April 14, 1892J 



NATURE 



559 



his eyes in the dark till the next observation. There were 

 twenty-five observers. The result seems worthy of notice. 



(i) Of the twenty-five all agree that the colour of the filament 

 is at first very pale. Thirteen call it very pale yellow, three 

 call it white, seven a faint pink, two a bluish white. 



(2) All agree that, as the temperature rises, the tint grows 

 deeper and redder, passing through orange before reaching 

 crimson. The words used to designate the final tint reached in 

 the experiment vary from deep reddish orange to copper colour, 

 dark red, blood red, crimson. 



I may add that some of the observers had had considerable 

 practice in observation, and their eyes were known to be normal 

 so far as the perception of the tints of the visible spectrum is 

 concerned. There is no reason to suppose that more than one, 

 or two at most, possessed any abnormal sense of colour. 



Assuming that in the cases of iron and carbon light oj greater 

 frequency of vibration is emitted as the temperalnre rises, in addi- 

 tion to 'the light emitted at lower temperatures (the vibrations 

 causing which are merely increased in amplitude), is it not 

 possible (i) that the selective power of the pigments of the retina 

 at first scarcely comes into play, the slower vibrations acting on 

 all to a certain extent, on the red more than the green, and the 

 green more than the violet, in the normal eye? or (2) Does not 

 the fact that all colours are more difficult to distinguish in a faint 

 light, e.g. moonlight, make it likely that very weak irritation of 

 any part of the retina (I mean a part which causes the sensation 

 of light, and that coloured, when the irritation is stronger) is 

 perceived as "light," the indication of specific absorption not 

 being strong enough in comparison with the total amount of 

 irritation to produce the sensation of any special colour in the 

 light perceived ? or lastly, if we do not make the above assump- 

 tion, it would seem that iron and carbon at all events emit, 

 when first visible, light of far wider limits of frequency of vibra- 

 tion than, so far as I know, is generally admitted. 



Some photographic experiments which I hope may throw 

 fresh light on the subject have been begun. 



Eton College Laboratory, April 4. T. C. Porter. 



Self-Registering Weather-cock. 



I SHOULD be grateful if any of your readers would kindly re- 

 commend me a simple, inexpensive instrument, to automatically 

 register the movements of a weather-cock above the roof. 



Such an appliance must roughly indicate the direction of the 

 wind at the time being. 



Some years ago, a London builder put me up a very ex- 

 pensive instrument, which, beyond making considerable noise, 

 was utterly useless. J. Lawrence-Hamilton. 



30 Sussex Square, Brighton, April 11. 



THE ROLLING OF SHIPS. 



ONE fact that often strikes the thoughtful traveller 

 by sea is that, notwithstanding the great and 

 numerous improvements of recent years which have 

 made life on shipboard pleasant and luxurious, little or 

 nothing has been done to steady a vessel when she meets 

 with waves that set her rolling heavily from side to side. 

 The tendency seems to be rather in the direction of in- 

 creased than of diminished rolling ; for the steadying 

 influence of sails, which makes the motion so easy and 

 agreeable in a sailing ship, is fast disappearing in large 

 steamers. Masts and sails add appreciably to the resist- 

 ance of large fast steamers ; so they have been cut down 

 in size year by year till such fragments of sail as still 

 remain are so small compared with the size of the ship as 

 to retain little power to reduce rolling. 



Shipowners and seamen do not show much sympathy 

 with the discomfort and misery that rolling causes to 

 most passengers. They perhaps get anxious about an 

 occasional vessel that acquires the evil reputation of 

 being a bad roller, because passengers may be frightened 

 away and the receipts fall off in consequence ; but beyond 

 wishing, or attempting, to deal with abnormal cases, 

 nothing seems to be thought of. Rolling is considered 

 incurable, or as not of sufficient '^importance to trouble 

 NO. I I 72, VOL. 45] 



about. Yet there is nothing which would contribute so 

 directly to the comfort of landsmen at sea, or do so 

 much to change what is for many misery and torture 

 into comfort, as to check and reduce as far as possible the 

 rolling proclivities of ships. 



The laws which govern rolling are now well understood, 

 and it is strange that this knowledge has not enabled an 

 effective means of control to be devised. What is 

 stranger still is that well-known means of mitigating 

 rolling — such as the use of bilge keels — are employed in 

 but very few cases. A ship rolls about a longitudinal 

 axis which is approximately at her centre of gravity, and 

 the rolling is practically isochronous at moderate angles 

 in ordinary ships. The heaviest rolling occurs when the 

 wave-period synchronizes with the natural period of 

 oscillation of the ship. Many vessels are comparatively 

 free from rolling till they meet waves of this period, and 

 if such meeting could be avoided, excessive rolling could 

 be prevented. Some vessels have periods as long as 

 fifteen to eighteen seconds for the double oscillation, and 

 as these would require to meet with waves 1300 to 1500 

 feet in length, in order to furnish the conditions of syn- 

 chronism, it is seldom that they suffer from heavy or 

 cumulative rolling. Such waves are, however, not rare 

 in the Atlantic. 



The limits of heavy rolling are fixed, of course, by the 

 resistance offered by the water and air to the transverse 

 rotation of the ship, which is very great because of the 

 large areas that directly oppose motion in a transverse 

 direction. But for this resistance, and the condition that 

 rolling is only isochronous within moderate angles of 

 inclination, a few waves of the same period as that of a 

 ship would capsize her. 



The two most obvious modes of preventing heavy 

 rolling are, therefore, (i) to make the period of rolling of 

 a ship as long as possible, so as to reduce the chances 

 of meeting waves whose period will synchronize with it, 

 and (2) to increase the resistance to rolling. The period 

 of a ship varies directly as her radius of gyration, and 

 inversely as the square root of her metacentric height. 

 Hence the period may be increased by increasing the 

 moment of inertia of the ship, or by decreasing the meta- 

 centric height. In armoured war-vessels the moment of 

 inertia is large, on account of the heavy weights of 

 armour on the sides, and the heavy guns that are either 

 placed at the side or high up above the centre of gravity. 

 Ordinary steamers have no such weights concentrated at 

 great distances from the centre of gravity, and their 

 moments of inertia are determined by the distribution of 

 material in the hull that is fixed by structural con- 

 ditions and by the stowage required for their voyages. 

 Metacentric height cannot be reduced below a certain 

 amount, which is necessary to prevent too easy inclina- 

 tion of the ship, or crankness, in still water. On the 

 whole, we may regard the longest periods that the largest 

 ships are likely to have with advantage to be about those 

 named above, i.e. fifteen to eighteen seconds. 



Length of period cannot give immunity against occa- 

 sional heavy rolling ; but increase of resistance reduces 

 the angles of roll at all times, and especially when the 

 angular velocity is greatest • and the rolling is worst. 

 Such resistance is furnished by the frictional resistance 

 of the bottom of a ship and by the direct resistance of 

 projecting parts of the bottom, such as the keel and the 

 large flat surfaces below at the stem and stern. This 

 resistance can be largely increased by means of bilge 

 keels. The value of bilge keels is recognized in the 

 Royal Navy, and the ships of the Navy have been fitted 

 with them for many years with highly .beneficial results. 

 The advantage of bilge keels was proved beyond all doubt 

 many years ago by careful experiments made in this 

 country and in France ; and the late Mr. W'm. Froude 

 showed, by the trials he made of H.M.S. Greyhound 



