height of the summit, and also of an important land boundary in the crater, viz: the corner where the four lands of Keauhou, Kahuku, Kapapala and Kaohe meet, which is at the cone in the central crater.

During the next month I ascended the mountain again, this time carrrying an excellent engineer's transit. In the clear frosty air at the summit station I was able to take the bearings of a dozen survey signals on the slopes and summit of Hualalai. The new spherical signal which I had erected was afterwards accurately determined by observations from more than twenty stations on Mauna Kea, Hualalai and in South Kona, and thus a trigonometrical station was at last located on the very summit of Mauna Loa.

On the second day I descended into the central crater, and found much of the bottom to consist of the most solid kind of "pahoehoe;" but in some large tracts the pahoehoe was covered with pumice, indicating the violence of the former surging and tossing of the lava. Just before reaching the cone we came to a deeper basin (E) twenty or more feet below the rest of the crater bottom and about 400 feet wide, covered with the most friable lava, swollen upward as though raised by air bubbles, and this basin extended into a lava How (LL) northeastward along the side of the crater. Probably this was the place of the last eruption and of most of the eruptions of this central crater. The cone, 140 feet high, was composed of pumice and friable lava still hot and smoking. We ascended it and set up a flag there for the boundary corner.

I returned to the second plateau to the north (B), and thence clambered out to the east of Mokuaweoweo by the route of a former cataract of lava from the summit into the crater, the black, shining spray of which lay spattered on the surrounding rocks. Farther south there were the courses of two other cataracts, which had poured directly into the central crater. At the summit I found the deep fissure from which these cataracts had been supplied with lava, and ascertained that it had also poured an immense stream north upon the first plateau and thence south into the central crater. Crossing from this place to the north over the first plateau I suddenly came to a circular crater in the bed of the plateau (A'), apparently 600 feet deep and 1,000 feet wide, with a cone in its center still smoking. The next day we took the transit to the sta tions in the crater, and the following surveyed along the western brink to the extreme south end, where we looked into the South Crater (D), which is about 800 feet deep and 2,500 feet wide. The length of the whole chasm I ascertained to be about 19,000 feet, the greatest breadth 9,000 feet, and the greatest depth 800 feet; and the area, three and six-tenths

square miles. The map of Plate II is reduced from the map sent to the Government as the result of the survey.

On the southwest side, near the junction of the central crater with the south plateau (C), I found that there had been another eruption from fissures that were still smoking, and that this eruption had sent an immense stream southward toward Kahuku, and had also poured cataracts into the South Crater from all sides.

I had everywhere observed that there had been great flows from the summit brink down the mountain, and questioned whether the chasm had filled up and overflowed its brim. This, however, turned out to be an incorrect view. The flows. have not been from the lowest parts of the brim, but from some of the highest, which could not have been the case in an overflow. The walls of the craters largely consist of loose, old weatherbeaten rocks, and large tracts of the plateau are composed of old pahoehoe, that has not been overflowed for ages, which would not be the case if the craters had filled and overflowed.

These outbreaks from fissures around the rim indicate that the lava has rather poured into the crater than out of it; and that it has flowed from such fissures in vast streams down the mountain side. The question arises, How has the lava risen high enough to pour in extensive eruptions through these fissures, almost a thousand feet above the bottom of the crater, without rising in the crater and overflowing it? The same question has often been asked in respect to the rise of liquid lava to the summit of Mauna Loa without overflowing the open crater of Kilauea, 10,000 feet below.

While surveying the region, I was extremely interested in the arrangement of the craters; and now, having determined the situation of more than fifty of them on Mauna Loa, Hualalai and Mauna Kea, I have ascertained that there is a method in their arrangement. They are not arranged relatively to the mountain on which they are situated, but relatively to the points of the compass. There seems to have been a series of nearly parallel fissures through which these craters have risen, in lines running from S. 40 deg. E. to S. 60 deg. E. There are a few arranged in lines running N. 50 deg. E.

It has been remarked by Mr. W. T. Brigham, in his memoir of 1868 on the Volcanoes of the Hawaiian Islands, that while the general trend of the Hawaiian group and of the major axis of each island is N. 60 deg. W., there is no crater on the Islands whose major axis is parallel to this line. "On the contrary," he continues, "a very interesting parallelism is ob

*To this map the depths of the different parts of Mokuaweoweo below the summit level have been added from estimates received in a letter from the author. The direction of the northern and southern halves of the longer diameter of the crater have also been added on the margin.—J. D. D.

served among all the craters, and invariably the longest diame- · ter is north and south." It would be more correct to say that the major axes of the great craters are usually at right angles to the general axis of the group, i. e., about N. 30 deg. E. Haleakala and the ancient Kipahulu crater appear to take the other direction, but the statement is certainly true of the great craters of Kilauea and Mokuaweoweo, which have other points of resemblance.

Thus in both the highest walls are on the western side, and in both the action is working toward the southwest, as is indicated by the fact that the northeast craters are nearly filled up, while the deepest and active craters are in the southwest end of the depression.

ART. IV. On an Explanation of the action of a Magnet on Chemical Action; by HENRY A. ROWLAND and LOUIS BELL.*

In the year 1881 Prof. Remsen discovered that magnetism had a very remarkable action on the deposition of copper from one of its solutions on an iron plate, and he published an account in the American Chemical Journal for the year 1881. There were two distinct phenomena then described, the deposit of the copper in lines approximating to the equipotential lines of the magnet, and the protection of the iron from chemical action in lines around the edge of the poles. It seemed probable that the first effect was due to currents in the liquid produced by the action of the magnet on the electric currents set up in the liquid by the deposited copper in contact with the iron plate. The theory of the second kind of action was given by one of us, the action being ascribed to the actual attraction of the magnet for the iron and not to the magnetic state of the latter. It is well known since the time of Faraday that a particle of magnetic material in a magnetic field tends to pass from the weaker to the stronger portions of the field, and this is expressed mathematically by stating that the force acting on the particle in any direction is proportional to the rate of variation of the square of the magnetic force in that direction. This rate of variation is greatest near the edges and points of a magnetic pole, and more work will be required to tear away a particle of iron or steel from such an edge or point than from a hollow. This follows whether the tearing away is done mechanically or chemically. Hence the points and edges of a magnetic pole, either of a permanent or induced magnet, are protected from chemical action.

* Read at the Manchester meeting of the British Association, September, 1887.

On

One of Prof. Remsen's experiments illustrates this most beautifully. He places pieces of iron wire in a strong magnetic field, with their axes along the lines of force attacking them with dilute nitric acid they are eaten away until they assume an hour-glass form, and are furthermore pitted on the ends in a remarkable manner. On Prof. Remsen's signifying that he had abandoned the field for the present, we set to work to illustrate the matter in another manner by means of the electric currents produced from the change in the electrochemical nature of the points and hollows of the iron.

The first experiments were conducted as follows: Two bits of iron, or steel wire about 1mm in diameter and 10mm long were imbedded side by side in insulating material, and each was attached to an insulated wire. One of them was filed to a sharp point, which was exposed by cutting away a little of the insulation, while the other was laid bare on a portion of the side. The connecting wires were led to a reflecting galvanometer, and the whole arrangement was placed in a small beaker held closely between the poles of a large electromagnet, the iron wires being in the direction of the lines of force. When there was acid or any other substance acting upon iron in the beaker, there was always a deflection of the galvanometer due to the slightly different action on the two poles. When the magnet was excited the phenomena were various. When dilute nitric acid was placed in the beaker and the magnet excited, there was always a strong throw of the needle at the moment of making circuit, in the same direction as if the sharp pointed pole had been replaced by copper and the other by zinc. This throw did not usually result in a permanent deflection, but the needle slowly returned toward its starting point and nearly always passed it and produced a reversed deflection. This latter effect was disregarded for the time being, and attention was directed to the laws that governed the apparent "protective throw," since the reversal was so long delayed as to be quite evidently due to after effects and not to the immediate action of the magnet.

With nitric acid this throw was always present in greater or less degree, and sometimes remained for some minutes as a temporary deflection, the time varying from this down to a few seconds. The throw was independent of direction of current through the magnet, and apparently varied in amount with the strength of acid and with the amount of deflection due to the original difference between the poles. This latter fact simply means that the effect produced by the magnet is more noticeable as the action on the iron becomes freer.

When a pair of little plates exposed in the middle were substituted for the wires, or when the exposed point of the latter was filed to a flat surface, the protective throw disappeared, though

it is to be noted that the deflection often gradually reversed in direction when the current was sent through the magnet; i. e., only the latter part of the previous phenomenon appeared under these circumstances.

When the poles, instead of being placed in the field along the lines of force, were held firmly perpendicular to them, the protective throw disappeared completely, though as before there was a slight reverse after-effect.

Some of Professor Remsen's experiments on the corrosion of a wire in strong nitric acid were repeated with the same results as he obtained, viz: the wire was eaten away to the general dumb-bell form, though the protected ends instead of being club-shaped were perceptibly hollowed. When the wire thus exposed was filed to a sharp point the extreme point was very perfectly protected, while there was a slight tendency to hollow the sides of the cone, and the remainder of the wire was as in the previous experiments. In both cases the bars were steel and showed near the ends curious corrugations, the metal being left here and there in sharp ridges and points. In one case the cylinder was eaten away on sides and ends so that a ridge of almost knife-like sharpness was left projecting from the periphery of the ends.

These were the principal phenomena observed with nitric acid. Since this acid is the only one which attacks iron freely in the cold, in Prof. Remsen's experiment, this was the one to which experiments were in the main confined. With the present method, however, it was possible to trace the effect of the magnet whenever there was the slightest action on the iron, and consequently a large number of substances, some of which hardly produce any action, could be used with not a little facility.

In thus extending the experiments some difficulties had to be encountered. In many cases the action on the iron was so irregular that it was only after numerous experiments under widely varying conditions that the effect of the magnet could be definitely determined. Frequently the direction of the original action would be reversed in the course of a series of experiments without any apparent cause, but in such case the direction of the effect due to the magnet remained always unchanged, uniformly showing protection of the point so long as the wires remained parallel to the lines of force. When, however, the original action and the magnetic effect coincided in direction, the repetition of the latter showed a decided tendency to increase the former.

When using solutions of various salts more or less freely precipitated by the iron, it frequently happened that the normal protective throw was nearly or quite absent, but showed itself when the magnet circuit was broken as a violent throw in the reverse direction, showing that the combination had been act