THE
order on the roof, is fixed on with laths, withies, or ropes. Ruft. Di&. in voc.
THEA, the tea-tree, in botany, the name of a genus of plants, the characters of which are thefe. The cup is a very fmall, plane, permanent perianthium, divided into fix roundifh, obtufe leaves. The flower conlilts of fix large roundifh, concave, and equal petals. The ftamina are numerous fi- laments, about two hundred, and are very (lender, capillary, and fhorter than the flower. The anthers; are ample. The germen of the piftil is globofe and trigonal. The ftyle is Tubulated, and of the length of the ftamina. The ftigma is fimple. The fruit is a capfule, formed of three globular bodies, growing together: it contains three cells, and opens into three parts at the top. The feeds are fmgle, globofe, and internally angulated. Linnet Gen. Plant, p. 233. Of this genus there is only one known fpecies, which is the tree whereon our common tea is produced. See the article Tea, Cyci. , , e
THEAVE, among country people, denotes an ewe lamb of the firft year. Ruft. Die*, in voc.
THERMOMETER (Cyd.)— In the Philofophical Tranfafli- ons b we have the defcription of a thermometer, made with a rod of metal, either brafs or iron. — [ b N° 485. p. 128- 130.]
This inftrument is compofed of an upright ftaff or bar of the bed iron, four feet long, and an inch and a quarter broad, having a polifhed brafs bar of the. fame length and width fcrewed to it before it, with four fteel fcrevvs, and being alfo capped with fteel, and thereon a lever moving up- on a find of fteel, which communicates with another lefs lever, which is alfo upon a flud, having a chain at the end of it, which laps round an axis, whereto the index is fixed, which fhews the degrees marked on a femicircular arch. Under the fteel fcrew heads there are fmall flits in the brafs bar, except the lowermoft which is fixed, which admit of its expanding, whereby it protrudes and operates on the firft mentioned lever, which being raifed moves the lefs le- ver, and thereby draws the chain which turns the axis af- fixed to the index, which fhews the degree of warmth of the weather marked on the femicircular arch. There is a fcrew through two ftuds to draw the great lever backwards and forwards, as occafion may be; and there is alfo a coun- terbalance to the fmall lever, to draw the hand back when the brafs bar fhrinks.
Dr. Mortimer laid claim to a like invention, and gave the defcription of his thermometer in the Phil. Tranf. N" 484, in the Appendix, p. 672.
The late Dr. George Martine, in his Book of Effays printed at London 1740, has treated the fubject of thermometers very fully. Among other curious things, he has given a plate of feveral thermometers, which he compares, in order to fhew the correfponding degrees in each. As a fixed and unalterable point of heat is not yet found, the coniiruction of thermometers flill remains imperfect. For the heat of boiling water is not always precifely the fame, neither can we depend abfolutely on the point of freezing. Fahrenheit has placed the freezing point at 32, and that of boiling water at 21 2; fo that he divides the diftance between the freezing point and that of boiling water into 180 de- grees.
Sir Ifaac Newton % in his fcale of the degrees of heat, marks the freezing point o, and that of boiling water 34. Hence one of his degrees correfponds to 5,^- degrees of Fahrenheit's thermometer. — [ e See Phil. Tranf. N° 270. J It is to be obferved, that if two thermometers be filled with different fpirits, they cannot be adjufted to correfpond by comparing together their fcales : for example, if the firft thermometer rifes 4 divifions, when the fecond rifes but 3, it is not to be expected that the fecond fhall juft rife 6, 9 or 12 divifions, when the firft riles 8, 12 or 16; becaufe the fpirit will not dilate in one in the fame proportion that it does tn the other; fo that unlefs the liquor be made to go its whole range in each of them, and the one be new mark- ed for every degree of the other, they will not be brought lb correfpond. Mr. de Reaumur being aware of this, at- tempted in a very ingenious way to eftablifh a general con- firmation of fuch thermometers, which might be copied at all times, and in all countries; and fo to fettle a general cor- refpondence of obfervations to be made by fuch inftruments. He took a large ball and tube, and knowing well the con- tents of the ball and that of the tube in every part, he gra- duated the tube, fo that the fpace from one divinon to an- other might contain -pr&G P art or " the liquor, which con- tained 1000 parts when it ftood at the freezing point. Then putting the ball of his thermometer and part of the tube into boiling water, he obferved whether it rofe 80 di- vifions, which if it exceeded, he changed his liquor, and adding water to it lowered it fo, that on the next trial from the freezing point to the point of boiling water it fhould only rife 80 divifions : but if the liquor being too low, fell fhort of 80 divifions, he raifed it by adding rectified fpirit to it. The liquor thus prepared fitted his purpofe, and would ferve for making a thermometer of any iize, whofe fcale would agree with his ftandard. Such liquor or fpirits
THE
being nearly of the ftrength of brandy may be eafily had, and may be made of a proper degree of deniity by railing or lowering it.
Dr. Martine finds fome faults in this thermometer; one of which is, that the ball or bulk of the thermometer being large, is not heated or cooled foon enough to fhew the quick va- riation of the weather. And, indeed, this is a faul: com- mon to all thermometers, which have bulbs to hold their li- quor; a cylinder being much better, whatever liquor is ufed, except where great degrees of heat are to be meafured, as in Sir Ifaac Newton's linfeed-oil thermometer. For though mo-fl fpir'it thermometers have the degree of the heat of boiling water marked upon them, as one of their boundaries; yet the heat of boiling water is always greater than that of boil- ing fpirits, and therefore they are unfit to meafure that de- gree of heat. But linfeed-oil is capable of fu (raining much greater degrees of heat; for it will bear a greater degree than what will melr lead, without firing or having the glafs bail of the thermometer melted. Whereas water is only capable of a certain degree of heat, much lower, at which it will evaporate; but this is only when water boils in open vcffels. Of late years qutckfilver has been made ufe : of for thermo- ?neters, and thefe are found to be the moft ufeful of any; becaufe they will bear fuch degrees of heat or cold as will burfl (pint-thermometers, or freeze the liquor in them. This Iaft inconveniency happened to the French philofophers, who went to the north polar circle to examine into the figure of the earth; for the fpirit in their thermometers froze, but their mercurial ones were as ufeful as any wheie elfe. Fahrenheit of Amfterdam may be looked upon as the inventor of this thermometer; and though Prins, and fome others in Eng- land, Holland, France, and other countries, have made this inftrument as well as Fahrenheit, yet ftill they may be called Fahrenheit's, as being graduated according to his fcale. For the different kinds of thefe thermometers, fee Defaguliers, Exper. Phil. Vol.11, p. 295. See alfo Dr. Martinis Eflays before mentioned.
Sir Ifaac Newton filled his thermometer with linfeed-oil, which will bear a very great heat. He aflumes the rare- faction of the oil to be proportional to its heat d; and this affumption feems juft from his experiments. However, it were to be wifhed that this fubject were farther examined. — [ d See the Phil. Tranf. loc. cit.j
Dr. Hales places the freezing point at 0, and the heat of water on which floating wax begins to melt at 100. In his thermometer the heat of boiling water anfwers to 1464. We fhall here infert a table of fome obfervations made, with the thermometers of Fahrenheit, Reaumur, Sir Ifaac Newton, and Dr. Hales, communicated by Mr. Labelye.
Obfervations by Fahrenheit's thermometer.
Boiling water.
Brandy boils.
Alcohol boils.
Serum of blood, and white of eggs hardens.
Killing heat for animals, in a few minutes.
A hen hatching eggs, but feldoin fo hot.
Heat of fkin in ducks, geefe, hens, pigeons, par- tridges and fwallows.
Heat of fkin in a common ague and fever.
Heat of fkin in dogs, cats, fheep, oxen, Twine and other quadrupeds.
Heat of the human fkin, in health.
Heat of a fwarm of bees.
A perch died in three minutes, in water fo heated.
Heat of the air in the {hade, in very hot weather.
Butter begins to melt.
Heat of the air in the ihade, in warm weather.
Temperate air, in England and Holland.
Oil of olive begins to ftiffen and grow opaque.
Water juft freezing, or fnow and ice juft thawing.
Milk freezes.
Urine and common vinegar freezes.
Blood out of the body freezes.
Good Burgundy, ftrong claret and Madera freezes. 1 One part of fpirits of wine mixt with three parts ' J water freezes.
5 Greateft cold in Penfilvania in 1731-2, 40 lat. 4 Greateft cold at Utrecht in 1728-9.
? A mixture of fnow and fait, which is able to freeze j oil of tartar per dcliquitfm, but not brandy.
We muft here obferve, that the heat of a hen hatching chickens is placed, by this table, at 108 of Fahrenheit's thermometer; but it appears from Mr. Reaumur's experi- ments, that eggs will hatch in a heat no greater than that of the human fkin. See Hatching, Append*
2. Obfervations by Reav?wr's thermometer.
9 7 -J- Anfwers to the heat of boiling water.
80 Spirit of wine in Reaumur's thermometer boils.
j . Gaateft
Deg. '
at 212 190 174 156
146 108
from 107 \ to 103 J at I oft
from 103 ) to 100 5
from 99 7 to 92 J at 97 96 80
74 64
43 3* 3°
28
25
