the force is measured by the space through which the heated particles recede, (vely) (space) or (force), and vely o expansive force, or as the excess of temperature. These heated particles, in receding, communicate their heat to the surrounding fluid; their place is supplied by fresh particles; and thus, by the renewal of contact, the refrigerating effect is accelerated. On a subject so intricate, and so liable to objection, it is right to quote the author's own words: Each portion of air or gaseous fluid which touches a hot surface must receive that same measure of heat and a corresponding increase of elasticity. It, consequently, dilates with a force proportional to the space through which it recedes, or to the elevation of temperature which it has assumed. But the square of the acquired velocity, as we formerly remarked, is compounded of the space and the actuating force in the present case, it is, therefore, as the square of either of these elements, or as the square of the degree of heat which is absorbed. The velocity of propulsion is hence proportional simply to the excess of temperature. The time of action is always evidently the same, because, if the space be enlarged, the rate of dilatation is likewise increased; and hence, from every exciting point of the hot sur face, a slender continued stream of air is emitted perpendicularly, whose velocity is proportioned to the measure of heat incessantly communicated. When the process is inverted, and the surface affectted is colder than the surrounding atmosphere, the contiguous portions suffer contraction and a diminution of their elasticity, which occasions a gentle perpendicular flow directed towards its source, and productive of a similar though an opposite effect. Thus the discharge of heat from a body is materially promoted by the soft propellent motion excited continually at its surface. This efflux extends to a very short distance, before it spends its force and loses itself in the atmosphere; yet it equally produces the refrigerating effect, by quickening the circulation and fresh contact of the ambient medium. Though it conspires with pulsation to accelerate the dispersion of heat, it differs essentially in its character from that species of energy. Pulsation is the same at all degrees of heat, and its intensity depends merely on the nature of the bounding surface: but the perpendicular flow is more vigorous in proportion to the excess of temperature, and has no relation whatever to the qualities, physical or mechanical, of that surface. It was shown that only a very few particles disseminated in the contiguous shell of air, feel at once the pulsatory influence: the other particles, which constitute the general mass, probably imbibe their share of heat, and passively obey the impression of their augmented elasticity.' To determine the law of the rate of cooling with regard to the temperature, recourse must be had to experiment; and Mr. Leslie, by repeated trials, found that at the temperatures 10°, 40°, 70°, the rates of cooling for a metallic surface were 2, 3, 4; and for a painted surface, 4, 5, 6. Consequently, if we increase the temperature by 30°, we increase the rate of cooling by 1, the pulsatory energy remaining the same. Hence, denoting by P the pulsatory energy, and by an unknown quantity x some unknown refrigerating cause, independent of the nature of the 8P+x=4 heated surface, we have P+x=2 temperature being 10°. Consequently, P}, and x=2—}=1, which two numbers 3, 15, therefore solely denote two effects, or losses of heat; one due to pulsation, a known cause, the other to causes not yet investigated. Suppose, now, the effect to be due to two causes y, z, one of which (y) varies as the excess of temperature; then, since at 30 the value of y = 1,at 10 it1. Consequently, 1}+2, or 2=14, the measure of a third unknown refrigerating cause. We have stated this reasoning in the above cautious terms, because we are aware that, if a person denies that besides pulsation a second refrigerating cause, varying as the temperature, and expounded in its operation by the perpendicular recession of heated particles, can be distinctly ascertained,-proofs sufficiently satisfactory cannot be collected from Mr. L.'s reasoning, to convince him of the contrary; indeed, we think that this is an assailable part of the author's doctrine. It is difficult to conceive how the heated surface can at once cause some particles to recede, forming pulses, and others to recede in lines perpendicular to the surface; and these latter particles, in consequence of their recession from an expansive force and increased rarity, must describe curve lines concave to a horizontal plane drawn above the heated body. In our opinion, Mr. L. too much neglects the dissipation of heat as proceeding from the ascent of the heated particles. The third cause of refrigeration, viz. z, whose effect we have deduced to be 1, the author thus assigns: The portion of heat thus consumed is most certainly not annibi. lated; neither is it transported to a distance, by any species of elastic motion excited in the encircling fluid. It is, therefore, absorbed by the contiguous shell of matter, and afterwards slowly diffused through the extended mass. Air is still the sole medium by which heat endeavours to maintain the balance among remote or detached bodies; but here its operation is of a passive nature, and it receives and conveys the calorific impressions through its substance in the same manner as a bar of iron or any solid material This completes the analysis of the refrigerating action of air, There are four distinct modes in which it produces the effect: three of these are always conjoined, and the fourth only throws in its occasional influence. They all conspire to the same end, but their relative shares of operation are various and mutable. One scource of com. munication depends on the quality of the heated surface, another on its elevation of temperature, a third on the permanent conducting disposition of the air, and the last arises from the celerity of impulse by which that active fluid may chance to be affected. The continua ascent of the hot, and consequently rarefied, air, must contribute in some degree, though indirectly, to accelerate the effect; for it is evident, that the stagnation of a warm encircling atmosphere would debilitate the operation of the combined refrigerating causes.' We have already noticed that the theoretical results from such a formula as dh-b.dt are not confirmed by experiments. According to Mr. Leslie, the true formula is db = — (a+b) b.dt, b excess of temperature, t time, a a constant quantity; and the following is his method of obtaining this formula. At the equidistant temperatures of 10°, 40°, 70°, in a variety of experiments, he says, he found the rates of cooling for a metallic surface to be, 2, 3, 4, and for a painted surface, 4, 5, 6: hence, to find the relation connecting the rates of cooling with the temperature, suppose to be a quantity to be investigated, then, when the temperature is 10 and 40°, v + 10 +40:23; consequently v 50;-and for a painted surface v +10: +40 :: 4:5; consequently v 10. Hence the rates of cooling, corresponding to any other temperature h, are for a metallic and painted surface respectively 50+h, and 110+b; and according to Mr. L. these quantities represent the combined action of three refrigerating causes, viz. the reiteration of aerial contact, pulsation, and the diffusive absorption of the atmospheric mass. Moreover, these three causes are, he says, always inseparably conjoined. Now the decrement of heat must vary in a ratio compounded of the refrigerating causes, and the increment of the time; or, in Mr. L.'s own words, in a ratio compounded of the refrigerating energy (a+b) and the intensity of impression (h): therefore dh=(a+b) h.dt (a denoting indifferently either 50 or 110). Now to this theory and connected formula we have some objections or rather we would merely state them as doubts, since we are aware that the author, quite at home with his own theory, may possibly have unwaringly omitted some circumstances of explanation that are essential to his readers. He says that the rates of cooling are as 2, 3, 4; now how are these rates calculated? They vary inversely as the ranges; and if they are cal-b.dt culated from fuch an expression as dh= they are calculated from a particular and imperfect hypothesis. If the expression dudh = (a + h) h.dt be correct, the true ranges and rates ought to be deduced from it: or does Mr. L. mean, on a particular hypothesis, to calculate one of the elements of the refrigerating process, and to apply that element, so calculated, in the general and true theory? x By intensity of impression, which varies as h, we suppose that the author means the refrigerating cause which arises from the & the perpendicular recession of the expanded particles from the heated surface: but he should have explained this phrase. The differential expression, from which the loss of heat is to be calculated, is db = (a+b) b.dt; consequently dt =and integrating t = hyp. dh dh b }, a log. (4+2)+ Corr (a+b). +Corr" = (if H be initial heat) = {h. 1. (+4) T a a+H -h.l. // } = { log. (4+1)—log.-1}, if M=.43429 &c. H a+H H From this formula, Mr. L. calculates the progressive cooling of a hollow tin ball, six inches in diameter, filled with boiling water but the table, as it appears to us, merely gives the proportional time; thus making a=50, H=100, 98, we have 2.9443 .Ifb=50,t=1249387 = 1000 Ma t= .0029443= I Ma 1000 Mai the proportion between which times agrees with that of Mr. Leslie: but, to know the actual time, we must learn from experiment the observed time corresponding to the loss of a certain portion of heat, 1 degree for instance; and we are surprised that the author should not have put his theory to the test, by comparing the times resulting from the preceding formula with observed times. By an application of the above form, Mr. L, calculates the range, and consequently determines the portion of heat spent in each particle of time. For a tin ball, it = of refrigeration, differing in the metallic and painted ball, on account of the inequality of their pulsatory energies. Hence 8P 6 Again, at an equili 29 brium of temperature between the conterminous surfaces, b=0; and the portion of heat spent (calculating by the formula) = ; hence 50 50.1000 M 351000M 35.1000 M sures the quantity of heat conducted away through the stationary mass of the surrounding air. Therefore, says Mr.L., from a 50 50.1000 M = mea hollow hollow sphere 6 inches in diameter, filled with boiling water, the portions of heat discharged every minute are thus represented: By abduction, the 524,1485th, By recession, the b×21714.725th, and By pulsation, the 2533.385th for metallic, and the 316.673d for the surface of paper. This is, we trust, a faithful account of Mr. L.'s analysis of the process of the cooling of heated vessels in atmospheric air:-the real cause of the difference in the rates of cooling is pulsation.-The author then applies his theory of the process of cooling to explain an anomaly that shewed itself in one of his experiments; viz. when the painted surface of the canister was turned towards the reflector, and the bulb of the thermometer was covered with tin foil, the effect indicated was 22 degrees when the metal surface was used, and the bulb was covered, the effect indicated was only 12 degrees: the procedure then being inverted, the same effects did not take place. The cause is thus assigned: the metallic bulb cools slower than the glass, and consequently is proportionally more affected by the same impression of heat. Here our limits oblige us to pause: but we hope to resume this report in our next Number. [To be continued.] R.W. MONTHLY CATALOGUE, For SEPTEMBER, 1804. POETRY. Art. 12. Syr Reginalde; or the Black Tower; a Romance of the 12th Century. With Tales and other Poems. By Edward Wedlake Brayley and William Herbert. Crown 8vo. pp. 170. 5%. Boards. Vernor and Hood. THE modest apology which is offered by the joint authors of this volume, for any defects in their performances, is adapted to conciliate the severest critic. The productions, themselves, however, with all their imperfections on their head," plead too forcibly for favour and indulgence to admit of much severity on our part; and though they certainly are not so correct in many respects as the laws of poetry and metre require, they exhibit many tokens of genius and talents, which, under due care and cultivation, may in time attain a sterling value. The story of the Devil and the Lawyer is more droll than poetical, and we extract a part of it to amuse the reader: A rogue of a Lawyer rode out one day; |