When Hume thought that a nation would be behind in the manufacture of cloth that had not studied astronomy, he perhaps did not mean to go the length of saying, that the study of astronomy has a real influence in making cloth cheaper, in lessening the cost of production, and in therefore increasing the number of consumers. But look at the direct influence of astronomy upon navigation. A seaman, by the guidance of principles laid down by the great minds that have directed their mathematical powers to the study of astronomy--such minds as those of Newton and La Place--measures the moon's apparent distance from a particular star. He turns to a page in the 'Nautical Almanac,' and, by a calculation directed principally by this table, can determine whereabout he is upon the broad ocean, although he may not have seen land for three months. Sir John Herschel, who unites to the greatest scientific reputation the rare desire to make the vast possessions of the world of science accessible to all, has given, in his 'Discourse on the Study of Natural Philosophy,' an instance of the accuracy of such lunar observations, in an account of a voyage of eight thousand miles, by Captain Basil Hall, who, without a single landmark during eighty-nine days, ran his ship into the harbour of Rio as accurately, and with as little deviation, as a coachman drives his stage into an inn-yard. But navigation not only depends upon lunar distances, but upon an instrument which shall keep perfect time under every change of temperature produced by variety of climate. That instrument is a chronometer. Every one who possesses a watch, however good, must have experienced the effects of heat or cold upon its accuracy, in making it go faster or slower--perhaps a minute in a week. Now if there were not an instrument that would measure time so exactly that between London and New York not a minute, or large fraction of a minute, would be lost or gained, the voyage would be one of difficulty and uncertainty. A Yorkshire joiner, John Harrison, at the beginning of the last century, found out the principle of the chronometer, which consists in the union in the balance-spring of two metals, one which contracts under increased temperature, and one which expands; and on the contrary under diminished temperature. Harrison worked for fifty years at his discovery; and he obtained a parliamentary reward of 20,000_l._ [Illustration: Greenwich Observatory.] The English chronometers are set by what is called Greenwich time. At the Greenwich Observatory a ball falls from a staff exactly at one o'clock; and by the application of electricity, a similar ball falls at the same instant at Charing Cross. The beautiful instruments that are constantly at work, and the laborious calculations which are daily proceeding, at the Observatory, are essentially necessary for the maintenance of a commerce that embraces the whole habitable globe.