THI 



THO 



heit, 32"= Fahrenheit being the freez- 

 ing of water, and 212= Fahrenheit 

 its boiling point. The {zero) is as- 

 certained by the cold produced from 

 mixing snow and salt so as to melt. 



The centigrade scale is used by the 

 French, in which the zero is the 

 freezing of water, and 100= cent, its 

 boiling point. 



The thennometcr is of great ser- 

 vice to the gardener in the hot-house, 

 to regulate its temperature. Ruther- 

 ford's thermometer marks the lowest 

 and highest degrees of temperature 

 during any given time. " It consists 

 simply of two thermometers : a mer- 

 curial thermometer. A, and a spirit 

 thermometer, B, attached horizontally 



o the same frame, and each provi- 

 ded with Its own scale. The index 

 of A is a bit of steel, which is pushed 

 before the mercury ; but, in conse- 

 quence of its horizontal position, re- 

 mains in its place when the mercury 

 recedes, and consequently indicates 

 the highest degree of the scale to 

 which the mercury has risen. The 

 index of B is of glass, with a small 

 knob at each end. This lies in the 

 spirit, which freely passes it when 

 the thermometer rises ; but when the 

 spirit recedes, the cohesive attraction 

 between the fluid and the glass over- 

 comes the friction arising from the 

 weight of the index, and the index is 

 consequently carried back with the 

 spirit towards the bulb. As there is 

 no force to move it in the opposite 

 direction, it remains at the point near- 

 est the bulb to wliich it has been 

 brought, and tlius indicates the low- 

 est temperature which has occurred. 

 By inclining the instrument, the in- 

 dices are brought to the surfaces of 

 liieir respective fluids, and prepared 

 for a new observation." 



Til Ell. M OS COPE. A general 

 term, including any implement for 

 measuring heat. 



THICKET. A dense accumula- 

 tion of shrubs or young trees. 

 X X s 



THILL. The central beam or 

 draught-tree of a wagon. 



THINNING OUT. Removing 

 some plants or branches of a tree to 

 give the rest a better opportunity of 

 growing large. It is an important 

 operation in the orchard, as a tree 

 overburdened with fruit will not pro- 

 duce such fine specimens as one that 

 has a few to perfect. 



THISTLE. The genus Carduus. 

 The commonest kinds in the United 

 States are the C. lanccolatus and ar- 

 Tcnsis : the latter is the well-known 

 Canada thistle. Considered as a 

 weed in our fields, our principal ob- 

 ject is to eradicate it, which, in con- 

 sequence of the ready dispersion of 

 the seeds by the wind, is not easily 

 done, as a slovenly farmer may seed 

 the whole country around. 



" Those crops which are usually 

 hoed can readily be cleared of this- 

 tles ; but where the seed is sown 

 broad-cast, the labour of weeding 

 them out is much greater. If they 

 are not extracted with the root, they 

 will soon grow again with redoubled 

 vigour. In a moist season they may 

 be pulled up by means of a wooden 

 or iron pincers, which grasps them 

 strongly near the crown of the root. 

 AMien a field has been long infested 

 with thistles, the best way of clear- 

 ing it is to watch when the thistle is 

 in full bloom and the seed just form- 

 ing ; if it be then cut off at the root, 

 it will die. Thus in two years a field 

 may be entirely cleared of thistles. 



" It is chiefly in arable land that 

 thistles are most troublesome. In 

 pastures it is sufficient to eradicate 

 them once, and to permit none to 

 grow along the hedges and ditches. 

 The seed does not readily vegetate, 

 unless it finds a loose soil ; and little 

 birds are so fond of it, that they will 

 leave none that is not covered with 

 earth, especially in the beginning of 

 winter." 



T H O R A C I C. Relating to the 

 chest. The thoracic duct is a vessel 

 which receives the contents of the 

 iacteals and absorbents, and con- 

 veys it to the blood by the subclavian 

 vein. 



793 



