obtained in practice. Double and independent coats of polished silver for the bulbs of thermometers have, we believe, been occasionally employed to resist radiation in all its forms; but where accurate experiments, on high temperatures, are to be made, we can recommend nothing equal to hanging the thermometer under the branches of a thick tree, abundantly surrounded by others; at a window little can be done when the air is not in a tolerable state of motion; pasteboard screens against particular reflecting objects, such as flat roofs, we have sometimes used, with advantage. Register thermometers are particularly liable to the effects of reflection, and all other disturbing causes in the indication of maximum temperatures, which has often caused us great inconvenience; polished copper cylinders, with free ventilation above and below, in which the instrument is placed, have been recommended for this purpose, particularly in tropical climates, where from the verticality of the sun, perfect shade is peculiarly difficult to be obtained. Reflection is so rarely separated from the influence of absorbed heat by surrounding objects, that the separation of them requires experiments peculiarly conducted, which, from the unparalleled unpropitiousness of the season, we regret not to have had an opportunity of completing. United with the effects of absorption, we shall presently give several examples. We have pleasure, however, in referring to the experiments of Captain Sabine and others, in Daniell's Meteorological Essays on this subject.

III. The first kind of ABSORPTION of heat we have to notice, is that by walls and other surrounding objects generally. Any extensive and fixed object in the vicinity of the thermometer, obviously tends to impart to it a constant share of heat, and subtract from it independent of the actual temperature of the air. The heat of an inhabited house is sufficient to influence considerably the indications of low temperatures made at the window. In the course of the diurnal rise of heat, the temperature of the mass of the building being lower, absorbs from the adjacent film of air, and the thermometer placed in it, a part of its heat. This cause, however, is generally not much felt, since the influence of reflection, or radiation, soon raises the temperature of the mass, which forming a receptacle for the accumulation of solar heat vaguely distributed through the atmosphere, rises, if its means of acquisition be considerable, to a temperature exceeding, by several degrees, the maximum in the shade, and as the diurnal temperature descends, the disproportionate slowness of the cooling of so great a mass, aided by new accessions of radiated heat, gives to the afternoon indications of the thermometer, a more than due height, a circumstance often of great inconvenience to the observers, especially as, in those seasons where considerable temperature becomes an object of scientific curiosity to measure, the sun's sweep in Azimuth in considerable latitudes, is so great as to make a series of observations at one station, during a considerable part of the day, very difficult, and still greater uncertainties are generally found in a new station; it

Exposed to bright horizontal sun.

In Table IV. No. 1, is a thermometer placed among trees in perfect shade. No. 2, a thermometer with metallic scale and glass case hung on a black painted pannel on the west side of a north window. No. 3, a large thermometer, mounted on metal, hung at the distance of one foot from the same window. No. 4, a small and delicate instrument, at two feet distance. It is worth observation that the reflection from grass at 30 feet below, was so considerable, that No. 4 always indicated a temperature higher than No. 3, which was much more liable, in every respect, to the effects of absorption. The last observation at 6h. 15' is inserted in confirmation of a remark we have already made upon the small influence of direct radiation upon a naked bulb when the sun is low,

The three thermometers used were constructed for the purpose, of small size and flattened bores, with light paste-board scales attached. No. 1 hung freely at the window sill already mentioned. No. 2 at 1 ft. 3 in. from the wall, and No. 3 at 2 ft. or double that distance. It will be observed that the height of No. 1 almost constantly exceeded that of the others by a less or greater quantity, but that the two last agreed perfectly till the shade of No. 3 was partially removed as the day advanced, a slight heat being communicated by the interruption of some of the rays by the frame to which the thermometers were attached; for the mere passage of the solar beams communicates no heat without the intervention of some object, and the direct distance of the grassy flat below, upon which they fell, was perhaps 40 feet. Had the day been unfavourable for the equilibrium of temperature, the differences would have been more considerable, but the wind being brisk from the N. W. diminished the sources of error. When the wind proceeds from points opposite to the sun at the hot part of the day, the instruments placed in the shade are of course most exposed to its influence, which often counterbalances all trifling causes; of course, in the N.E. point this reaches a maximum. The two following Tables will demonstrate its influence in annihilating the effect of absorption last illustrated, while they will show the constancy of the difference between observations made in contact with a building, and one defended from radiation in all its forms by the shelter of thick trees, the instruments being within not much exceeding 100 yards from each other. No. 1 was the large thermometer already mentioned, hung one foot from the window. No. 2 a small and delicate instrument at 2 feet. No. 3 the ac

12.0 70.4 70.25 70.5 109

1.0

2.0

3.0

69.169+ 68.5 104.5 70.4 70.5-70.75 104 68.869

The second kind of absorption we have mentioned is caused by the scales and mounting of thermometers, which is often done in a very unskilful way, and in the measurement of high temperatures is as fertile a source of error as any other. Not merely do the scales of thermometers, when large, act as increasing the mass of matter to be heated or cooled, but, as we have already explained in the case of buildings, magnify the influence of radiation and reflection, by accumulating the calorific rays which find their way to the instrument; and if the scale be metal, as is frequently the case, its rapid conducting power facilitates the communication. This is our principal objection to metallic scales. The adoption of glass cases also, to protect the instru ment from the effect of weather, is, to the last degree, prejudicial in retarding an equilibrium of temperature; the small hole at the bottom of such cases being a very tardy medium of communication. These remarks will be confirmed by the following table of the indications of three instruments, extremely differently mounted, and placed together at a free exposure at a window. The total effect of absorption will be seen by a comparison with a thermometer in absolute shade, No. 4.

No. 1 was a large thermometer, by Adie, attached to the case of a sympiesometer, of considerable mass. No. 2, a thermometer by Jones, with a metal scale of the usual size. No. 3, a small and delicate one, with light ivory scale by Cary. These were all hung together. Nos. 1 and 2, by the superior conducting power of their scales, kept nearly together till the maximum, but at 3h. 30m. the effect of the mass of No. 1 becomes evident by keeping up the temperature, while No. 2, losing its heat with facility, speedily began to decline. Taking also into consideration the third modification of absorption, as connected with the colour of the object upon which the instrument is suspended, we present the following table; in which No. 1 is the instrument No. 2 of the last table, but inclosed in a glass cylinder, and hung against a pannel of wood painted black, at the west side of a north window. No. 2 is the thermometer of a sympiesometer, hung on the opposite pannel, exposed without cover to the N.W. wind. No. 3 is the thermometric indication of the gaseous bulb of the same instrument, which, as usual, is protected by a polished brass cylinder, but admitting free ventilation. It was used by correcting the instrument for pressure instead of temperature, as is done when we wish to extract the barometric indications. No. 4 was a small thermometer with detatched scale freely suspended at the window sill. The differences are sufficiently marked, but had the day been more sultry and perfectly free from clouds, (which sometimes intervened) they would have been more striking.

ture of the atmosphere by observations at any one spot. Another illustration will be found in Table IV.

IV. HUMIDITY.-The first source of error, which we have classed rather from convenience than by strict theory under this head, includes the many irregularities which disturb the indications of the thermometer at the time of dew falling, a subject one of the deepest in meteorology, and which we merely introduced as one of the most important sources of error requiring to be thus scantily mentioned in this place. And we cannot help observing, that in the attempt to register faithfully the true atmospheric temperature for twenty-four hours in summer, the most difficult point of management is about sunset, when we are driven from the trees by the copious deposition of humidity, and temporary decline of local heat, to the alternative of a building, the whole of which is more or less affected by the absorbed heat of the preceding day.

No. 1 is in this situation, being the thermometer with a metallic scale, the same as No. 1 in the last table, placed, as there described, in a common glass cylinder, on a black pannel facing the east, at a north window. No. 2 is No. 2 of last table, and freely suspended for comparison, the observations commencing above an hour after sunset, and the differences regularly diminishing, and scarcely vanished after an hour and a half, a curious fact, showing the intrinsic difficulty of obtaining the tempera

Table XII. is illustrative of the same facts, and the differences of the temperature of the grass; and the air four feet above it (the numbers being of exactly the same import as in the last table) are still more remarkable. But we have not room at present to enter into the consideration of these facts.

With regard to the proximity of water, the other source of humidity mentioned, we have nothing particular to add; the cold produced by the solution of water is well known to be subjected to certain hygrometric laws, and this influence communicates an absolute degree of cold to any film of air actually in contact with water. The proximity of the sea has besides a general influence on climate,

by tending to flatten the form of the annual curve, its temperature diverging little from that corresponding to the latitude.

With these details, for the imperfection of which we have already apologised, we must conclude at present the subject of thermometric errors, and with them the present article. The meagre illustration we have been able to give of these remarks, show that abundant scope is given for extended researches upon a systematic plan, with the certainty of results of practical value, which ere this the author of the present article had hoped to have filled up, but at least we are convinced that we should not have done justice to the thermometer as an instrument, without some attempt to direct the observer in the practical use of it. Our limits do not permit us to enter more fully upon the means of preventing errors in practice, but the detail of the sources of these has almost formed a sufficient guide.

We here subjoin a very complete table of phenomena occurring at particular temperatures,

122

Phosphorus burns vividly (Fourcroy) 148 (Thomson).

126

Bisulphuret of carbon boils.

127

Tallow melts (Nicholson) 92 (Thomson).

140

Liquid ammonia boils.

145

Camphor sublimes (Venturi).

149

150

155

145 Ambergris melts (La Grange).

Bees wax melts.

Potassium melts.

Bleached wax melts (Nicholson).

165 Albumen coagulates 156 (Black).

170 Sulphur evaporates (Kirwan).

176 Alcohol boils, 174 (Black) 173 (specific

gravity 800).

30

Nitric acid, specific gravity 1.407.

23

22

Acetous acid freezes.

212 Bismuth, 5 parts, tin 3, lead 2, melt.

218

11

2 alcohol 1 water freezes.

Sulphur melts (Thomson), 238 (Hope), 212 (Fourcroy), 185 (Kirwan).

Phosphorus distils (Pelletier).

Water saturated with common salt boils.

For the higher or pyrometric temperatures we refer to the article PYROMETER, as likewise for the expansion of different substances.

On the subject of the THERMOMETER, the following works may be consulted:-Sanctorii Commentaria in Avicennam. Saggi di Naturale Esperienze, vol. i. Van Helmont Opera; Sturmius, Collegium Naturale Curiosum, Nuremburg 1666; Martine on Thermometers; Newton, Principia; Hooke, Micrographia; Boerhaave, Chemiæ, i. 720; De Luc, Modifications de l'Atmosphere; Leslie on Heat; Rumford's Essays; Crawford on Animal Heat; Dalton's Essays; Nicolson's Chemistry; Henry's Chemistry, vol. i. and Appendix to vol. ii.; Daniell's Meteorological Essays; Biot, Traité de Physique, vol. i.; Parry's Second Voyage, Appendix.

Boyle, Hook, Newton, Fahrenheit, Cavendish, Hutchins, Wilson, Six, Rumford, and others, in the Philosophical Transactions; Keith, Hope, Blackadder and Brewster, in the Edinburgh Royal Society's Transactions; Bouvard on the Meteorological Observations made in the Observatory at Paris, in the Memoirs de l'Institut; sundry papers in the Journal

of the Royal Institution; Nicolson's Journal; Edinburgh Philosophical Journal; Dr. Brewster's Jour nal; the Bulletin des Sciences; the Annales des Mines; the Annales de Chemie; the Bibliotheque Universelle; the Giornale de Fisica, &c. &c.

See also the articles ATMOSPHERICAL CLOCK, CHEMISTRY, HEAT, METEOROLOGY, PYROMETER. And upon particular subjects, see the references in the course of the preceding paper. (A) THERMOPYLE, Battle of, See GREECE, Vol. X. p. 72.

THETFORD, a market town of England in Norfolkshire, situated at the confluence of the Little Ouse and Thet, the first of which flows through the town. It consists of several streets, the principal of which are in the form of a cross. In the time of Edward III. it received the name of Hierapolis and Monachapolis, from its having twenty churches and eight monasteries. At the same time, it had twenty-four streets and six hospitals. The principal buildings are three churches, St. Peter's, St. Cuthbert's, and St. Mary the less, the guildhall, a handsome market-house, a new bridge, a county gaol, a bridewell and workhouse. There is also here a free school, an hospital and several alms-houses. As the Little Ouse is navigable, Thetford enjoys considerable trade, exporting woollen goods, corn, malt, beer, paper, &c. There are three breweries in the town, a circulating library, and a printing office. The chief manufactures are woollen goods, malt, paper, wire tools, and agricultural machinery. Population of the burgh, in 1821, 579 houses, 703 families, and 2922 inhabitants. See Beauties of England and Wales, vol. xi. p. 241.

THIBET or Tibet, an extensive country of central Asia, is bounded on the south by the Birman Empire, Assam, Bootan, Sirinagur, and Hindostan: on the west by Great Buckharia; on the north by Eastern and Western Tartary; and on the east by China. These boundaries, particularly on the north and south-east, have not been exactly ascertained. On the north, there appears to be provinces, to which our geographical knowledge does not yet reach. The extent of this country must, of course, be a little uncertain; but its length from east to west has been estimated at 1500 miles: its breadth from north to south at 500.

Thibet, the name by which this extensive territory is known to us, is a term not much used in the country itself. It seems indeed to be known to different people under different names. The Chinese, for example, call it Dshan. The Mongols or inhabitants of Tartary term it Baran-tala. The natives themselves call it Püe or Puë Koachim-the word Püe signifying north, and Koachim, snow; terms, as we shall see, admirably descriptive of the country. (Captain Turner's Account of an Embassy to the Court of Thibet in the year 1783, p. 305.) The

appellation Thibet, which in the east is pronounced Tibbit or Tibt, may probably signify, as has been conjectured, the kingdom of Boot or Budha, or the divine kingdom, in reference to its being the chief seat of a religion which prevails over the whole of central Asia. But on conjectures such as these, no great confidence can be placed.

An uncertainty, similar to that which attaches to its name and boundaries, prevails as to its divisions: an uncertainty which the mixture of Thibetian, Chinese, and Mongolian names render the more inveterate. It is said to contain eight provinces; but the most common division is Upper, Middle, and Lower Thibet; or Little Thibet, and Thibet Proper or Great Thibet.

Thibet forms the highest table-land in Asia, and is the nucleus from which the largest rivers of that continent take their rise, and flow towards every point of the compass. The country exhibits few high mountains in its interior, but its frontiers, particularly on the south and west, can boast of mountain chains, the peaks of some of which are the most elevated known in the world. Thibet is remarkable rather for its general elevation as an