diminished to its present value of 67° 39′. The period in which the cycle is completed is not known, but the rate of variation of the dip is less at the present time than it was fifty years ago. In all parts of the earth both declination and inclination are changing similarly. The following table gives the data of the secular changes at London. TABLE OF SECULAR MAGNETIC VARIATIONS. The Total Magnetic force, or "Intensity," also slowly changes in value. As measured near London it was equal to 4791 dyne-units in 1848, 4740 in 1866, and at the beginning of 1880, 4736 dyne-units.1 Owing to the steady decrease of the angle at which the needle dips, the horizontal component of this force (ie. the Horizontal Intensity") is slightly increasing. It was 1716 dyne-units in 1848, and 1797 dyne-units at the beginning of 1880. 1 That is to say, a north magnet pole of unit strength is urged in the line of dip, with a mechanical force of a little less than half a dyne. 142. Daily Variations.-Both compass and dippingneedle, if minutely observed, exhibit slight daily motions. About 7 a.m. the compass needle begins to travel westward with a motion which lasts till about 1 p.m.; during the afternoon and evening the needle slowly travels back eastward, until about 10 p.m.; after this it rests quiet; but in summer-time the needle begins to move again slightly to the west at about midnight, and returns again eastward before 7 a.m. These delicate variations-never more than 10' of arc-appear to be connected with the position of the sun; and the moon also exercises a minute influence upon the position of the needle. 143. Annual Variations.-There is also an annual variation corresponding with the movement of the earth around the sun. In the British Islands the total force is greatest in June and least in February, but in the Southern Hemisphere, in Tasmania, the reverse is the case. The dip also differs with the season of the year, the angle of dip being (in England) less during the four summer months than in the rest of the year. 144. Eleven-Year Period.-General Sabine discovered that there is a larger amount of variation of the declination occurring about once every eleven years. Schwabe noticed that the recurrence of these periods coincided with the eleven-year periods at which there is a maximum of spots on the sun. Professor Balfour Stewart and others have endeavoured to trace a similar periodicity in the recurrence of aurora1 and of other phenomena. 145. Magnetic Storms.-It is sometimes observed that a sudden (though very minute) irregular disturbance will affect the whole of the compass needles over a considerable region of the globe. Such occurrences are known as magnetic storms; they frequently occur at the time when an aurora is visible. 146. Self-recording Magnetic Apparatus.—At 1 See Lesson XXIV., on Atmospheric Electricity. Kew and other magnetic observatories the daily and hourly variations of the magnet are recorded on a continuous register. The means employed consists in throwing a beam of light from a lamp on to a light mirror attached to the magnet whose motion is to be observed. A spot of light is thus reflected upon a ribbon of photographic paper prepared so as to be sensitive to light. The paper is moved continuously forward by a clockwork train; and if the magnet be at rest the dark trace on the paper will be simply a straight line. If, however, the magnet moves aside, the spot of light reflected from the mirror will be displaced, and the photographed line will be curved or crooked. Comparison of such records, or "magnetographs," from stations widely apart on the earth's surface, promises to afford much light upon the cause of the earth's magnetism and of its changes, of which hitherto no reliable origin has been with certainty assigned. The phenomenon of earth - currents (Art. 403) appears to be connected with that of the changes in the earth's magnetism, and can be observed whenever there is a display of aurora, and during a magnetic storm; but it is not yet determined whether these currents are due to the variations in the magnetism of the earth, or whether these variations are due to the currents. It is known that the evaporation (see Art. 63) always going on in the tropics causes the ascending currents of heated air to be electrified positively relatively to the earth. These air-currrents travel northward and southward toward the colder polar regions, where they descend. These streams of electrified air will act (see Art. 337) like true electric currents, and as the earth rotates within them it will be acted upon magnetically. Whether this will account for the gradual growth of the earth's magnetism is an open question. The action of the sun and moon in raising tides in the atmosphere might also account for the variations mentioned in Art. 142. It is important to note that in all magnetic storms the intensity of the perturbations is greatest in the regions nearest the poles; also, that the magnetic poles coincide very nearly with the regions of greatest cold; that the region where aurora (Art. 309) are seen in greatest abundance is a region lying nearly symmetrically round the magnetic pole. It may be added that the general direction of the feeble daily earth - currents (Art. 403) is from the poles toward the equator. CHAPTER III. CURRENT ELECTRICITY.. LESSON XIII.-Simple Voltaic Cells. 147. It has been already mentioned, in Lesson IV., how electricity flows away from a charged body through any conducting substance, such as a wire or a wetted string. If, by any arrangement, electricity could be supplied to the body just as fast as it flowed away, a continuous current would be produced. Such a current always flows through a conducting wire, if the ends are kept at different electric potentials. In like manner, a current of heat flows through a rod of metal if the ends are kept at different temperatures, the flow being always from the high temperature to the lower. It is convenient to regard electricity as flowing from positive to negative; or, in other words, the direction of an electric current is from the high potential to the low. It is obvious that such a flow tends to bring both to one level of potential. The "current" has sometimes been regarded as a double transfer of positive electricity in one direction, and of negative electricity in the opposite direction. The only evidence to support this very unnecessary supposition is the fact that, in the decomposition of liquids by the current, some of the elements are liberated at the point where the potential is highest, others at the point where it is lowest. Continuous currents of electricity, such as we have described, are usually produced by voltaic cells, or batteries of such cells, though there are other sources of currents hereafter to be mentioned. 148. Discoveries of Galvani and of Volta.The discovery of electric currents originated with Galvani, a physician of Bologna, who, about the year 1786, made a series of curious and important observations upon the convulsive motions produced by the "return-shock" (Art. 26) and other electric discharges upon a frog's leg. He was led by this to the discovery that it was not necessary to use an electric machine to produce these effects, but that a similar convulsive kick was produced in the frog's leg when two dissimilar metals, iron and copper, for example, were placed in contact with a nerve and a muscle respectively, and then brought into contact with each other. Galvani imagined this action to be due to electricity generated by the frog's leg itself. It was, however, proved by Volta, Professor in the University of Pavia, that the electricity arose not from the muscle or nerve, but from the contact of the dissimilar metals. When two metals both in contact with the air or other oxidising medium are placed in contact with one another, the surface of one becomes positive and of the other negative, as stated on p. 67. Though the charges are very feeble, Volta proved their reality by two different methods. 149. Contact Electricity: Proof by the Condensing Electroscope.-The first method of proof devised by Volta involved the use of the Condensing Electroscope, alluded to in Art. 71. It can be used in the following way to show the production of electrification. A small bar made of two dissimilar metals, zinc and copper soldered together, is held in the hand, and one end is touched against the lower plate, the upper plate being at the same time joined to "earth" or touched with the hand (Fig. 68). During the contact electrical separation has taken place at the point |