22 



REPORT — 1876. 



narrow water-tight trough formed of three or four rectangular washers of 

 caoutchouc laid on a shest-caoutchouc floor, upon which the sides of the rectan- 

 gular tin bath, open at the top and bottom, are pressed down. The tin bath is 

 5 inches long (the same as the width of the rock-sections), nearly the same 

 height, and 2 inches wide ; and it is provided with a false bottom, through 

 the perforations of which the water reaches the wires, and is kept agitated 

 above by a thermometer passing through a longitudinal slit in the lid and 

 attached to a small tin blade, without injuring them. The twenty-four 

 extremities of iron wire projecting 1 or 2 inches beyond the bottom of the 

 bath are there soldered to the feet of twenty-four teeth of the commutator, 

 and the twelve iron wedges of the hand-rack being inserted between the 

 points of these teeth, completes the circuit connexion in the ordinary way for 

 observing a diffei'ence of temperature between the two principal sets of 

 junctions of the thermopile. As a proof of the trustworthy action of the 

 instrument, it may be mentioned that when, in the course of an experiment, 

 the reading of the galvanometer with the thermopile thus joined up was 

 being noted, and water of various temperatures from 60° F. to 160° F. was 

 poured into the bath where the twenty-four supplementary junctions are 

 placed and are all included in the circuit, not the smallest effect was pro- 

 duced upon the reading as soon as the water in the bath had by gentle 

 agitation become uniform throughout in temperature. Not only are the two 

 opposing sets of twelve junctions heated in the bath on the average all of 

 exactly equal force, so as to balance each other, but the false currents, which 

 in such ranges of temperature must be evoked with sensible intensity if any 

 of them should prevail, either neutralize each other exactly or are entirely 

 absent, as it appears equally probable to conjecture, in this portion of the 

 apparatus. As regards formation of the circuit through one of the principal 

 sets of junctions only, accompanied by a corresponding set of junctions in the 

 bath, this is accomplished as is represented in the annexed outline sketch, 

 where two pairs of junctions only 

 (a b, a V), above and below the rock- 

 plate, are shown, thin lines represent- 

 ing iron and thick lines German-silver 

 wire. B is the bath in which the 

 supplementary junctions, s'sss', ob- 

 tained by severing the loops of German- 

 silver wire, as at ss, are immersed. 

 The two extreme half-loops and cor- 

 responding teeth of the commutator 

 serve to complete the circuit with the 

 galvanometer; and the arrangement 

 for every additional severed loop of 

 German-silver wire introduced be- 

 tween them will easily be apprehended 

 from the single intermediate one, 5 s, 

 here shown. The iron wedges, lu lu iv, 

 of the rack-piece pushed downwards 

 between. the yielding iron blades of the 

 commutator are shown by black dots, 

 forming a circuit in the usual manner 

 for obtaining a reading of difference 

 of temperature between the junctions 

 rt rt', h b'. Each loop or half- turn (bfa, 

 b'f'a') of iron wire is continued past 



