here, because these pages are intended not merely for the limited number of those who participate in the simultaneous observations, but for all who are engaged in investigating the laws of magnetic phenomena.
Those less delicate instruments which were employed for magnetic measurements before the invention of the magnetometer, may not only be used for the same purposes as formerly, but may also be applied to those absolute measurements of intensity which owe their origin to the invention of the magnetometer. It is true that these instruments are far from affording such accurate results as the magnetometer; but the results they give are more easily obtained. On this account they have not lost all their value by the later invention; they may still be usefully employed, though in a more limited sphere. Wherever, from want of means or time, or from any other circumstances, magnetometers cannot be employed, these instruments may still be used with advantage. This will be the case most frequently in voyages and journeys to remote parts of the world. It is true that magnetometers may be carried on journeys, as was done by M. von Waltershausen and Dr. Listing in their Italian tour; but this is only possible for those who are highly favoured by external circumstances; and it is therefore not to be expected that many will follow this praiseworthy example. If, therefore, we wish to collect observations from the whole surface of the earth, we must be content with such as are not made with magnetometers; and it is important to extend the application of portable instruments to the absolute measurement of the intensity, which has been hitherto performed with the magnetometer only. The difference in respect to accuracy between the absolute measures with such instruments and those made with the magnetometer, is nearly the same as between measurements of declination with the two kinds of instruments. A skilful hand will be able to obtain useful results even with the smaller apparatus; and it appears desirable, therefore, that it should be extensively employed.
We shall consider successively,
- The parts of the small apparatus.
- The observations to be made with it.
- The application of the observations.
- The calculations required.
- The result of the calculation.
