352 



NATURE 



{Feb. 28, 1878 



Lord John Manners stated in the House of Commons on 

 Thursday last that experiments have been made by officers of 

 the Post -Office with the telephone, the result being that the 

 instrument is not at present considered suitable for public 

 telegraphy. 



In Prof, Lebour's letter on Marine F ossils in the Gannister 

 Beds of Northumberland, in last week's Nature, the word 

 country should have been county. It is the first time marine 

 forms have been found in this series in Northumberland. 



The additions to the Zoological Society's Gardens during the 

 pas week include two Black-winged Pea-Fowls {Pavo tiigri- 

 pennis) from Cochin China, presented by the Hon. A. S. G. 

 Canning, F.Z.S. ; a Javan Parrakeet [Palaornis javanica) from 

 Muttra, North-West India, presented by Mr. Barthorp ; two 

 Ked-vented Bulbuls (^Pycnonotus hcetnorrhous) from India, pre- 

 sented by Col. A. L. Annerley, F.Z.S. ; two Leopards (^^/w 

 pardus) from Persia, deposited ; two Barbary Wild Sheep {Ovis 

 tragelaphus) from North Africa ; two I'ale-headed Parrakeets 

 {Platycercus pallidiceps) from North-East Australia ; four Tur- 

 quoisine Parrakeets {Eupkej?ia pulchella) from New South Wales, 

 pu'chased ; two Tigers {Felis tigris), bom in the Gardens. 



ON COMPASS ADJUSTMENT IN IRON SHIPS 1 

 II. 



AN important objection was made to me some years a^o by 

 Capt. Evans against the use of quadrantal correctors in the 

 Navy, that they would prevent the taking of bearings by the 

 prismatic azimuth arrangement, which forms part of the Admiralty 

 standard compass. The azimuth mirror (Fig. 5) applied to the com- 

 pass before you was designed to obviate that objection. Its use 

 even for taking bearings of objects on the horizon is not interfered 

 with by the globes constituting the quadrantal correctors, even 

 if their highest points rise as high as five inches above the glass 

 of the compass-bowl. It is founded on the principal of the 

 camera lucida. The observer when taking a bearing turns the 

 instrument round its vertical axis until the mirror and lens 

 are fairly opposite to the object. He then looks through 

 the lens at the degree divisions of the compass-card, 

 and turns the mirror round its horizontal axis till 

 he brings the image of the object to fall on the card. 

 He then reads directly on the card the compass bearing of 

 the object. Besides fulfilling the purpose for which it was 

 originally designed, to allow bearings to be taken without im- 

 pediment from the quadrantal correctors, the azimuth mirror has 

 a great advantage in not requiring any adjustment of the instru- 

 ment, such as that by which, in the prism compass the hair is 

 brought to exactly cover the object. The focal length of the 

 lens in the azimuth mirror is about 12 per cent, longer than the 

 radius of the circle of the compass-card, and thus, by an ele- 

 mentary optical principle, it follows that two objects a degree 

 asunder on the horizon will, by their images seen in the azimuth 

 mirror, cover a space of i°*i2 of the divided circle of the com- 

 pass-card seen through the lens. Hence, turning the azimuth 

 instrument round its vertical axis through one degree will only 

 alter the apparent bearing of an object on the horizon by '12°. 

 Thus it is not necessary to adjust it exactly to the direct position 

 for the bearing of any particular object. If it be designedly put 

 even as much as 4° awry on either side of the direct position, the 

 error on the bearing would hardly amount to half a degree. If 

 the instrument were to be used solely for taking bearings of 

 objects on the horizon, the focal length of the lens should be 

 made exactly equal to the radius of the circle, and thus even the 

 small error of •12° in the bearing for one degree of error in the 

 setting would be avoided. But one of the most important uses 

 of the azimuth instrument at sea is to correct the compass by 

 bearings of sun or stars at altitudes of from 0° to 50° or 60° 

 above the horizon. The actual focal length is chosen to suit an 

 altitude of 27°, or thereabouts (this being the angle whose 

 natural secant is I*i2). Thus if two objects whose altitudes are 



• Report of paper read to the Royal United Service Institution, February 

 4, by Sir Wm. Thomson, LL.D., F.R.S., P.R.S.E., Professor of Natural 

 Philosophy in the University of Glasgow, and Fellow of St. Peter's College, 

 Cambridge. Revised by the Author. [The Council of the R. U.S.I, have 

 kindly permitted us to publish Sir W. Thomson's paper in advance, and have 

 rmted us th« use of the illustrations, — Ed.] Continued from p. 334. 



27°, or thereabouts, and difference of azimuths 1°, are taken 

 simultaneously in the azimuth mirror, their difference of bearings 

 will be shown as one degree by the divided circle of the com- 

 pass-card seen through the lens. Hence for taking the azimuth 

 of star or sun at an altitude of 27°, or thereabouts, no setting of 

 the azimuth mirror by turning round the vertical axis is neces- 

 sary, except just to bring the object into the field of view, when 

 its bearing will immediately be seen accurately shown on the 

 divided circle of the compass-card. This is a very valuable 

 quality for use in rough weather at sea, or when there are flying 

 clouds which just allow a glimpse of the object, whether sun or 

 star, to be caught, without allowing time to perform an adjust- 

 ment, such as that of bringing the hair, or rather the estimated 

 middle of the space traversed by the hair in the rolling of the 

 ship, to coincide with the object. The same degree of error a:s 

 on the horizon, but in the opposite direction, is produced by 

 imperfect setting round the vertical axis in taking the bearing of 

 an object at an elevation of 38°. 



Thus for objects from the horizon up to 38° of altitude the 

 error in the bearing is less than 12 per cent, of the error of the 

 setting. For objects at a higher elevation than 38° the error 

 rapidly increases ; but even at 60° altitude the error on the 

 bearing is a little less than half the error of the setting ; and it is 

 always easy, if desired, to make the error of the setting less than 



Fig. s. 



a degree by turning the instrument so that the red point, which 

 you see below the lens, shall point within a degree of the posi- 

 tion marked on the circle of the compass-card by the image of 

 the object. 



For taking star azimuths the azimuth mirror has the great 

 advantage over the prism compass, with its then invisible hair, 

 that the image of the object is thrown directly on the illuminated 

 scale of the compass-card. The degree of illumination may be 

 made less or more, according to faintness or brilliance of the 

 object, by holding a binnacle lamp in the hand at a greater or 

 less distance, and letting its light shine on the portion of the 

 compass-card circle seen through the lens. Indeed, with the 

 azimuth minor it is easier to take the bearing of a moderately 

 bright star by night than of the sun by day : the star is seen as a 

 fine point on the degree division, or between two, and it is easy 

 to read of its position instantly by estimation to the tenth of a 

 degree. The easiest, as well as the most accurate of all, however, 

 is the sun when bright enough and high enough above the 

 horizon to give a good shadow on the compass-card. For this 

 purpose is the stout silk thread which you see, attached to the 

 framework of the azimuth mirror in such a position that when 

 the instrument is properly placed on the glass of the compass- 

 bowl, the thread is perpendicular to the glass and through the 

 central bearing-point of the compass. 



Another advantage of the azimuth mirror particularly im- 

 portant for taking bearings at sea when there is much motion, is 



