Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/398

 level, except in the size of the graduated arch, while he observed the bearing at the same time by a compass carried separately. Mr. Jardine of Edinburgh made the important improvement of having the compass mounted upon the radial bar, and placed it between the center and the graduated arch. Soon afterwards I had an instrument made in imitation of this, but, in order to reduce the radius of the arch, I had the compass fixed at the end of the radial bar. There was a difficulty in applying the base of this instrument to the surface of a rock, so as to place it with precision in the line of greatest depression, and the endeavour to remedy it led me to the idea of the plate. Two other clinometers were successively made for me upon this plan, with such further improvements as experience gave rise to; and the last was adapted for observation upon an under surface, upon the suggestion of Mr. Jardine. This last contained all the principles of the instrument in its present state, and was given as a model to Mr. Troughton, who, in making that which has now been described, introduced a more simple construction of the radial bar, and shewed that ingenuity which distinguishes every object of his labours, by other alterations conducive to strength and lightness as well as to greater ease and accuracy of observation.

The chief objection to the instrument in its present form, is the weight of the plate. But it is necessary that it should have a certain diameter, in order to admit of its adjustment by the eye to the mean plane of the stratification, and it must have a thickness sufficient to prevent it from bending much under the pressure required to hold it against a steep surface. No substance occurred besides a metal, that was not liable to some objection, particularly that of warping.

The clinometer may be employed for other purposes, besides