316 



NATURE 



A ugiist 



1883 



Scientifically, however, the most important ichthyological 

 collection exhibited in the Indian department is beyond 

 doubt that shown by Dr. Day — fine specimens in alcohol 

 of several hundred species illustrated in his great work, 

 "The Fishes of India." Dr. Day also exhibits a set of 

 his coloured drawings of Indian fish. 



The Straits Settlements exhibit a fair sample of the 

 sea-fish of that region, unfortunately unnamed ; there are 

 also a few freshwater fishes from Singapore. 



China has a rich and interesting collection of fish, and 

 also some very good drawings of them. Unfortunately 

 they also are unnamed. The fishes exhibited are princi- 

 pally in alcohol, and come mostly from Swatow; some 

 are very rare, and others appear to be new to science ; 

 amongst those of some interest I may mention : Elacate. 

 niger, Rhynchobatis ancylostomus, Zygana malleus, Ces- 

 tracion zebra, and some fine species of Pteroplatea, 

 //, Raja. One fish of special importance is Polyo- 

 don gladius, from Tchang. 



I have now finished, and hope I have been successful 

 in giving a fair general sketch of the Vertebrata shown in 

 the International Fisheries Exhibition ; some of the 

 contributions might, no doubt, have been better, but on 

 the whole we may well be content with the opportunity thus 

 given of seeing many good things. 



London, July 17 Henry H. Giglioli 



STELLA R NA VIGA TION 

 Stellar Navigation, with New A, B, and C Tables for 



Finding Latitude, Longitude, and Azimuth by Easy 



Me/hods. By W. H. Rosser. (Published by Norie 



and Wilson, 1883.) 

 '"T'HERE can be no doubt that star observations, when 

 •1- the horizon is clenr and well defined, are the best 

 means by which the position of a ship at sea can be 

 ascertained ; as, by altitudes of two or more stars, in 

 suitable positions with regard to the observer, the latitude 

 and longitude can be obtained at the same moment, 

 whereas single observations of heavenly bodies only give 

 one element, and consequently it is not possible to obtain 

 simultaneous observations for both elements during the 

 day, unless either the moon, Venus, or Jupiter passes the 

 meridian whilst the sun is above the horizon. 



It is true that when the azimuth of the sun is changing 

 rapidly the latitude as well as the longitude can be 

 obtained from two sets of observations, taken at a given 

 interval of time, provided the alteration in the position of 

 the ship, during that interval, can be accurately deter- 

 mined ; but this supposes a knowledge, not only of the 

 course and distance traversed during the interval, but 

 also of the tidal stream or current affecting the ship, 

 which is usually uncertain. 



Any writer or teacher, therefore, who impresses on 

 navigators and students the importance of obtaining 

 star observations is deserving of praise, for it is impos- 

 sible to take too much precaution in ascertaining the 

 position of a ship ; cloudy or foggy weather may set in at 

 any moment, and an opportunity lost can never be 

 recovered. 



Mr. Rosser has in the Nautical Magazine drawn atten- 

 tion to the value of Sumner's method of working out 

 simultaneous observations of two or more stars, and there 



is little doubt that it is the best, as it is the only method 

 by which results obtained from simultaneous observations 

 of three or more heavenly bodies can be readily combined. 

 It has been for years constantly used by the naval officers 

 employed on surveying service, and in fact by most navi- 

 gators, though, perhaps, they seldom take observations of 

 more than t»vo stars at the same time. We however 

 prefer three for precisely the same reason we prefer three 

 to two chronometers. 



Sumner's method may be thus briefly described. As 

 at a given moment of time each heavenly body is at the 

 z- nith at some points on the earth's surface, so at that 

 moment circles may be described on which its altitude 

 will be 8o°, 70 , 60°, &c. If then the altitudes of two 

 stars are obtained at the same instant, and the Greenwich 

 time be known, the two circles of altitude may be drawn 

 on the earth's surface with the points where the stars are 

 in the zenith as centres, and the point where these circles 

 cut will be the position of the observers. In actual 

 practice it is not necessary to draw the circles, it is merely 

 necessary to be able to draw the arc of a small portion of 

 each circle ; for the position of the observer being gene- 

 rally known to within twenty miles, the arc of the circle 

 of altitude on which he is situated can be readily drawn. 



The method of obtaining this arc of altitude formerly 

 practised was to calculate the longitude with two latitudes, 

 using the same two latitudes for each star, which gave 

 four resulting longitudes ; then, by plotting these four 

 longitudes on the two parallels, and drawing lines joining 

 the longitudes given by each star, two circles of altitude 

 were obtained, which either cut in a given point, or would 

 do when produced, which point was the position of the 

 observer. 



This method of calculation was however quickly dis- 

 carded for a more simple one, where one latitude only was 

 used ; for as the azimuth of a heavenly body can be 

 readily calculated at the same time as its hour angle, and 

 the azimuth being the bearings of the place where the 

 star is at the zenith from the observer, it is evident that a 

 line drawn at right angles to the azimuth will be the arc 

 of the circle of altitude on which the observer is situated, 

 as practically the arc is, for so short a distance as twenty 

 or thirty miles, a straight line. The two longitudes on one 

 parallel with the azimuth enable the two arcs of altitude 

 to be plotted as before. 



The importance of Sumner's method has not as yet 

 been pointed out in any treatise on navigation, principally 

 because since the time of Lieut. Raper, R.N., no treatise 

 has been written by a practical navigator. It is true the 

 method is mentioned in Riddle's " Navigation," and was 

 taught by him many years since,- though not in the form 

 now adopted, and we think Mr. Rosser has done good 

 service by urging its importance and the importance of 

 stellar navigation generally. All navigators should in our 

 opinion obtain star observations every night and morning, 

 during twilight, as constant practice will alone render 

 them expert in these observations, and familiar with the 

 positions of the stars. 



The extra work entailed by such observations will be 

 amply repaid if, when standing in towards the land, after 

 three or four days' thick weather, a partial break in the 

 clouds enables the expert navigator to secure a couple of 

 star observations which give him his position and enable 



