perfect, since he describes the constellation of the great Bear as never setting, which is only the case in certain latitudes.

"Plac'd at the helm he sat, and mark'd the skies,

Nor clos'd in sleep his ever watchful eyes.

There view'd the Pleiads, and the Northern team,
And great Orion's more refulgent beam,
To which, around the axle of the sky,
The Bear revolving, points his golden eye;
Who shines exalted on th' etherial plain,
Nor bathes his blazing forehead in the main.”

ODYSSEY, B. V.

LETTER X.

THE SAME SUBJECT CONTINUED.

ANOTHER method for finding the longitude of places, is by means of a chronometer, or an instrument for measuring time. This excellent machine, so useful in the common affairs of life, is capable of affording us information upon subjects which seem to have little or no connection with it; but genius and industry find analogies where vulgar apprehension is blind. If a watch or clock could be so constructed, as to measure time with accuracy and precision, at all seasons, and in all places, such an instrument might be used to discover the longitude with the greatest ease and facility.

To illustrate this subject, we must consider the manner in which time is estimated. When the sun, in his apparent daily course round the earth, comes over any particular meridian, it is then twelve o'clock, or noon, at all places which lie under that meridian; but at all other places, it is either before noon, or after noon, according to their situation. The sun, moving from east to west, must pass over the meridian NBS before he comes to the meridian NAS (Pl. iv. fig. 12.); and consequently, when it is noon to the inhabitants who live under the meridian NBS, it will be some time before noon to the inhabitants who live under the meridian NAS. And, on the contrary, when it is noon at those places which are situated under the meridian

1

NAS, it will be some time after noon to the places which are situated under the meridian NBS.

But as the sun appears to move uniformly round the earth, and to describe a circle, which contains 360 degrees, in twenty-four hours, he will of course move through an arc of fifteen degrees in an hour. When it is noon, therefore, at London, and at all other places which lie under the same meridian, it will be one o'clock in the afternoon at those places which lie under the meridian fifteen degrees to the east of that of London; and eleven o'clock in the morning, at all those places which lie under the meridian fifteen degrees to the west of that of Lon. don. If the distance of the meridians be thirty degrees, it will make two hours difference in the time; if forty-five degrees, three hours; and so on, reckoning according to the situation of the places.

To make this matter still plainer, let us consider the situation of any two particular places; for instance, that of London and Paris. The meridian of Paris is two degrees twenty minutes east of the meridian of London; and this difference, reckoning after the rate of fifteen degrees to an hour, is nine minutes and twenty seconds of time. When it is noon at London, therefore, it will be nine minutes and twenty seconds after noon at Paris ; and on the contrary, when it is twelve o'clock at Paris, it will be only fifty minutes and forty seconds after eleven at London: so that the clocks at London, ought to be slower than those at Paris by nine. minutes and twenty seconds. In like manner, the meridian of New York, in North America, being

seventy-four degrees nine minutes and forty-five seconds west of that of London, the difference of time at those places will be four hours fifty-six minutes and thirty-nine seconds; so that when it is noon at London, it is only three minutes and twenty-one seconds after seven o'clock in the morn ing at New York.

From these circumstances you will readily observe, that as places differ in longitude, or are situated under different meridians, so the clocks and watches of those places, supposing them to be well regulated, will show different hours at the same moment of absolute time; a difference of fifteen degrees in longitude, always producing a difference of an hour in the time shown by those machines. Knowing, therefore, the difference of time between two clocks, or watches, well regulated, at different places, it will be easy to find the difference of longitude of those places, by reckoning after the rate of fifteen degrees for every hour, and a quarter of a degree for every minute.

The difference of time shown by the clocks and watches of different places, will not appear in the least surprising, when you consider, that whilst it is noon with us, there are countries towards the east, where the sun is just setting; and others towards the west, where he is just rising; so that it is evening with the one, and morning with the other, at the same instant of absolute time that it is midday with us. With our antipodes, or those who live directly opposite to us upon the globe, the difference is still greater: it is noon with us, when it

is midnight with them, and noon with them when it is midnight with us; their time, as well as their situation, being always directly opposite to ours. These considerations will clear the subject from alambiguity, and you will now be able to perceive in what way a clock or watch may be used to discover the longitude.

Suppose I had a watch of such excellent workmanship, that, being once regulated for that purpose, it would always show me, in whatever part of the world I might be, the exact time which it was then at London; by means of such a watch, I should be presently enabled to tell the longitude of the place I was in. For this purpose, I have only to find when the sun comes to the meridian of the place, or, which is the same thing, when he is exactly south; and as I then know it to be twelve o'clock at that place, the time thus found, being compared with the time shown by my watch, and the difference, turned into degrees and minutes, will give the longitude of the place required.

To illustrate this by an example; suppose I was at Petersburg in Russia, and wanted to find the longitude of that city from London. Not choosing to trust to the common account of time, as it is shown by the clocks and watches at Petersburg, I find when the sun comes to the meridian, and know it to be then exactly twelve o'clock at that place. At this moment I look at my watch, and find that, instead of its being twelve o'clock by that, it is only nine hours fifty-eight minutes and forty-three seconds. From this I conclude that, when it is