The eighteenth century was chiefly distinguished by its scientific improvements in time-keepers, to adapt them for astronomical observations and for use at sea, in not only accurately determining the time, but the degrees of longitude. Chronometers were invented, distinguished from watches and clocks, by means by which the fluctuation of the parts caused by the variations in temperature are obviated or compensated. In clocks what are known as the mercurial and gridiron pendulums were invented respectively toward the close of the eighteenth century by Graham and Harrison, and the latter also subsequently invented the expanding and contracting balance wheel for watches. The principle in these appliances is the employment of two different metals which expand unequally, and thus maintain an uniformity of operation. The Dutch, with Huygens in the lead, were long among the leading clock-makers. Germany ranked next. It was in the seventeenth century that a wonderful industry in clock-making there commenced, which lasted for two centuries. The Black Forest region of South Germany became a famous locality for the manufacture of cheap wooden clocks. The system adopted was a minute division of labour. From fourteen to twenty thousand hands twenty years ago were employed in the Schwarzwald district. Labour-saving machines were ignored almost entirely. The annual production finally reached nearly two million clocks, of the value of about five million dollars. Switzerland in watch-making followed precisely the example of Germany in clock-making. It commenced there in the seventeenth and culminated in the nineteenth century. Many thousands of its population were engaged in the business and it flourished under the fostering care of the government--by the establishment of astronomical observations for testing the adjustment of the best watches, the giving of prizes, and the establishment and encouragement of schools of horology conducted on thorough scientific methods. A quarter of a century ago it was estimated that in Switzerland 40,000 persons out of a population of 150,000 were engaged in watch-making, and that the annual production sometimes reached 1,600,000 completed movements. The whole world was their market. The United States alone was in 1875 importing 134,000 watches annually from that country. As in Germany, so one characteristic of the Swiss system was a minute sub-division of the labour. Individuals and entire families had certain parts only to make. It is said that the Swiss watch passed through the hands of one hundred and thirty different workmen before it was put upon the market. The use of machines was also, as in Germany, ignored. By this national devotion to a single trade and its sub-division of labour, the successful production of complicated watches became great and their prices comparatively low.