THE 



THE 



how, accordintr to flic taste of the 

 tlmtclicr. ]\.()ds willi straw lopc^s 

 twisleti roiiiul thcni are inserted near 

 the edge of the slanting side and all 

 along the eaves, winch prevent the 

 wind from hlowing oflthe thatch. 



"The only diderence in the thatch 

 of a round rick is, that it is brought 

 to one point, where it is tied with 

 straw rope wound round it, and form- 

 ed into a kind of how ; the rods are 

 inserted a little below in a circle, and 

 a straw rope twisted round them, 

 and likewise around the circular 

 eaves. Barley is generally put into 

 square stacks, and wheat in round 

 ones. When the outside is neatly 

 trimmed and cut smooth, so that no 

 birds can lodge in it, wheat may be 

 kept for years, without danger of in- 

 jury or loss, much belter than in a 

 barn, or even in a granary. 



" In thatching sheds and buildings 

 which are to last many years, the 

 straw is prepared in the same man- 

 ner, but the ends of the handfuls, as 

 they are put on a lathed roof, are 

 kept down by means of long rods, 

 which are tied to the laths of the roof 

 by means of strong tar twine. A 

 much thicker coat of straw is put on ; 

 and rye straw, which has a solid 

 stem, is preferred as more lasting, 

 and less liable to be filled with water 

 than hollow straw. Instead of straw 

 ropes, split willow is used, and the 

 rods which are inserted are much 

 nearer each other and more carefully 

 secured. As this kind of thatching 

 is a peculiar trade, it requires a reg- 

 ular apprenticeship to be master of 

 it. The thatching of temporary ricks 

 may he done from inere description, 

 and a very little practice will enable 

 any one to protect his stacks suffi- 

 ciently by a thatched covering." 



THECA. A case, usually the urn 

 of mosses, in which their spores are 

 situated. 



THECOSTOMES. Those insects 

 which have their suckers surrounded 

 bv a sheath or case. 



THEODOLITE. A surveying in- 

 strument for measuring both vertical 

 and horizontal angles, and necessary 

 iu accurate surveys. 

 792 



THEORY. The e.xpression of a 

 general law based on numerous as- 

 certained facts. An hypothesis is 

 merely a guess, without any basis on 

 fact. 



THERAPEUTICS. The science 

 which treats of the action of medi- 

 cines. 



THERMO-ELECTRICITY. The 

 study of the conditions for the produc- 

 tion of an electrical current by heat. 



" 'VA'hcn one part of a metallic bar 

 is heated and another cooled, an elec- 

 tric current is generated in its sub- 

 stance, which may be rendered evi- 

 dent, and its direction ascertained by 

 the galvanometer. When two metals 

 of different temperatures are brought 

 into contact, similar electric currents 

 are generated, the quantity and di- 

 rection of the electricity varying with 

 the nature of the metals and their re- 

 spective temperatures. The best ap- 

 paratus for exhibiting tiiese thermo- 

 electric currents consists of alternate 

 bars of antimony and bismuth sol- 

 dered together at their ends, so as to 

 form a compound bar or parallelo- 

 gram, the junctions of which may 

 be alternately heated and cooled. In 

 this case, the direction of the current 

 is from the antimony to the bismuth ; 

 so that these metals bear the same 

 relation to each other in the thermo- 

 electric series as the zinc and silver 

 in the simple voltaic circuit. The 

 term stcreo-elcclrjc has also been ap- 

 plied to these currents, implying their 

 production in solid bodies independ- 

 ent of a fluid, and as opposed to the 

 hydro-electric or voltaic current." — 

 {Rrandc.) 



THERMO-ELECTRIC PILE. 

 See above. 



THERMOMETER (from i?fp/^of, 

 irarm, and /xerpov, a measure). An in- 

 strument to measure sensible heat. 

 It most usually consists of a tube of 

 small bore, with a bulb at the lower 

 end containing mercury, which, by its 

 dilatation from heat, ascends witliin 

 the tube. There is a scale attached, 

 marked into degrees, and the altitude 

 of the mercury is measured by its 

 position opposite the degrees. The 

 marking is after the plan of Fahren- 



