FINDING THE WAT AT SEA. 7 i 9 



lence, the Nautical Almanac. The astronomer, in his fixed observa- 

 tory, finds this almanac essential to the prosecution of his observa- 

 tions ; the student of theoretical astronomy has continual occasion to 

 refer to it ; but, to the sea-captain, the Nautical Almanac has a far 

 more important use. The lives of sailors and passengers are depend- 

 ent upon its accuracy. It is, again, chiefly for the sailor that our 

 great nautical observatories have been erected, and that our astron- 

 omer-royal and his officers are engaged. What other work they 

 may do is subsidiary, and, as it were, incidental. Their chief work is 

 to time this great clock, our earth, and so to trace the motions of 

 those celestial indices, which afford our fundamental time-measures, 

 as to insure as far as possible the safety of our navy, royal and mer- 

 cantile. 1 



Let us see how this is brought about, not, indeed, by inquiring into 

 the processes by which, at the Greenwich Observatory, the elements 

 of safety are obtained, but by considering the method by which a sea- 

 man makes use of these elements. 



In the measures heretofore considered, the captain of a ship in 

 reality relies on terrestrial measurements. He reasons that, being on 

 such and such a day in a given place, and having in the interval sailed 

 so many miles in such and such directions, he must at the time being 

 be in such and such a place. This is called " navigation." In the 

 processes next to be considered, which constitute a part of the science 

 of nautical astronomy, the seaman trusts to celestial observations in- 

 dependently of all terrestrial measurements. 



The points to be determined by the voyager are his latitude and 

 longitude. The latitude is the distance north or south of the equa- 

 tor, and is measured always from the equator in degrees, the distance 

 from equator to pole being divided into ninety equal parts, each of 

 which is a degree. 2 The longitude is the distance east or west of 

 Greenwich (in English usage, but other nations employ a different 

 starting-point for measuring longitudes from). Longitude is not meas- 

 ured in miles, but in degrees. The way of measuring is not very 



1 This consideration has been altogether lost sight of in certain recent propositions 

 for extending government aid to astronomical inquiries of another sort. It may be a 

 most desirable thing that government should find means for inquiring into the physical 

 condition of sun and moon, planets and comets, stars and all the warious orders of star- 

 clusters. But, if such matters are to be studied at government expense, it should be un- 

 derstood that the inquiry is undertaken with the sole purpose of advancing our knowl- 

 edge of these interesting subjects, and should not be brought into comparison with the 

 utilitarian labors for which our Royal Observatory was founded. 



8 Throughout this explanation all minuter details are neglected. In reality, in conse- 

 quence of the flattening of the earth's globe, the degrees of latitude are not equal, being 

 larger the farther we go from the equator. Moreover, strictly speaking, it is incorrect 

 to speak of distances being divided into degrees, or to say that a degree of latitude or 

 longitude contains so many miles ; yet it is so exceedingly inconvenient to employ any 

 other way of speaking in popular description, that I trust any astronomers or mathema- 

 ticians who may read this article will forgive the solecism. 



