VOL. IV.] PHILOSOPHICAL TRANSACTIONS. 34/ 



be greater than that shown by the watches, you are come under a more easterly 

 meridian ; and if less, you are come under a more westerly. And counting for 

 every hour of difference of time 15 degrees of longitude, and for every minute 

 1 5 minutes or ^ of degree, you will then know how many degrees, minutes, 

 &c. the said meridians differ from each other. 



7. To find the time of the day at sea. — ^This, it has been shown above, is 

 necessary for finding the longitude, which time you must observe as precisely 

 as possible; for every minute of time that you misreckon makes a fourth part 

 of a degree in longitude, which amounts, near the equator, to above 15 English 

 miles, but less elsewhere. Wherefore, to find the time of the day with cer- 

 tainty, you are not to trust to the observation of the sun's greatest altitude, 

 thence to conclude that it is just noon, or that the sun is in the south, unless, 

 being between the tropics, you have it just in the zenith ; for the sun being 

 near the meridian, remains for some time without any sensible alteration of his 

 altitude ; much less are you to rely on the sea-compasses for finding the precise 

 time of noon ; neither are the astronomical rings, or other sorts of sun-dials, 

 exact enough for showing the time to minutes and seconds. But it is better to 

 observe the sun's altitude when he is in the east or west, for there his altitude 

 changes in a short time more sensibly than before or after; and thus from the 

 height of the pole and the declination of the sun, the hour may be calculated. 

 Yet as this calculation is rather troublesome, and there may be some errors in 

 taking the sun's altitude, here follows an easier way. 



8. How to find the longitude at sea, by observing the rising and setting of 

 the sun, and the time by the watches. — ^This way neither requires the know- 

 ledge of the height of the pole, nor of the declination of the sun, nor the use 

 of any astronomical instruments ; neither can the refractions of the sun or stars 

 cause any considerable error; the refraction of the morning differing but little 

 or nothing from that of the evening of the same day, especially at sea. 



At the rising and setting of the sun, when half above the horizon, mark the 

 time of the day shown by the watches. Then reckon by the watches what time 

 is elapsed between the two observations, and add the half of it to the time of 

 the rising, and you will have the time by the watches when the sun was at 

 south; to which is to be added the equation of the present day by the table. 

 And if these together make 12 hours, then was the ship at noon under the same 

 meridian where the watches were set with the sun. But if the sum be more 

 than 1 2, then was she at noon under a more westerly meridian ; and if less, then 

 under a more easterly one; and that by as many times 15 degrees as that sum 

 exceeds or falls short of 12 hours, 



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