~—- — 
THEAVE. 
preparing it is simply to take a sheaf of wheat, 
and to place it in a press, made of two pieces of 
timber 10 feet long, and put on a stool, and to 
have women to lay hold of the ears and draw 
them out and cut them off, and then to bind up 
the sheaf for thatch. A covering with this straw 
is not only neat but elegant, while one with the 
thrashed straw is ragged and slovenly ; and it 
has, in many instances, been preferred to all 
other coverings for cottages ornées in conse- 
quence of its picturesque effect. Rye straw is 
also neater, more durable, and less liable to be 
filled with water than common thrashed wheat- 
straw, in consequence of its having a solid stem. 
The art of fixing thatch on houses is similar to 
that of fixing it on ricks, but is much nicer and 
more elaborate, and requires thicker layers, 
stronger fastenings, and split willows or some 
similar materials instead of straw-ropes, and is 
properly the work of a separate class of artifi- 
cers, who qualify for it by a regular apprentice- 
ship. 
THEA. See Tua. 
THEAVE. An ewe of the first year. 
THEEZAN TEA. See Buckruorn. 
THELYGONUM. See Doe’s Canpace. 
THELYMITRA. A genus of curious, tuber- 
ous-rooted, herbaceous, Australian plants, of the 
orchis order. Hight or nine species, principally 
blue-flowered, and all about a foot high, bloom- 
ing from April till June, loving a soil of loamy 
peat, and propagable by division of the root, 
have been introduced to the greenhouses of Bri- 
tain. See the article Orcuis. 
THEOBROMA. See Cacao. 
THEOPHRASTA. A small genus of orna- 
mental, exotic, ligneous plants, of the dog’s-bane 
family. Jussieu’s species, 7. Jussice2, is a white- 
flowered, evergreen shrub, of about 3 feet in 
height, introduced 30 years ago to the green- 
houses of Britain from Central America. It re- 
quires a soil of rich mould, and is propagated 
from seeds. 
THERMOMETER. An instrument for mea- 
suring heat, founded on the principle that the 
expansions of matter are proportional to the 
augmentations of temperature. With regard to 
aeriform bodies, this principle is probably well 
founded; and hence our common thermometers 
may be rendered just by reducing their indica- 
tions to those of an air thermometer. Solids, 
and still more liquids, expand unequally, by 
equal increments of heat, or intervals of temper- 
ature. With regard to water, alcohol and oils, 
this inequality is so considerable as to occasion 
their rejection for purposes of exact thermome- 
try. But mercury approaches more to solids 
than ordinary liquids in its rate of expansion, 
and hence, as well as from its remaining liquid 
through a long range of temperature, is justly 
preferred to the above substances for thermome- 
tric purposes. A common thermometer, there- 
fore, is merely a vessel in which very minute 
THERMOMETER. 497 
expansions of mercury may be rendered percep- 
tible, and, by certain rules of graduation, be 
compared with expansions made on the same 
liquid by other observers. The first condition is 
fulfilled by connecting a narrow glass tube with a 
bulb of considerable capacity filled with mercury. 
As this fluid metal.expands one sixty-third by 
being heated in glass vessels, from the melting 
point of ice to the boiling point of water, if ten 
inches of the tube have a capacity equal to one 
sixty-third of that of the bulb, it is evident that, 
should the liquid stand at the beginning of the 
tube at 32°, it will rise up and occupy ten inches 
of it at 212°. Hence, if the tube be uniform in 
its calibre, and the above space be divided into 
equal parts by an attached scale, then we shall 
have a centigrade or Fahrenheit’s thermometer, 
according as the divisions are 100 or 180 in num- 
ber. Such are the general principles of the con- 
struction of thermometers. 
The tubes drawn at glass-houses, for making 
thermometers, are all more or less irregular in 
the bore. Hence, if equal apparent expansions 
of the included mercury be taken to represent 
equal thermometric intervals, these equal expan- 
sions will occupy unequal spaces in an irregular 
tube. The attached scale should, therefore, cor- 
respond exactly to these tubular inequalities ; 
or, if the scale be uniform in its divisions, we 
must be certain that the tube is absolutely uni- | 
form in its calibre. The first step in the forma- 
tion of this instrument, therefore, is to graduate 
the tube into spaces of equal capacity. A small 
caoutchouc bag, with a stop-cock and nozzle, 
capable of admitting the end of the glass tube 
when it is wrapped round with a few folds of | 
tissue paper, must be provided, as also pure 
mercury, and a sensible balance. Having ex- 
pelled a little air from the bag, we dip the end 
of the attached glass tube into the mercury, and 
by the elastic expansion of the caoutchouc, we 
cause a small portion of the liquid to rise into 
the bore. We then shut the stop-cock, place 
the tube in a horizontal direction, and remove it 
from the bag. The column of mercury should 
not exceed half an inch in length. By gently 
inclining the tube, and tapping it with the fin- 
ger, we bring the mercury to about a couple of 
inches from the end where we mean to make the 
bulb, and, with a file or diamond, mark there the 
initial line of the scale. The slip of ivory, brass, 
or paper, destined to receive the graduations, 
being laid on a table, we apply the tube to it so 
that the bottom of the column of mercury coin- 
cides with its lower edge. With a fine point, we 
then mark on the scale the other extremity of 
the mercurial column. Inclining the tube gently, 
and tapping it, we cause the liquid to flow along 
till its lower end is placed where the upper pre- 
viously stood. We apply the tube to the scale, 
taking care to make its initial line correspond to 
the edge as before. A new point for measuring 
equal capacity is now obtained. We thus pro- 
wy. — a 
