T ii i; 





T ii i: 



point* 



Their i. utly indicate the 



at which the i-niU of th. have rtood 



:lu- expansion iil' (hi- spirit in 



' 





' 

 









cxp 



are 



bf'n 

 and 



ards, ' /; remaining : 



..it in 

 in fo. 'i 



>rale belo 1 : ich ( ' 







indicate the same degree on the two 

 the indices in and /; 



may hi n to tlne points: from thence afler- 



!>y a decrease, and the hit; 

 an inc. npvrature. 



An instrument of this kind is generally used for ascer- 

 taining the temperature of tl 

 depths, or of the atmosphere at great heights. 



l)if,-rcntKil 'i r. This instrument, which was 



invented by M. Sturmins, of Altdorf, 

 and v d by Professor Leslie' in 1SO-4. 



tvT thermometer tubes, terminating, at one extremity of 

 each, in a hollo 



phuric acid: tli- extremities are united by the 



flame of a blow-pipe, and an enlargement of the b. 



made at the place of junction. The 



O/~\ tube is then bent so as to form three 

 \ J sides of a rectangle, the two 



which arc of equal diameter, form- 

 ing the upper extremities of two 

 sides; and the instrument is on a 

 stand with the branches of the tube 

 in vertical positions. Wl 

 temperature of the air in the two 

 i he same, the acid occupies 

 one side and the base, and 

 little way up the other side of the 

 |[ rectangle. To the latter . side is at- 



tached a graduated scale, with the 

 zero of which the upper extremity 

 ' ^ . of the aciJ in that branch should 



iiiir..i*niiai Ttimnometw. coincide. In HID event of th 



inTeiiM liy M. Murrains, of justment '. 



Alldorf. antity 



of air to pass from one ball to done 



simply by the warmth of a hand applied to that ball from 

 when' .en. 



The variations of temperature in the apartment wil 

 dently have no effect on the instrument, since the action 

 upon the two balls will be equal : but if one ball alone be 

 if the acid in the other will immediately 

 indicate the difference between the tern, of the 



media about, the two balls by th f the 1 i xpu 



of the air in one ball above the expansion in the < 

 The <: ,,-h that the least dif- 



ferenc. 



nt of tl 



pose of mea.Miring t 

 tion, an inst 



; a boll 

 one end to a therm 



is immediately made sensible by 



i~ or the pur- 

 radia- 



:- nulled at 

 at the upper extremity by a ball, wi. 



,|) with wax, which is to I > ; when 



is to be eli ai lid. The other 



the s;, itK in a collar of v 



will, a ,|et ,, i 



and is enclosed in a box, which is blackened on 

 interiorly, and ha* a thick gla^s m I 

 crew is to diminish (ir in 

 di-r if nccewar)', driving a i 

 with. 

 Utr and tl.. : ut the th 





r with the liquid 



in an unbroken column. When the instrument is to be 

 I in a horizontal position with the glass 



In : lent is disposed 



n the 



liquid will mour ' in tlie thermometei 



should be allowed to do so lor three or four mi: 

 which, bv turning tin the liquid 



tie zero of r nnds 



to minute is ii. .. the 



'he instrument. ' 



low with his eye the t .and, 



counting i wnteh. when the six- 



Ihe number 



to the top of the column ; 

 then, waiting till the 



niniife. t! -ing eoliium as 



ntinp the beats by his ear. at the end of 

 the minute he . ately the height < 



column ot liquid. The instrument b'eiiiit then drawn into 

 the shade, or covered with a screen, a pair of observations, 

 at the inleival of one minute, are made and n 

 before, the liquid descending in the tube ! 

 two observations. The instrum> that 



the sun may shim- on it. and afterwar> when 



two other pairs of observations are made, and so on. 

 A mean of the two differences between the read in 



two ni. rvations while the sun shone on the i 



rnent, added to the difference between the readings at the 

 intermediate observations while the instrument was in the 

 shade, is taken as a mra.-urc of the intensity of tin 

 radiation at the middle time between the tirM and third 

 observations ; and a mean o'.' such results for all the triplets 

 of observations is considered as the general mean. 



approximation to the measure of solar radiation may 

 be obtained by simply exposing a register thermometer 

 with a blackened ball to the direct action of the sun's 

 [RADIATION.] The thermometer should be p!: 

 inches above the ground, and 1 a currents 



of air; and the graduations should be made on the 

 of the thermometer, in o: 

 from the expansion or warping of the- scale. 



The force of terrestrial radiation may be measured by 

 the minimum temperature of a register HUTUUM. 

 ball is placed in the focus of a parabolical mi. 

 o of the mirror is to be turned towards the face of 

 . but awav from the rays of the sun. 



THERMOMETER, DIFFERENTIAL. [THEMIOMK- 



TEU.] 



THERMOTYL/E. [ZuiTOt'N, GULF OF.] 

 THERMOSTAT, or In, ,-. an apparatus in- 



vented and patented in 1831 by Dr. tire, for rcgu! 

 temperature in the ) ion and distilla- 



tion, in heating baths and hothouses, in adjusting 1 1n- 

 draft of .stoves and furnaces, in ventilating apartm 

 &c. It acts upon the principle that, when two thin 

 tallic bars, of different degrees of expansibility, are n. 

 or soldered together side by side, any change of tempera- 

 ture will CM in the compound bar: 

 the side consisting of the least extensible metal becoming 

 concave, and the other convex. By this flexure of the 

 compound bar. which takes place wiih considerable force. 

 a movement is effected, which, by the intervention of 

 may be made to open or close stop-cock*, dampers, 

 scntilators, ~crip1iou oi valves, and thereby to 

 regulate the How of heated liquids or the admission and 

 emission of air. Tin- compound bars frequently consist of 

 thin pieces of >teel and hard hammered brass, iheted to- 

 : but various other metals may be employed, anil 

 nay be used for "one part of 'the bar. 



The principle of the thermostat may be applied in many 

 different, \\a\s. of which the following may 

 examples. In /'/i. I, a is the compound bar. which is 

 firmly ti\cd at //. and, when exposed to the ordinary tem- 

 thc atmosphere, remains in the straight hori- 

 zontal position shown in the cut. To tin 



the compound \ eoli- 



; rod. the short end of a lever mounted upon the 

 axis of a circular revolving vai :. c ; and 



from the longer end of tl. 'Hug 



valve, or damper. '/. Uy increasing . rature of 



the chamber or vessel in which the them. ;ieed, 



the compound bar will assume the curved form indicated 



