occur contemporaneously, and with almost equal magnitude, over great districts, the means are furnished to divest them almost entirely of any injurious practical effect. The surveyor need only make all his operations with the compass accurately according to time, and cause contemporaneous observations to be made at some other not very distant place; and it will be easy to eliminate the effects of these disturbances by comparison, just as travelling observers render their barometrical determinations of height independent of the irregular variations of the barometer, by comparative observations at fixed stations. Of course this has no reference to disturbances of the compass by mineralogical causes.
The preference given to the Declination over the other elements of terrestrial magnetism is less however to be ascribed to these motives than to the present state of our means. The investigation of the laws of nature has for the philosopher its own value and its own reward; and a peculiar charm surrounds the recognition of measure and harmony in that which at first sight appears wholly irregular. In following the constantly varying changes of the Declination, the apparatus at present employed leaves, as to certainty and precision, nothing more to wish; but the same cannot be said of the present means of observation of the other two elements. The time is therefore not yet come for including the latter in the circle of combined inquiry; as soon, however, as the means of observation shall be so far perfected, that we can recognise with certainty, follow with ease, and measure with accuracy, the variations, and chiefly the rapidly varying changes, in the other two elements of terrestrial magnetism, these variations will have the same claims on the united activity of natural inquirers as the variations of the declination now possess. We venture to hope that this day is not far distant.
I.
Remarks on the Arrangement of Magnetical Observatories, and Description of the Instruments to be placed in them.
The instruments with which the observations were made, which are to be mentioned in these pages, differ in many respects from all previously employed, and a more accurate knowledge of their construction is indispensable, in order to judge of the results
