(387.) In any enumeration of causes which have contributed to the recent rapid advancement of science, we must not forget the very important one of improved and constantly improving means of observation, both in instruments adapted for the exact measurement of quantity, and in the general convenience and well-judged adaptation to its purposes, of every description of scientific apparatus. In the actual state of science there are few observations which can be productive of any great advantage but such as afford accurate measurement; and an increased refinement in this respect is constantly called for. The degree of delicacy actually attained, we will not say in the most elaborate works of the highest art, but in such ordinary apparatus as every observer may now command, is such as could not have been arrived at unless in a state of the mechanical arts, which in its turn (such is the mutual re-action of cause and effect) requires for its existence a very advanced state of science. What an important influence may be exercised over the progress of a single branch of science by the invention of a ready and convenient mode of executing a definite measurement, and the construction and common introduction of an instrument adapted for it cannot be better exemplified than by the instance of the reflecting goniometer. This simple, cheap, and portable little instrument, has changed the face of mineralogy, and given it all the characters of one of the exact sciences. (388.) Our means of perceiving and measuring minute quantities, in the important relations of weight, space, and time, seem already to have been carried to a point which it is hardly conceivable they should surpass. Balances have been constructed which have rendered sensible the millionth part of the whole quantity weighed; and to turn with the thousandth part of a grain is the performance of balances pretending to no very extraordinary degree of merit. The elegant invention of the sphærometer, by substituting the sense of touch for that of sight in the measurement of minute objects, permits the determination of their dimensions with a degree of precision which is fully adequate to the nicest purposes of scientific enquiry. By its aid an inch may be readily subdivided into ten or even twenty thousand parts; and the lever of contact, an instrument in use among the German opticians, enables us to appretiate quantities of space even yet smaller. For the subdivision of time, too, the perfection of modern mechanism has furnished resources which leave very little to be desired. By the aid of clocks and chronometers, as they are now constructed, a few tenths of a second is all the error that need be apprehended in the subdivision of a day; and for the further subdivision of smaller portions of time, instruments have been imagined which admit of almost unlimited precision, and permit us to appreciate intervals to the nicety of the hundredth, or even the thousandth part of a single