mans regulated their time by it for the space of ninety-nine years, when Q. Marcus Philippus, who was cenfor with L. Paulus, caufed another dial, conftructed for the latitude of Rome, to be erected near the old one. But as a fun-dial did not serve in cloudy weather, Scipio Nafica, five years after, remedied this defect, by introducing a method of dividing the night as well as the day into hours, by means of a water machine, a clepfidra, which Pliny calls an horologium. I do not indeed conceive how a fun-dial, or any other inftrument, could point out the various hours, as time was computed by the ancient Romans. The time the earth takes to revolve once round its axis, or the space between the rifing of the fun till its next rifing, which makes a day and a night, divided into twenty-four equal parts, we call hours. Now, the Romans divided the day and the night into twenty-four hours. Twelve of thefe, from the rifing of the fun to its fetting, conftituted their day; and the other twelve, from the fetting of the fun to its rifing, conftituted their night. Thus as the feafons changed, the length of their hours must have varied. In winter the twelve hours of the day were fhort, and thofe of the night long: in fummer they were the reverfe. How then could thefe hours, of an unequal length, and which daily varied, be measured by an inftrument? I have not been able to discover any method by which this could be done. However, they had two fixed points, viz. mid-day and mid-night, which they called the fixth hour: fo that a meridian line would always point out the fixth hour, or mid-day. Neither have I been able to difcover when the modern Romans changed this method of computing time. In the course of the day and night they reckon twenty-four hours, which are all of an equal length in every season of the year. Noinconvenience can arife in reckoning twenty-four hours in place of twelve and twelve, as we do. Perhaps to far the modern Roman method is preferable to ours. But the difficulty is, that they do not begin to reckon their hours from a fixed point, viz. from mid-day, when the fun croffes the fame meridian line every day in the year. Thus they call half an hour after fun-set the twenty-fourth hour; and an hour and a half after fun - fet the firft hour, or one o'clock. Hence the nominal hour of mid-day conftantly changes with them: in June it is called fixteen, and in December nineteen o'clock. To regulate, therefore, a time - piece by this method of computing, it must be daily altered. *To reckon time from the fetting of the fun, was a very ancient cuftom: it was practifed particularly by the Germans and Gauls. It feems to be connected with the ideas which establish the existence of a chaos or night, before the world or day, See "Recherches fur l'Origine et les Progrès des Arts de la Grèce, par M. d'Hankerville." 1.1, C. 2, p. 131. MISCELLANEOUS. MISCELLANEOUS ESSAYS. Efay on the Means of confining Heat, and wood will manifest itself in a man directing its Operations. From Count Rumford's Effays, vol. 2, chap. 3. been difcovered. The utility of giving a wooden handle to a tea-pot or coffee-pot of metal, or of covering its metallic handle with leather, or with wood, is well known; but the difference in the conducting powers of various bodies with regard to heat, may be shown by a great number of very fimple experiments;-fuch as are in the power of every one to make at all times and in all places, and almost without either trouble or expence. If an iron nail and a pin of wood, of the fame form and dimensions, be held fucceffively in the flame of a candle, the difference in the conducting powers of the metal and of ner in which there will be no room left for doubt. As foon as the end of the nail, which is expofed in the flame of the candle, begins to be heated, the other end of it will grow fo hot as to render it impoffible to hold it in the hand without being burnt; but the wood may be held any length of time in the fame fituation without the leaft inconvenience; and, even after it has taken fire, it may be held till it is almoft entirely confumed; for the uninflamed wood will not grow hot, and, till the flame actually comes in contact with the fingers, they will not be burnt. If a fmall flip or tube of glass be held in the flame of the candle in the fame manner, the end of the glafs by which it is held will be found to be more heated than the wood, but incomparably lefs fo than the pin or nail of metal-and among all the various bodies that can be tried in this manner, no two of them will be found to give paffage to heat through their fubftances with exactly the fame degree of facility *. To show the relative conducting power of the different metals, Doctor Ingen. houz contrived a very pretty experiment. He took equal cylinders of the different metals (being straight pieces of stout wire, drawn through the fame hole, and of the fame length) and, dipping them into melted wax, covered them with a thin coating of the wax. He then held one end of each of thefe cylinders in boiling water, and obferved how far the coating of wax was melted by the heat communi cated through the metal, and with what celerity the heat paffed. Dd4 Το To confine heat is nothing more than to prevent its efcape out of the hot body in which it exifts, and in which it is required to be retained; and this can only be done by furrounding the hot body by fome covering compofed of a substance through which heat cannot pafs, or through which it paffes with great difficulty. If a covering could be found perfectly impervious to heat, there is reafon to believe that a hot body, completely furrounded by it, would remain hot for ever; but we are acquainted with no fuch fubftance; nor is it probable that any fuch exifts. Those bodies in which heat paffes freely or rapidly, are called conductors of heat; thofe in which it makes its way with great difficulty, or very flowly, non-conductors, or bad conductors of heat. The epithets, good, bad, indifferent, excellent, &c. are applied indifferently to conductors and to non-conductors. A good conductor, for inftance, is one in which heat paffes very freely; a good non-conductor is one in which it pafles with great difficulty; and an indifferent conductor may likewise be called, with out any impropriety, an indifferent non-conductor. Those bodies which are the worst conductors, or rather the beit nonconductors of heat, are beft adapted for forming coverings to confine it. All the metals are remarkably good conductors of heat; wood, and in general all light, dry, and fpungy bodies, are non-conductors. Glafs, though a very hard and compact body, is a non-conductor: mer cury, water, and liquids of all kinds, are conductors; but air, and in general all elaftic fluids, fteam even not excepted, are non-conductors. Some experiments which I have lately made, and which have not yet been publifhed, have induced me to fufpect that water, mercury, and all other non-elaftic fluids, do not permit heat to pass through them from particle to particle, as it undoubtedly paffes through folid bodies, but that their apparent conducting powers depend effentially upon the extreme mobility of their parts. In fhort, that they rather tranfport heat than allow it a paffage. But I will not anticipate a fubject which I propofe to treat more fully at fome future period. The conducting power of any folid body in one folid mafs, is much greater than that of the fame body reduced to a powder, or divided into many fmaller pieces. An iron bar, or an iron plate, for inftance, is a much better conductor of heat than iron filings; and saw-duft is a better non-conductor than wood. Dry wood-aflies is a better non-conductor than either; and very dry charcoal, reduced to a fine powder, is one of the best non-conductors known: and as charcoal is perfectly incombuftible when confined in a space where fresh air can have no accefs, it is admirably well calculated for forming a barrier for confining heat where the heat to be confined is intense. But among all the various fubftances of which coverings may be formed for confining heat, none can be employed with greater advantage than common atmospheric air. It is what Nature employs for that purpofe; and we cannot do better than to imitate her. The warmth of the wool and fur of beafts, and of the feathers of birds, is undoubtedly owing to the air in their interftices; which air, being ftrongly ftrongly attracted by thefe fubftances, is confined, and forms a barrier which not only prevents the cold winds from approaching the body of the animal, but which oppofes an almost infurmountable obftacle to the escape of the heat of the animal into the atmosphere. In the fame manner does the air in fnow ferve to preferve the heat of the earth in winter. The warmth of all kinds of artificial clothing may be fhown to depend on the fame caufe; and were this circumftance more generally known, and more attended to, very important improvements in the management of heat could not fail to refult from it. A great part of our lives is fpent in guarding onrfelves against the extremes of heat and of cold, and in operations in which the ufe of fire is indifpenfable; and yet how little progrefs has been made in that moft ufeful and most important of the arts, the management of heat! Double windows have been in ufe many years in moft of the northern parts of Europe; and their great utility, in rendering the houfes furnifhed with them warm and comfortable in winter, is univerfally acknowledged; but I have never heard that any body has thought of employing them in hot countries to keep their apartments cool in fummer yet how eafy and natural is this application of fo fimple and fo ufeful an invention! If a double window can prevent the heat which is in a room from paffing out of it, one would imagine it could require no great effort of genius to difcover that it would be equally efficacious for preventing the heat without from coming in. But na tural as this conclufion may appear, I believe it has never yet occurred to any body; at leaft, I am quite certain that I have never seen a double window either in Italy, or in any other hot country, I have had occafion to vifit. But the utility of double windows and double walls, in hot as well as in cold countries, is a matter of fo much importance, that I fhali takę occafion to treat it more fully in another place. In the mean time I fhall only obferve here, that it is the confined air fhut up between the two windows (not the double glafs plates) that renders the paffage of heat through them fo difficult. Were it owing to the increased thickness of the glafs, then a fingle pane twice as thick would anfwer the fame purpofe; but the increased thickness of the glafs of which a window is formed, is not found to have the leaft fenfible effect in rendering a room warmer. But air is not only a non-conductor of heat, but its non-conducting power may be greatly increafed. To be able to form a juft idea of the manner in which air may be rendered a worfe conductor of heat, or, which is the fame thing, a better non-conductor of it than it is in its natural unconfined state, it will be neceffary to confider the manner in which heat paffes through air. Now it appears, from the refult of a number of experiments which I made with a view to the inveftigation of this fubject, and which are publifhed in a paper read before the Royal Society, that though the particles of air, each particle for itfelf, can receive heat from other bodies, or communicate it to them, yet * See the Philofophical Tranfa&tions and our Register for 1792. there there is no communication of heat between one particle of air and an other particle of air. And from hence it follows, that though air may, and certainly does, carry off heat, and tranfport it from one place, or from one body to another, yet a nafs of air in a quiefcent ftate, or with all its particles at reft, could it remain in this ftate, would be totally impervious to heat; or fuch a mafs of air would be a perfect nonconductor. Now if heat paffes in a mass of air merely in confequence of the motion it occafions in that air, if it is tranfported, not fuffered to pafs, -in that cafe it is clear that whatever can obftruct and impede the internal motion of the air, muft tend to diminish its conducting power: and this I have found to be the cafe in fact. I found that a certain quantity of heat which was able to make its way through a wall, or rather a fheet of confined air, half an inch thick in nine minutes and threefifths, required twenty-one minutes and two-fifths to make its way through the fame wall, when the internal motion of this air was impeded by mixing with it one-fiftyfixth part of its bulk of eiderdown, of very fine fur, or of fine filk, as fpun by the worm. But in mixing bodies with air, in order to impede its internal motion, and render it more fit for confining heat, fuch bodies only must be chofen as are themselves non-conductors of heat, otherwife they will do more harm than good, as I have found by experience. When, in ftead of making ufe of eider-down, fur, or fine filk, for impeding the internal motion of the confined air, I used an equal volume of exceedingly fine filver-wire flatted (b.ing the ravellings of gold or filver lace) the paffage of the heat through the barrier, fo far from being impeded, was remarkably facilitated by this addition; the heat paffing through this compound of air and fine threads of metal much fooner than it would have made its way through the air alone. Another circumstance to be attended to in the choice of a fubftance to be mixed with air, in order to form a covering or barrier for confining heat, is the fineness or fubtility of its parts; for the finer they are, the greater will be their furface in proportion to their folidity, and the more will they impede the motion of the particles of the air. Coarfe horse-hair would be found to answer much worse for this purpose than the fine fur of a beaver, though it is not probable that there is any effential difference in the chymical properties of thofe two kinds of hair. But it is not only the fineness of the parts of a fubftance, and its being a non-conductor, which render it proper to be employed in the formation of covering to confine heat; there is still another property, more occult, which feems to have great influence in rendering fome fubstances better fitted for this use than others; and this is a certain attraction which fubfifts between certain bodies and air. The obftinacy with which air adheres to the fine fur of beafts and to the feathers of birds, is well known; and it may eafily be proved that this attraction muft affift very powerfully in preventing the motion of the air concealed in the interstices of thofe fubftances, and confequently in impeding the paffage of heat through them. Perhaps there may be another |