- mena show the operation of subterranean forces, acting
either dynamically in earthquakes, in the tension and agitation of the crust; or in volcanos, in the production and chemical alteration of substances. They also show that these forces do not act superficially, in the thin outermost crust of the globe, but from great depths in the interior of our planet, through crevices or unfilled veins, affecting simultaneously widely distant points of the earth's surface.
The greater the variety of structure in volcanos, or in the elevations which surround the channel through which the molten masses of the interior of the earth reach its surface, the greater the importance of submitting this structure to strict investigation and measurement. The interest attaching to these measurements, which formed a particular object of my researches in another quarter of the globe, is enhanced by the consideration that at many points the magnitude to be measured is found to be a variable quantity. The philosophical study of nature endeavours, in the vicissitudes of phenomena, to connect the present with the past.
If we desire to investigate either the fact of a periodical return, or the law of progressive variations or changes in phenomena, it is essential to obtain, by means of observations carefully made and connected with determinate epochs, certain fixed points which may afford a base for future numerical comparisons. If we only possessed determinations made once in each period of a thousand years, of the mean temperature of the atmosphere and of the earth in different latitudes, or of the mean height of the barometer at the level of the sea, we should know whether, and in what
