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ohms when the wires were coupled for observing a difference of temperature; and it was assumed that, with this resistance and with the probable tendency of twelve wires similarly circumstanced to neutralize each other's false effects, no sensible errors from local disturbances would arise. The instrument was submitted to some careful tests, with a result that, at the highest temperatures of the experiments, errors in the temperature-difference amounting to about 1° F. may have been committed. At the ordinary temperatures of the wires between 100° and 120° F. it was found, by substituting a heated iron disk (coated on the faces with thin paper) in the place of a rock-plate, so as to heat both sets of wires equally, that the only permanent deviations produced as the plate sunk very slowly in temperature also sunk gradually with it from an equivalent value of about jo to about jo upon the scale. As the correctness of the small temperature-differences (of 6° and upwards) lying usually between the above two temperatures was thus fairly checked, and for exceptionally higher differences and temperatures the conditions could not easily be more exactly assimilated to those of the actual experiments so as to control and estimate them, the effects of these small errors have not been further regarded in the calculations; but in order to avoid changes of value in the divisions of the scale, and to enable the actual temperature of each set of wire-junctions to be directly observed, an arrangement of the thermopile was made by which each set of junctions could be separately combined with a similar set in continuous circuit with the galvanometer placed in a small rectangular waterbath. The latter is made of tin, and, as well as its lid, is well jacketed with cork, and provided with an agitator; so that by adding hot or cold water, which can be withdrawn below, any temperature of the water in the bath can be obtained. A simple commutator enables the circuit with the galvanometer to be closed, either through the two principal sets of junctions or through one of them and through a set corresponding to it in the bath; so that by changing the temperature of the latter until no current passes through the circuit the actual temperature of each rock-face could be observed. This mode of observation is free from all objections, excepting those of false currents arising in long wires and plates of the same metal maintained at very various temperatures ; but with the exception of the twelve loops of German-silver wire projecting on one side from the rock-plate, the corresponding loops on the other side, and all the rest of the circuits made to the galvanometer, were formed from the same piece of iron wire freshly annealed. The comb-like teeth of the commutator are pieces of narrow hoop-iron about 3 inches long, closely set together in wood, and also thoroughly annealed, to which the proper terminals of iron wire are soldered at their feet, while the upper ends are filed to chisel-edges; and a small hand-rack of iron wedges set on wood at proper distances apart, thrust between them in different positions, completes the connexion in the three different orders that are required. The additional branch wires used in the arrangement are few, and, as will be seen from the following description, add very little to the total lengths of iron wire which conduct the currents. The twelve-turn coil of wire in which the rock is pressed consists of twelve half-turns or loops of German silver and the same number of iron loops. The twelfth loop of German silver (see figure, p. 22) completes the circuit or connexion from the beginning to the end of the coil through the medium of the galvanometer. There are thus twelve junctions of dissimilar metals above, and twelve below the rockplate in a closed circuit with the galvanometer. To produce a new set of twelve junctions corresponding to each of these, the loops of German-silver wire are all cut through in the middle, and the free ends soldered to twentyfour short pieces of iron wire, the junctions being laid side by side across a

narrow water-tight trough formed of three or four rectangular washers of caoutchouc laid on a sheet-caoutchouc floor, upon which the sides of the rectangular 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 difference 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 produced upon the reading as soon as the water in the bath had by gentle agitation become uniforin 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' l'), above and below the rockplate, are shown, thin lines representing iron and thick lines German-silver wire. Bis the bath in which the supplementary junctions, s's s s', obtained by severing the loops of German

I lol lol 10 C silver wire, as at ss, are immersed. The two extreme half-loops and corresponding 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 between them will easily be apprehended from the single intermediate one, ss, here shown. The iron wedges, w w w, of the rack-piece pushed downwards

al is an ib! 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 å a', 67'. Each loop or half-turn (bfa, B'f'a') of iron wire is continued past

the upper junctions (a, a') and carried through the bath to a separate tooth of the commutator; and by moving the wedges of the rack-piece together one tooth-space to the right or left (as shown in new positions by a x in the figure), combinations of junctions in the bath with junctions (a a') above or (6 b') below tho rock-plate are put into connexion with the galvanometer.

By the same mode of trial as before, a heated iron plate coated with thin paper being substituted in the place of an experimental plate, the temperatures of its two faces, as exhibited by the thermometer in the bath when the commutator was shifted from one of its two supplementary positions to the other, were sensibly the same as the heated plate slowly cooled, and no false difference of temperature arising from false currents differently excited in the two circuits thus joined up were found to be indicated as a result of several such determinations of the really equal temperatures of the two faces of the plate. This mode of observing the actual temperatures and the temperature-differences of the rock-faces in the present series of experiments was therefore constantly employed, and the values of the scale-divisions in degrees for the other more usual method of employing the thermopile were not determined with special care, although this adjustment of the commutator was also used to check and follow the gradual variations of temperaturedifference that were less speedily, although more certainly, measured by the absolute method of determination. The only case of failure to observe a sensible difference of temperature between the two sides of an experimental plate occurred with iron-pyrites, which (as well as galenn), being a good conductor of electricity, it was found necessary to coat with two thicknesses of the thinnest tissue-paper on each face; and the apparent difference of temperature recorded (which was decidedly less than 19) may have arisen from the resistance offered by the slight obstructions of these thin paper sheets (soaked with water) to the passage of the heat : although certainly very great, no definite value of the thermal conductivity of ordinary iron-pyrites can therefore be assigned. It was also necessary to use oil junctions instead of wet bladders, from the galvanic effects produced by the saturated salt solution, when rock-salt was tested ; and it appears probable from some measurements of quartz with the same kind of luting that the conductivity of rock-salt thus found is somewhat less than, rather than likely to be in excess of, the real thermal conductivity of that substance. As a good assurance that when membranes wetted with water were used to press the thermopile against the rocks the true temperatures of their faces. were very nearly marked, the experiment with iron-pyrites may be instanced, as the small temperature-difference of less than 1° could not have been observed if the wires were not very nearly indeed at the same temperature as the two papercovered faces of the pyrites against which they were pressed ; and as the circumstances of their adjustment in other cases were exactly the same as in this instance, it may be assumed that the method of pressing the thermopile against the rocks with wet bladders adopted in the present series of experiments exhibited the true temperature-difference of the faces, and afforded correct values of the thermal conductivities. The pressure was applied by means of strong spiral springs (instead of the weights described in the last Report), whose extensions in a graduated tube indicated the pressures which they were made to exert. The pressure thus applied was usually 80 lbs. upon a surface of nearly 20 square inches of the rock-plates, or about 4 lbs. per square inch. The general agreement of the results with those formerly obtained also serves to verify the correctness both of the thermal conductivities now assigned and of those previously observed. The principal differences in the two methods of determination consist in the use of an im,

Comparisons with former

observations, 1875.

Rock specimen tested, 1876. (Water-satura

tion in vacuo.)

Grains and Absolute conducti-
per cent. vity observed.
(on rock-
weight) Luting of Wet-
of water | Oil and bladder
absorbed. Red-lead. junction.

Absolute
Resis-
tance.

Absolute Conducti-' Rock-specimen vity (1875). tested (1875).

[blocks in formation]

wet.

[blocks in formation]

Kenton sandstone,

thoroughly wet

(5-7 per cent.). Do. dry (or moist

by 1st experiment).

[blocks in formation]

The same specimen.

[blocks in formation]

Rock-salt (observed )......

0.01 154 Do. allowing for radiation

0:01130 Fluorspar .............. Opaque white quartz......

0.00753 Do. a new specimen ......

0.00738 Galena (interspersed with

a little quartz). Pennant sandstone (near 80 grns. Bristol), thoroughly =13 per

cent. Do. dry ........ Hard 'grit (Lee Abbey | 358 grns.

quarry, Linton, N. De- =6:3 per von), thoroughly wet. I cent. Do. moistened by lst ex- 98 grns.= periment.

(1.75 per ct. Festiniog slate (specimen

A, cut across the cleav

age). Festiniog slate (specimen

A, cut parallel to the

cleavage). Calcite (soft crystalline

vein-stuff in red sand

stone, Clifton). Trap-rock, Pokham quar-11

ry, near Exeter. Firebrick (fine ground 859 grns.

Newcastle, thoroughly = 17:0 per wet).

I cent. Do. moist by lst experi- 432 grns.= ment.

8:6 per ct. Cornish elvan (Christow

Lead-mines, near Exe

ter). Clay-slate from same lo

cality cut across the

cleavage. Do. a specimen cut paral

lel to the cleavage. Another do. do.... English plate-glass ......

0.00204 Heavy spar, opaque crys

tallized (Christow, Exe

ter): two experiments. Pumicestone thoroughly 1374 grns. wet.

= 70-3 p. ct. Do. dry (or moist by 1st 110 grns. experiment).

=5-6 p. ct. Newcastle house-coal...... ..........

0-00462

to
0:00488
( 0.00332

[blocks in formation]

1 0.00366
0:00247

Calton Hill Trap

rock. | Whinstone. Fine red brick

thoroughly wet

(15-6 per cent.). Do. dry (or moist

by ist experiment).

[blocks in formation]

Welsh slates cut

parallel to the cleavage.

[blocks in formation]

English alabaster

(or gypsum).

[blocks in formation]

0.00065 Cannel-coal.

proved thermopile, and in the substitution of wet membranes for the pre-
viously employed moist luting of wet linseed-meal in the present series.
Where considerable discrepancies still exist, the discordance is rather to be

ascribed to the numberless small precautions required to ensure perfect accuracy, than to any constant errors of the methods with which either of the two series of determinations is now believed to be affected.

In concluding this description, some remarks on the results of the new experiments that have been carried out will serve to show what new data have been obtained, and how far the observations made last year are corroborated and confirmed by the slightly modified apparatus and method of procedure that has been adopted to extend the series.

Among the points of principal importance noticed last year the following facts of great interest already ascertained have now been verified and confirmed. Quartz is still found to have about the same high thermal conductivity (85-88 concisely expressed, as explained at the beginning of this Report) compared to the other rocks which had been previously observed. The direction in which heat is transmitted through slate is a very important condition in regard to its conducting-power-the conductivity of good Welsh (Festiniog) slate cut across the cleavage being, however, to that of a plate of the same stone cut parallel to the cleavage-planes, as 5:3 from this year's experiments, instead of 6:3 very nearly, as observed in the same slate specimens last year. The notable part of this difference of the two years' observations is in the better-conducting cross-cut plate (54 instead of 66), although the other less-conducting plate (31:5 and 32:5) has nearly the same conductivity as it appeared to have last year. Cleavage-fractures which the cross-cut plate has suffered, and their repairs, rendering its surfaces uneren and the water-junction contacts consequently somewhat imperfect, have probably caused this apparent loss of conductivity in the transversely cut specimen of slate. But this latter still exhibits a much higher thermal conductivity than that shown by the plate from the same piece of slate cut parallel to its cleavage-planes. A less distinct difference was found this year in similarly sawn and tested plates of clay-slate cut across and parallel to the planes of cleavage or of foliation ; but the stronger kinds of the stone which supplied a transverse section (as well as the less fragile plates cut parallel to the planes of cleavage) presented the appearance of cleavage and foliation only very imperfectly, and much less remarkably than the specimens of ordinary slate from Wales. The thermal conductivity of the soft clayslate is also less in all directions (26-28.5) than the least observed conductivity (31.5) of Welsh slate cut parallel to its cleavage-planes.

The observations of the effect of moisture in increasing the conductivity of the porous rocks, when thoroughly saturated with water, entirely corroborate the similar observations made last year. When the great pressure required to force a sensible quantity of water through such rocks as sandstone and others which were tested is compared with the very feeble currents which differences of temperature and of density of the water in their cavities can produce, it appears evident that the very marked increase of conductivity observed in such cases cannot be owing to convection- or gravitationcurrents in the water which the saturated rocks contain, although the mobility of the liquid by diffusion and consequent intermixture of its molecules probably assists the direct conducting-power of water in the transmission of the heat; and the resulting conductivity of water, free from the action of convection-currents, appears to be at least equal to that of some rock-species whose thermal conductivities are either the last or nearly the lowest in the present list.

The thermal conductivities of certain new species of rocks are now also assigned, the values of which, although they are few in number, appear to possess considerable interest from a mineralogical as well as from a geological

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