728 THE POPULAR SCIENCE MONTHLY. 



an exact description of any technical matter, it is as well to secure 

 correctness by submitting the description to some friend acquainted 

 with the principles of the subject. For, singularly enough, people pay 

 much more attention to these descriptions when met with in novels, 

 than when given in text-books of science, and they thus come to re- 

 member thoroughly well precisely what they ought to forget. I think, 

 for instance, that it may not improbably have been some recollection 

 of " Foul Play " which led Mr. Lockyer to make the surprising state- 

 ment that longitude is determined at sea by comparing chronometer 

 time with local time, which is found "at noon by observing, with the 

 aid of a sextant, when the sun is at the highest point of its path." 

 Our novelists really must not lead the students of astronomy astray in 

 this manner. 



It will be clear to the reader, by this time, that the great point 

 in determining the longitude is, to have the true time of Greenwich 

 or some other reference station, in order that, by comparing this time 

 with ship-time, the longitude east or west of the reference station may 

 be ascertained. Ship-time can always be determined by a morning 

 or afternoon observation of the sun, or by observing a known star 

 when toward the east or west, at which time the diurnal motion 

 raises or depresses it most rapidly. The latitude being known, the 

 time of day (any given day) at which the sun or a star should have 

 any particular altitude is known also, and, therefore, conversely, when 

 the altitude of the sun or a star has been noted, the seaman has learned 

 the time of day. But to find Greenwich time is another matter ; 

 and, without Greenwich time, ship-time teaches nothing as to the lon- 

 gitude. How is the voyager at sea or in desert places to know the 

 exact time at Greenwich or some other fixed station ? We have seen 

 that chronometers are used for this purpose ; and chronometers are 

 now made so marvellously perfect in construction that they can be 

 trusted to show true time within a few seconds, under ordinary con- 

 ditions. But it must not be overlooked that in long voyages a chro- 

 nometer, however perfect its construction, is more liable to get wrong 

 than at a fixed station. That it is continually tossed and shaken is 

 something, but is not the chief trial to which it is exposed. The 

 great changes of temperature endured, when a ship passes from the 

 temperate latitudes across the torrid zone to the temperate zone 

 again, try a chronometer far more severely than any ordinary form of 

 motion. And then it is to be noted that a very insignificant time- 

 error corresponds to a difference of longitude quite sufficient to occa- 

 sion a serious error in the ship's estimated position. For this reason 

 and for others, it is desirable to have some means of determining 

 Greenwich time independently of chronometers. 



This, in fact, is the famous problem for the solution of which such 

 high rewards were offered and have been given. 1 It was to solve this 



1 For invention of the chronometer, Harrison (a Yorkshire carpenter, and the son of 



