Aug. 21, 1885.] 



♦ KNOWLEDGE ♦ 



^3^ "an illustrated' " W^' 



^^ MAGAZINE OF SCIENCE ^1 

 1 Plaindtworded-exactudscrebm 



LONDON: FRIDAY, AUGUST 21, 1885. 



Contents of No. 



ThoDRht and Language. 



FirstV'ar LTsons'.' (IT 



ByK. A. Proctor 



The Philosophy of Cloth 



FINDING THE WAY AT SEA. 



Bt Richard A. Proctor. 



I PROPOSE to give a brief sketch of the methods in 

 use for finding the way at sea, in order that the 

 general principles on which safety depends may be 

 recognised by the general reader. 



It is known, of course, to everyone, that a ship's 

 course and rate of sailing are carefully noted throughout 

 her voyage. Every change of her course is taken account 

 of, as well as every change in her rate of advance, 

 whether under sail or steam or both combined. If all 

 this could be quite accurately managed, the position of 

 the shiji at any hour could be known, because it would 

 be easy to mark down on a chart the successive stages of 

 her journey, from the moment when she left port. But 

 a variety of circumstances render this impossible. 



To begin with, the exact course of a ship cannot be 

 known, l<reause there is only the ship's compass to de- 

 terniine lier course by, and a ship's compass is not an 

 instruiueut iiii'ording perfectly accurate indications. Let 

 anyone on a sea voyage observe the compass for a short 

 time, being careful not to break the good old rule which 

 forbids speech to the " man at the wheel," and he will 

 presently become aware of the fact that the ship is not 

 kept rigidly to one course even for a short time. The 

 steersman keeps her as near as he can to a particular 

 course, but she is continually deviating, now a little on 

 one side now a little on the other of the intended direc- 

 tion ; and even the general accuracy with which that 

 course is followed is a matter of estimation, and depends 

 on the skill of the individual steersman. Looking at the 

 compass card in steady weather, a course may seem very 

 closely followed ; perhaps the needle's end may not bo a 

 hundredth part of an inch (on the average) from the 

 position it should have. But a hundredtli part of an 

 inch on the circumference of the compass card would 

 correspond to a considerable deviation in the course of 

 a run of twenty or thirty knots ; and there is nothing to 

 prevent the errors so arising from accumulating in a long 

 journey until a ship might be thirty or forty miles from 

 her estimated place. To this may be added the circum- 



^l 111, I til it the direction of the needle is diilerent in 

 ililt n HI 1 lifts of the earth. In some jilaces it points to 

 ill, I I I f the north, in others to the west. And although 

 the .iLiu.il " variation of the compass," as this peculiarity 

 is called, is known in a general way for all parts of the 

 earth, yet such knowledge has no claim to actual exact- 

 ness. There is, also, an important danger, as recent 

 instances have shown, in the possible change of the 

 position of the ship's compass on account of iron in her 

 cirgo. 



But a far more important cause of error, in determina- 

 tions merely depending on the log-book, is that arising 

 from uncertainty as to the ship's rate of progress. The 

 log-line gives only a rough idea of the ship's rate at the 

 time when the log is cast ;* and, of course, a ship's rate 

 does not remain constant, even when she is under steam 

 alone. Then, again, currents carry the ship along some- 

 times with considerable rapidity; and the log-line affords 

 no indication of their action : while no reliance can be 

 placed on the estimated rates even of known currents. 

 Thus the distance made on any course may differ con- 

 siderably from the estimated distance ; and when several 

 days' sailing are dealt with, an error of large amount may 

 readily accumulate. 



For these and other reasons, a ship's captain places 

 little reliance on what is called " the day's work," that 

 is, the change in the ship's position from noon to noon 

 as estimated from the compass courses entered in the 

 log-book, and the distances supposed to be run on these 

 courses. It is absolutely essential that such estimates 

 should be carefully made, because under unfavourable 

 conditions of weather there may be no other means of 

 guessing at the ship's position. But the only really re- 

 liable way of determining a ship's place is by astronomical 

 observations. It is on this account that the almanac 

 published by the Admiralty, in which the position and 

 appai'cnt motions of the celestial bodies are indicated 

 four or five years in advance, is called, par e.rclhnce, 

 the " iSTautical Almanac." The astronomer iu his fixed 

 observatory finds this almanac essential to the pnisecntion 

 of his observations ; the student of theoretical astronomy 

 has continual occasion to refer to it ; but to the sea- 

 captain the "Natitieal Almanac" has afar more important 

 use. The lives of sailor.? and passcnn-ors arc dependent 

 upon its accuracy. It is, :ig:iin, cliii l!\ f", i ; ' , - !' r that 

 our great nautical ol s. i'v,it,'rli s li:!\i ' ; ;ind 



that our AstronomtT- luy.il ami lii^ < ' _'i'd. 



Wliat othtT work till y'lniy ,1,1 ;> -ii-:r ■- . ; ns it 



l.stial 



ivy, national and ii 



possible, the sal 





* The log is a fiat piece of wood of quadrantal shape, so loaded 

 at the rim as to float with the point (that is, the centre of the 

 quadrant) nppennost. To this a line about three hundred yards 

 long is fastened. The log is thrown overheard and comes almost 

 immediately to rest on the surface of the sea, the line being 

 suffered to run freely out. M;irka on the log-line divide i 



efpia 





X Go' 



in of SI 



ai,,l li. i^ ^. .: . ■■ ■ .. ■> arious orders of 



Btur ,- Ii.-.i- r-^. 1' ;• ''■ ^'•. '-; :mi'-;,ji ^ in ' : ' ^ -■ . ' i iit Government 

 expense, it should bo understood that the imiuiry is undertaken 

 with the solo purpose of advancing our knowledge of these interest- 

 ing subjects, and the work should not bo brought into comparison 



