current went through it when the spirals were together slid off the magnet. The action of one spiral alone was sufficient to deflect the galvanometer needle about 60°. This deflection was reduced to 1° 18' by placing in its circuit a helix of 735 ft. of no. 18 copper wire; the mean of six experiments (the range of which was only div.) giving 104 div. The ends of the back spiral were now so connected that an equal current flowed through it in a direction the reverse of the other. The mean of six deflections, produced by sliding together the spirals off the magnet, equalled 10-4 div., the same as in the previous experiment; thus showing that the mutual inductive action of the spirals had no effect on the intensity of the induced magnetoelectric currents.

It was also found that on passing the induced current from a spiral through another spiral on which rested a third spiral whose ends were connected with the galvanometer, that no deflection ensued when the magneto-electric current was passed through the inducing spiral.

However, the magneto-electric currents were of such low intensity that probably they were not able to produce an induced current in the second spiral capable of deflecting the needle, and that therefore the experiments here narrated are of little value; nevertheless, I think the reasoning given above will be supported by experiments made with more powerful magnets and with larger spirals.

4. The degree of Precision of the method.

The degree of precision of this special apparatus was determined in the following manner. A copper wire 123 ins. long had opposed to it a resistance which was about equal to 120 ins. of its length and the mean deflection of the galvanometerneedles was carefully determined. The copper wire was now shortened 1 in. and the deflection again determined; this was repeated, determining the amount of deflection produced after each shortening of 1 in.,-until 6 in. had been cut off. These experiments showed that a diminution or increase of resistance of part in one of the wires caused a deflection of 4 div. of the scale, or of 3' of arc, in the galvanometer-needles. But we have seen that 1 div. can be read on the scale, therefore, we can, with this special apparatus, detect and measure an increased or diminished resistance of part. But as the galvanometer can be removed to even twice the distance at which we read its deflections, I think I am safe in saying that with this method, as applied with the above apparatus, we can measure a dif ference of resistance in two conductors of part; which is far within the variations observed in different samples of wires of the same lengths and diameters.

If a galvanometer formed of 6 or 8 turns of 1 in. wire were used in connection with a powerful magnetic battery and larger spirals of thicker wire, while the galvanometer is placed at a greater distance, I have no doubt that a variation of part can thus be detected and measured.

5. Examples of the determinations of electrical conductivities by this method.

The object of these determinations was not to furnish science with new and accurate data,-for that would have required a careful personal supervision of the operations of preparing chemically pure metals, but it was to give examples setting forth the practice of the method.

I had prepared "hard-drawn" wires, of No. 18 B. W. G. (='049 in. diam.), of copper, silver, iron, and German silver. These wires were found to have the same diameter. They were all covered with a double wrapping of silk.

Silver. The spirals were balanced, by the introduction of an increased resistance in the back-spiral, so that no deflection took place on sliding off the spirals. A length of 120 in. of the silver wire having been placed in the circuit of one spiral, it was found that 127 in. of copper wire were required in the other circuit, in order to equal it in resistance. Taking the copper wire as the standard of comparison, at 100, we have

127: 120 :: 100 :: 94·48.

Matthiessen (Phil. Trans., 1858, 1862) makes the ratio of the conductivity of silver to copper, both hard-drawn, as 100: 99.95 or about equality; but in my determination the silver is 5.5 per cent below the copper. I therefore suspected impurities in the silver, and an examination of the wire kindly made by my colleague, Dr. Wetherill, showed that it contained about 01 per cent of gold and a trace of iron. This accounts for the low number found, and affords a good illustration of Pouillet's remark, that the purity of a metal is most readily determined by a measure of its electrical conductivity. The electrical test of purity, however, exceeds in delicacy the chemical examination; for a very minute percentage of alloy causes a great increase of resistance, and if we could be sure that the wires we compared were in the same physical condition as to annealing or hardness, we could probably use this method as a means of determining the percentage of a known metal which formed the alloy.

Pouillet shows (Traité de Physique, 1856, vol. i, p 606) that silver whose conductivity is 100 when pure, is only 51 when it contains 037 of alloy, and is 47, 42 and 39 when it contains respectively 100, 143, and 253 of alloy. Pure gold gave 39, but 049 of alloy reduced its conductivity to 13; and Jenkin

has found that an alloy of 1 part of silver and 2 of gold presents almost as much resistance as German silver.

Iron.-The three following determinations were made of the conductivity of the best quality of iron wire relatively to the standard copper wire.

(1) The resistance of 240 in. of copper wire

"111.6"
60

64

36.7 iu. of iron wire. =16·16"

64 66 66

Giving for the relative conductivity of iron,

(1) 240 : 36.7 = 100: 15.29

(2) 1116 16:16

= 100 14:48

(3) 60 : 8.67 = 100 14:45

14.74 Mean.

E. Becquerel (Ann. de Ch. et Phys., III, xvii, 266) gives 13-6 for the conductivity of iron, copper being 100; and both wires hard-drawn; while Matthiessen (Phil. Trans. 1858, 1862) determines 16.81 as the conductivity of iron, copper being 100, and both hard-drawn.

The mean of Becquerel and Matthiessen = 15-20
My determination = 14.74
Difference

= •46

The copper and iron wires in (3) were cut off from the lengths used in (2); but the wires used in (1) were taken from parts of the coils removed from the lengths (2) and (3). This accounts for the close agreement of (2) and (3) and the higher number obtained in (1).

My determination therefore appears to compare favorably with those made with different methods by these experimentalists. I say "appears," because although the copper was of excellent quality and the iron the best procurable, yet they were not chemically examined as to their purity.

Another series of determinations was obtained by comparing the lengths of copper and of iron wires which would equal in resistance one and the same length of German silver wire, used as a term of comparison. The result agreed with the above determinations.

6. On a modification of the method.

As long ago as 1832 Faraday (Exp. Res. 170-180) first obtained an electric current, directly induced by the earth's magnetism, by rotating a closed wire circuit around an axis at right angles "to the line of the dip;" an experiment whose theoretic beauty has ever been the admiration of natural philosophers.

A length of 38 ft. of in. insulated copper wire was wound into a coil of 3 ft. in diameter, containing 4 turns. The terminals of this coil were connected by binding screws with the

wires leading to the galvanometer and the coil placed in a plane at right angles to the line of the dipping needle. On quickly rotating the coil through 180° the needles were deflected 25°, and by making the rotations correspond in direction and time with the oscillations of the needle, I found that six rotations brought the deflection to over 45°. Faraday (Exp. Res. 202-213 and 3145 et seq.) has shown that the intensities of the magneto-electric currents induced in wires of different metals are as their electrical conductivities, therefore a coil of iron wire similar in all respects to the above copper coil will give a deflection of about 4° for the first rotation; but by increasing the number of turns of the coil to 10 or more and by using a galvanometer with a shorter and thicker wire coil, the angle of deflection can no doubt be doubled.

The above facts show that we can substitute for the steel magnets, previously used, the magnetism of the earth, and can replace the spirals by two similar coils made of the two specimens of wire to be compared. The coils are placed on each other so that their convolutions are in opposite directions; and having been firmly tied together their plane is made to coincide with a direction at right angles to the dipping needle, while their terminals are so connected with the galvanometer that the currents induced in the two coils tend to traverse it in opposite directions.

By

Things being arranged as above, it is evident, as the wire coils are similar in all other respects, that if the conductivities of the wires are the same, there will follow no deviation of the galvanometer needle when the coils are quickly rotated through 180°; but if the wire of one coil offers a greater or less resist ance than that of the other the needle will be deflected. ascertaining what differential actions correspond to knowl differences of conductivity of coils of a certain diameter, number of turns and thickness of wire, we can, by always using similar coils in these relative measures, ascertain what difference in relative conductivity corresponds to a certain angle of deflection; the chords of these angles, or, the sines of half of the angles, being to each other as the intensities of the currents.

Minute differences of resistance in the two coils may be made to cause a deflection in the galvanometer needle by knowing the time of its oscillation, and by reversing the motion of rota tion of the coils so as to correspond to the swing of the needle; thus after several reversals a motion is given to the needle which could not have been observed after a single rotation.

In point of ready application,-and especially in reference to the determination of the resistances of lengthy conductors,-I doubt whether this method will be generally adopted; but after the conception of the idea it appeared worth investigating; this

I have done, and have thus developed at least any value it may possess. It certainly presented an interesting problem and the pleasure afforded in its solution has repaid me for the considerable labor which it required.

S. Bethlehem, Pa., July 15, 1870.

ART. XXX. On the supposed absence of the Northern Drift from the Pacific Slope of the Rocky Mountains; by Dr. ROBERT BROWN, M.A., F.R.G.S., etc., Edinburgh, Scotland.

IN some interesting remarks addressed to the California Academy of Sciences on the 4th of June, 1868, and published in their Proceedings' for that year (vol. iii, pp. 271, 272), Professor J. D. Whitney denies that there is any true Northern Drift within the State of California. "Our detrital materials," the learned Professor remarks, "which often form deposits of great extent and thickness, are invariably found to have been dependent for their origin and present position on causes similar to those now in action, and to have been deposited on the flanks and at the bases of the nearest mountain ranges by currents of water rushing down their slopes. While we have abundant evidence of the former existence of extensive glaciers in the Sierra Nevada, there is no reason to suppose that this ice was to any extent an effective agent in the transportation of the superficial detritus now resting on the flanks of the moun tains. The glaciers were confined to the most elevated portions of the mountains, and although the moraines which they have left as evidences of their former extension are often large and conspicuous, they are insignificant in comparison with the detrital masses formed by aqueous erosion. There is nothing anywhere in California which indicates a general glacial epoch during which ice covered the whole country, and moved bodies of detritus over the surface independently of its present configuration, as is seen through the Northeastern States.

Fur

Mr. Whitney goes on to observe that the same condition of things prevails in Nevada and Oregon, the detritus seeming always to be accumulated at the base of the mountains. ther, from the observations of Messrs. Ashburner and Dall, he 1emarks that "it would appear that no evidences of a Northern Drift have yet been detected on this (Pacific) coast, even as jar north as British Columbia and Russian America (Alaska). Neither of these gentlemen has observed any indication of a transportation of drift materials from the north toward the south, or any condition of things similar to that which must have existed in the Eastern States during the diluvial epoch." Mr.