March 8, 1877] 



NA TURE 



403 



SIR WILLIAM THOMSON ON NAVIGATION^ 



■pOPULAR lectures rarely contain much that deserves 

 ■■- repetition or notice in a review. But when the 

 lecturer is Sir William Thomson and his subject navi- 

 gation, we may be sure that we shall hear something 

 that we have not heard before, and that we should hear, 

 if we wish to keep abreast of the advance of nautical 

 science. When a reformer contents himself with merely 

 rnaking suggestions and leaving it to others to test them, 

 his work is comparatively easy and its results are propor- 

 tionally valueless. The suggestions of Sir W. Thomson 

 have the very special merit that they are submitted to a 

 practical test before he gives them utterance, and after 

 he has done so he is far from considering his connection 

 with them over. Every part of every crude idea or novel 

 appliance is submitted to a searching process of natural 

 selection which must cost the author as much labour as 

 to watch it gives the onlooker pleasure, and those who see 

 only the final survival of the fittest cannot form anything 

 like a just conception of the time and pains which have 

 been bestowed on the rejection of the less fit. 



We cannot here find space to notice those parts of 

 the lecture which are the feproduction of old and 

 received truths, interesting though these are by virtue of 

 their skilful dressing. We must pass on at once to men- 

 tion one or two points which are either in themselves 

 new, or which have as yet failed to secure the recognition 

 they deserve. 



The first important novelty we come to is the discovery 

 by Mr. Hartnup, astronomer to the Harbour Board of 

 Liverpool, of a system of rating chronometers, which 

 gives an almost perfect means of compensating for 

 change of rate due to change of temperature. It had 

 long been known that no compensation balance could be 

 made to keep time correctly through wide ranges of 

 temperature : — 



"Thus the best chronometers of the best makers in 

 modern times are practically perfect only within a range 

 of 5° or 10° Fahrenheit on each side of a certain tempera- 

 ture, infinitely near to which the compensation is perfect 

 in the individual chronometer. 



" The temperature for which the compensation is per- 

 fect, and the amount of deviation from peifection at tem- 

 peratures differing from it are different in different chro- 

 nometers. Mr. Hartnup finds that at the temperature 

 for which the compensation is perfect, the chronometer 

 goes faster than at any other temperature, and that the 

 ra»e at any other temperature is calculated with mar- 

 vellous accuracy (if the chronometer be a good one) by 

 subtracting from the rate at that critical temperature the 

 number obtained by multiplying the square of the differ- 

 ence of temperature by a certain constant coefficient." 



Two chronometers recently carried from Liverpool to 

 Calcutta, when rated on Mr. Hartnup's plan, gave a mean 

 error of six seconds, while by the ordinary method the 

 reckonings of Greenwich time from them differed by 

 4 minutes 35 seconds. The navigator could easily secure 

 the advantages of Mr. Hartnup's system by noticing the 

 temperature of his chronometer-case daily, and entering 

 a few figures in a note-book. His work would be much 

 facilitated if the thermometer used were graduated to 

 squares of numbers of degrees from the temperature of 

 maximum rate. 



The lecturer discusses at considerable length various 

 modifications of the pressure-log, the invention of Mr. 

 J. K Napier and Mr. Berthon, the principle of which is 

 to measure the speed of the ship by observing to what 

 height a column of water rises in a vertical tube, the 



' Navigation. A Lectnfe delivered under the atfspices of the Glasgow 

 Science Lectures Association. By Sir William Thornson, DC. L., LL.D., 

 F.R.S., Professor of Natural Philosophy in the University of Glasgow, 

 and Fellow of St. Peter's College, Cambridge. (London and Glasgow '; 

 William Collins, Sons, and Company, 1876.) 



bottom end of which dips into the sea and faces forwards. 

 It shows the ship's velocity through the water at any 

 instant, instead, like all other logs, of telling the dis- 

 tance run during a known length of time. The latter 

 piece of information is what is chiefly wanted for the 

 purposes of ordinary navigation, but the former could not 

 fail to be of immense use in the navy when ships are 

 sailing in squadron. Even now, a rough approximation 

 to a knowledge of velocity is got in the navy by the use of 

 indicators showing the number of revolutions per minute 

 made by the screw, and these satisfy very imperfectly the 

 requirements of the case, as appears from the evidence 

 given at the court martial on the loss of the Vanguard. 

 The Admiral signalled to the squadron that his ship was 

 about to go at thirty-three revolutions, which he after- 

 wards explained to mean that he desired the squadron to 

 go as nearly as possible at a speed of seven knots. Had 

 each of the ships been provided with a pressure log, he 

 might at once have given an order of whose meaning 

 there could have been no possible doubt, and which it 

 would have been perfectly easy for every ship to obey. 



The taking of soundings to determine the depth is one 

 of the most important of nautical operations. In surveys 

 of the ocean's bed and for guidance in cable laying, 

 soundings have to be made in great depths, often of 

 several thousand fathoms. The trouble and time in- 

 volved in taking a deep-sea sounding have been greatly 

 reduced by Sir W. Thomson by the substitution of a 

 single steel pianoforte wire for the hemp rope formerly 

 used as a sounding line. The advantage of the wire is 

 the comparatively small resistance it meets with in pass- 

 ing through the water. When hemp rope is used for 

 sounding in deep water a weight of three or four hundred 

 pounds must be attached to it, and even then it descends 

 very .slowly. When it reaches the bottom the weight 

 is detached by a trigger and is therefore lost. When 

 wire is used a weight of about thirty pounds suf- 

 fices ; it descends very much more rapidly, and there 

 is nothing to prevent its recovery each time. For 

 very small depths such as are met with in the imme- 

 diate neighbourhood of land, the hand lead is convenient 

 and sufficient, but there is a third class of soundings, 

 those which are (or should be) made in depths of about 

 twenty fathoms and upwards when a ship is approaching 

 land. To be able to take " flying " soundings— that is, to 

 find the depth without stopping the ship— in any depth 

 from 20 to 1 50 fathoms, is a matter of the greatest possible 

 importance in ordinary navigation. Sir W. Thomson has 

 succeeded in making it easy to do this, by the aid of his 

 pianoforte wire in combination with another apparatus 

 which he described at the recent meeting of the British 

 Association. This consists of a pressure gauge of very 

 simple construction, which is attached close to the end of 

 the sounding4ine, and which, by registering the maximum 

 pressure to which it has been subjected duiing immersion, 

 registers the maxim i-m depth it has attained. This indi- 

 cation is of course quite independent of the length of wire 

 out, and is not affected by the fact that the ship is in 

 motion. The pressure gauge consists of a small glass 

 tube, of about ^xs i"ch bore, open at the lower end, but 

 closed at the top. As this descends, the water rises in 

 the tube compressing the column of air. In order that a 

 permanent record may be left of the maximum height to 

 which the water rises, the interior of the tube is coated 

 along its whole length with starch, in which red prus- 

 siate of potash has been dissolved, and just at the 

 mouth of the tube are placed a few crystals of sulphate 

 of iron, which are held in position by an outer guard 

 tube. The water which rises in the tube carries with it a 

 little sulphate of iron in solution, and so leaves a per- 

 manent record of its height by staining the tube with 

 Prussian blue. The system of sounding by wire has now 

 had abundant trial, and its success is thoroughly esta- 

 blished. Its author was, no doubt, quite within the 



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