In one of the public gardens of Paris a little cannon is set up with a burning-glass attached to it in such a manner that the sun itself fires the cannon as it reaches the meridian. This, of course, is the time of Paris noon--apparent noon--but it would be exceedingly imprudent of any traveller through Paris who wished, say, to catch the one o'clock express, to set his watch by the gun. For if it happened to be in February, he would find when he reached the railway station that the station clock was faster than the sun by nearly a full quarter of an hour, and that his train had gone; whilst towards the end of October or the beginning of November, he would find himself as much too soon. Until machines for accurately measuring time were invented, apparent time--time, that is to say, given by the sun itself, as by a sun-dial--was the only time about which men knew or cared. But when reasonably good clocks and watches were made, it was very soon seen that at different times in the year there was a marked difference between sun-dial time and that shown by the clock, the reason being simply that the apparent rate of motion of the sun across the sky was not always quite the same, whilst the movement of the clock was, of course, as regular as it could be made. This difference between time as shown by the actual sun and by a perfect clock is known as the 'equation of time.' It is least about April 15, June 15, August 31, and December 25. It is greatest, the sun being after the clock, about February 11; and the sun being before the clock, about November 2. Flamsteed, before he became Astronomer Royal, investigated the question, and so clearly demonstrated the existence, cause, and amount of the equation of time as entirely to put an end to controversy on the subject. We had thus, early in the century, the two kinds of time in common use, apparent time and mean time, or clock time. But as the sun can only be on one particular meridian at any given instant, the time as shown by the clocks in one particular town will differ from that of another town several miles to the east or west of it. It is thus noon at Moscow 1 hr. 36 min. before it is noon at Berlin, and noon at Berlin 54 min. before it is noon in London. This was all well enough known, but occasioned no inconvenience until the introduction of railway travelling; then a curious difficulty arose. Suppose an express train was running at the rate of sixty miles an hour from London to Bristol. The guard of the train sets his watch to London time before he leaves Paddington, but if the various towns through which the train passes, Reading, Swindon, etc., each keep their own local time, he will find his watch apparently fast at each place he reaches; but on his return journey, if he sets to Bristol time before starting, he will in a similar way find it apparently slow by the Swindon, Reading, and Paddington clocks as he reaches them in succession.