rotten ice falls in- 

 ta small fragments, and, being violently 

 agitated, in conjunction with the warmer 

 water beneath, it all dissolves and van- 

 ishes in the course of a few hours. 



There is one phenomenon, which is of 

 common occurrennce in many of our 

 streams, during the coldest part of win- 

 ter, and which may not at first appear 

 reconcilable with what has been said 

 above, and that is, the formation of ice 

 upon the stones at the bottom of the 

 streams, usually called anchor ice. An- 

 chor ice is formed at falls and places 

 where the current is so rapid that ice is 

 not formed upon the surface. In the case 

 of running water, and particularly where 

 the water is not deep and the current 

 rapid, over a rough bottom, the tempera- 

 ture of the whole mass is probably reduced 

 nearly or quite to the freezing point be- 

 fcirc any ice is formed ; and then, where 

 the current is so rapid that the ice cannot 

 form at the surface, the ice-cold waters 

 of the surface, in their tumultuous de- 



* A remarkable phenomenon attending this dis- 

 integration of the ice b> the influence of the sun's 

 rays, and one wliich we think worthy of investiia- 

 tioi), is its separation into parallel icicles, or can- 

 dles, as they are sometimes called, exiendinjj per- 

 pendicularly from the upper to the lower surface of 

 the ice, giving the mass, particularly the lower por- 

 tions, somewhat the appearance of a hcuey comb. 



scent, are successively brought in contact 

 with the stones at the bottom, which, 

 themselves, soon become ice-cold, after 

 which they serve as nuclei upon which 

 the waters are crystilized and retained by 

 attraction, forming anchor ice. 



SmoLij dtmosplicrc, — From the earliest 

 settlement of this country there have been 

 observed a number of days, both in spring' 

 and autumn, on which the atmosphere 

 was heavily loaded with smoke. The 

 smoke has generally been supposed to re- 

 sult wholly from extensive burnings in 

 some unknown part of the country. There 

 is no doubt but that much of the smoke 

 often is produced in this way, but it has 

 appeared to us, that, since smoke is not a 

 product, but a defect, of combustion, it 

 may be possible for it to be produced even 

 where there is no fire. We have been 

 led to this conclusion by observino- that 

 the amount of smoke has not always been 

 greatest in those years in which burnings 

 were known to be most extensive; and 

 by observing, moreover, that the atmos- 

 phere was usually most loaded with smoke 

 in those autumns and springs which suc- 

 ceeded warm and productive summers. 

 These circumstances have led us to the 

 ojiinion that tlie atmosphere may, by ita 

 solvent power, raise and support the mi- 

 nute particles of decaying leaves and 

 plants, with no greater heat than is ne- 

 cessary to produce rapid decomposition. 

 When, by the united action of the heat 

 and moisture of autumn and spring, tJie 

 leaves are separated into minute particles, 

 we sui>pose these particles may be taken 

 up by the atmosphere, before they are en- 

 tirely separated into their original ele- 

 ments, or permitted to form new com- 

 pounds. This process goeson insensiblv, 

 until, by some atmospheric chano-e, a con- 

 densation takes place, which renders the 

 effluvia visible, with all the appearance 

 and properties of smoke. 



Dark Days. — It sometimes happens 

 that the atmosphere is so completely fill- 

 ed witli smoke as to occasion, especially 

 when accompanied by clouds, a darkness 

 in the day-time, approaching to that of 

 night. The most remarkable occurren- 

 ces of this kind, within our own recollec- 

 tion, were in the fall of 1819, and in tJio 

 spring of 1820. At both of these seasons, 

 the darkness was so great, for a while 

 near the middle of the day, that a book of 

 ordinary print could not be read by the 

 sun's light. The darkness in both cases 

 was occasioned principally by smoke, and 

 without any known extensive burnino-g ; 

 but the summer of 1810, is known to have 

 been remarkable for the abundant growth 

 of vegetation. But the most remarkable 



