38 



ON THE POSITION OF FIRE-PLACES. 



[1854. 



than 7000 feet, and as tliey are still 2,500 feet thick in New York, and 

 do not die away before reaching the Mississippi, it is to be expected 

 " that tliey would follow the Upper Silurian zone in its southwestern 

 course from the eastern extremity of GaspS, and display a conspicuous 

 figure either in a metamorphic or unaltered condition, between it and 

 the carboniferous areas of Eastern America ; to one of which New 

 Brunswick belongs, while another is met with in the state of Rhode 

 Island, and in a metamorphic condition in Massachusetts." (Report 

 for 18-18, p. 58.) The lower part of the Devonian, farther west, em- 

 braces beds of limestone, but in Gaspfi the formation consists almost 

 entirely of siliceous and argillaceous beds. In Mr. Logan's section of 

 the whole 7000 feet on the Gulf of the St. Lawrence, he observed only 

 one small bed of limestone, and a few thin bands of limestone conglo- 

 merate. When we consider the geographical position of the Upper 

 Silurian rocks in the Connecticut valley on the one hand, and the coal 

 field of southeastern Massachusetts on the other, we can scarcely doubt 

 that the intermediate gueissoid, and hornblendic rocks v;ith their 

 accompanying limestones, are the Devonian strata in an altered condi- 

 tion. Prof. Agassiz from his own examination of the region, was led 

 to a similar conclusion as to the age of the so-called syenites, and in 

 Auo-ust, 1850, presented to the American Association for the Advance- 

 ment of Science at New Haven, a paper on the age of the Metamorphic 

 rocks of Eastern Massachusetts, which has never I believe been pub- 

 lished. The less altered limestones which, according to Dr. Hitchcock 

 are found interstratified with red slates at Attleborough and Walpole, 

 may correspond to those which with similar slate and sandstone, are 

 met with at the base of the carboniferous formation in Canada on the 

 Bay de Chaleurs, and in New Brunswick. 



We have then distinguished four classes of crystalline limestones: 

 first, those of the Laurentian series with their acoomp.anying garneti- 

 ferous gneiss, labradorite and hypersthene rooks ; secondly those of the 

 Lower Silm-ian formation, with their attendant auriferous rocks, talcose 

 slates and chromiferous serpentines ; thirdly those of the Upper Silu- 

 rian age, with their associated'calcareo-micaceous schists; and fourth- 

 ly, those which belong to the gneissoid rocks of eastern Massachusetts, 

 and are probably of the Devonian period. 



I have endeavoured in this paper to bring together the facts known 

 with regard to the different crystalline limestones, and their associated 

 strata in this portion of the continent, and to show how far these m.ay 

 serve as a guide in the geological investigation of the metamorphic 

 rocks While the result confirms the observatic us of European Geolo- 

 gists that similar crystallized minerals may occur in the metamorphic 

 limestones of very diiferent geological epochs ; it also shows that 

 within certain limits, the mineral characters of the altered silicious 

 strata, may serve as important guides to our investigation. 



Ou tlic Position of Pire-Places. 



By Dk. Neil Aenott, F.R.S.* 



This is the fit place for remarking on the fashion lately introduced 

 in this country of placing the fire-grates much lower down than form- 

 erly in some cases, on the vei-y hearth — the reasons usually assigned 



being that a lower fire burns better, or gives out more heat from the 

 same quantity of fuel, than a higher ; and, because lower and nearer 

 the floor, that it must warm the carpet better, and so lessen the evil 

 of cold feet. Now, both these suppositions are curious errors or de- 

 lusions, having their origin, iupopular misconceptions respecting heat, 

 and particularly respecting the radiation of heat. 



Radius is the Latin word for the spoke of a wheel, and anything 

 which diverges or spreads around from a centre, in some degree like 

 spokes, is said to radiate. Light and heat are of this nature ; the 

 portion of either which passes in a straght line from the centre is 

 called a ray. 



The simplest observation teaches all that a lamp placed in the mid- 

 dle of a room radiates its light nearly equally in all directions ; 

 and most persons are aware that if an opaque mirror be placed close to 



« The subject of this article was referred to by Dr. Arnott, during 

 the reading of his paper on the Smoke-consuming Fire-grate, on the 

 lUth of May last before the Society of Arts ; and as it has been deemed 

 important, he has been good enough to give his remarks in writing, 

 ■which may be taken as forming part of his original paper. 



a lamp on one side, it not only intercepts all the rays that fall upon it 

 — and that means nearly half of the light given out — but it returns or 

 reflects these rays back in contrary corresponding directions, and 

 nearly doubles the illumination in those directions. 



Most persons also have observed that if a fire, or a red-liot mass bf 

 metal, be placed in free space, it radiates its heat as well as its light 

 nearly equally in all directions ; but many do not learn, by their un- 

 aided observation, that if a surface of any substance, like fire-brick, 

 which strongly resists the passage of heat through it, be placed near 

 a fire, it not only intercepts the heat-rays falling on it, but after ab- 

 sorbing them, and so becoming heated, often to redness, it then reflects 

 and radiates back the greater part of the heat, almost as if it were 

 additional hot fuel in the fire, and thereby nearly doubles the warmth 

 felt in directions away from the surface, 



Neither does common observation make persons aware of the truth 

 that of the heat pioduced by combustion in a common fire, one part — 

 being somewhat more than half' — is diffused, like the light, by radia^ 

 tion, into the open space around, and the remainder is given, by con- 

 tact and conduction, to the air which supports the combustion, and to 

 the solid material of the fire-place; Thus, with a common open fire- 

 place, it is the radiant heat almost alone which warms the room, the 

 remainder either at once combining with the burned air or smoke, 

 and passing up the chimney, or being given by Ihe heated giate to 

 pure air, which touches that, then passing into the chimney with the 

 smoke. 



And, lastly, manj' persons do not at first learn the truth, that the 

 rays of heat passing through pure or transparent air do not at all 

 warm that air, but warm only the solid or opaque bodies by which the 

 rays are intercepted, and that thus the air of a room is warmed only 

 at second-hand, liy contact with the solid walls and furniture, which 

 having intercepted the heat rays, have themselves first become heated. 

 Yet most educated persons know similar facts, such as that the sun- 

 beams, bringing both light and heat to the earth, as they descend to 

 warm the hottest valleys or plains of the earth, pass through the upper 

 strata of the atmosphere, which are always of a temperatui-e much be- 

 low freezing. This is proved by the fact that all lofty mountains, even 

 under the equator, are capped with never-melting snows, and that the 

 higher the peaks are — and, therefore, the nearer to the sun — the colder 

 they are. Thus, also, all person s who have attended to the subject 

 know that aeronauts, in their balloon-car, if they amount very high, 

 would be frozen to death, but that they are protected by very warm 

 clothing. Another fact of the same kind is, that a glass globe, filled 

 with cold water, or even ice, may in the sun's ray be used as a burn- 

 ing-lens. 



These explanations being premised, the two popular delusions re- 

 specting the low fires become at once apparent. 



1st. The supposition that fuel burnt in a low fire gives out more 

 heat, has arisen from the experimenter not reflecting that his hand 

 held over the low fire feels not only the heat radiated from the fire 

 itself, but also that reflected from the hearth close beneath it, which 

 second portion, if the grate were high, would have room to spread or 

 radi.ate downwards and outwards to the' more distant floor or carpet, 

 and to warm them. 



2nd. The notion that the fire, because near the floor, must warm 

 the carpet more, springs from what may be called an error in the logic 

 of the reasoner, who is assuming that the hearth, floor, and carpet be- 

 ing parts of the same level, are in the same predicament — the truth 

 being, however, that in such a case the hearth within the fender gets 

 nearly all the dowuAvard rays, and the carpet almost none — as a can- 

 dle held before a looking-glass at a moderate distance diffuses its heat 

 pretty uniformly over the whole, but if moved close to one part of the 

 glass it overheats and probably cracks that part, leaving the rest un- 

 affected. A low fire on a heated hearth is to the general floor or car- 

 pet of a room nearly what the sun, at the moment of rising or setting, 

 is to the sm-face of a field. The rays are nearly all shooting upwards 

 from the surface, and the few which approach it slant obliquely along 

 or nearly parallel to the surface, without touching, and therefore with- 

 OHt warming it. 



Striking proof of the facts here set forth is obtained by laying ther- 

 mometers on the floors of a room with a low fire, and of a room with 

 the fire, as usual of old at a height of about 15 or 16 inches above the 

 hearth. An experiment, tried in two such rooms, in both of which 

 thermometers on the pisinofortes , four feet above the floor, stood at 

 62°, showed the carpet, not far from the hearth, to be at 56° with 

 the low and at 73 ° with the high fire. 



