In order to really profit by his visit, he should have gained, beforehand, some idea of what is to be observed and the manner of observing. This information I shall try to give the readers of this article; and then proceed with a description of the observatory and its equipments.
The force called terrestrial magnetism is subject to variations both in intensity and in direction. There are three ways in which a varying force of this kind may be measured:
1. It may be resolved into components acting along three axes, and the intensity of these components measured. I am not aware that this method has ever been applied to the measurement of terrestrial magnetism; probably, because one of the components would be so small as not without great difficulty to be directly measured.
2. We may measure its intensity along some fixed axis, and its angular variation of direction from that axis, in each of two planes intersecting the same. This method is frequently employed. The fixed axis taken is the intersection of the plane of the magnetic meridian and the plane of the horizon, and the angular variations from the axis are measured in these planes, the variation in the horizontal plane being called the "declination," and that in the plane of the magnetic meridian the "dip."
3. We may measure the intensity of its components along two axes and its angular variation in direction from the plane of those two axes. This last is the method in use at the observatory. The axes assumed are horizontal and vertical, and their plane is that of the magnetic meridian. Angular variations from this plane may be measured in any plane at right angles to it, as the plane of the horizon, and are, therefore, changes of declination.
The instruments used for making the measurements are the declinometer, the bi-filar magnetometer, and the balance magnetometer.
The declinometer consists, essentially, of a bar magnet so suspended as to turn freely in the horizontal plane. Changes in the position assumed by the bar show changes in declination.
The magnet of the bi-filar magnetometer likewise turns in the vertical plane; but, while the magnet of the declinometer is free to assume any position in that plane, the magnet of this instrument is pulled by a constant force into a position at right angles to the magnetic meridian.
The magnet of the balance-magnetometer, like that of the last two instruments, is in a position at right angles to the magnetic meridian; but, unlike either of the other two, it turns in the vertical plane.
The only effect of the horizontal force is to press the magnet against its bearings, and were the magnet suspended at its centre of gravity, the north-seeking pole would point directly downward in obedience to the vertical force. In reality, the magnet is so suspended as to assume a position approximately horizontal. The force of gravity remaining constant, the magnet will not change its posi-
