July 13, 1882] 



NATURE 



246 



(66 - 15*2) = 354 per cent. 



Efficiency = l*lf 



If the same formula is applied to the Waterivitck, at 

 9 '3 knots — 



Efficiency = '— — Ll (30 - 157) = 499 per cent., 



giving about 20 per cent, less efficiency to that vessel, 

 than is given by the accepted formula first stated. 



It has been explained that the assumptions upon which 

 the first formula rests are not fairly representative of the 

 conditions of practice. For example, the deduction 

 therefrom (stated above), that it is advantageous to 

 operate upon larger quantities of water, and to reduce 

 the 'excess in speed of outflow above the speed of the 

 ship requires an important qualification in practice This 

 deduction would be absolutely correct were it not for the 

 waste-work which has to be done in giving the motion to 

 the water; but in actual practice the growth in that waste 

 work may exceed the gain obtained by dealing with larger 

 quantities of water. The parallel case in a screw steamer 

 is that wherein screws of too large diameter or too large 

 surface may involve so much more waste work on fric- 

 tional or edgewise resistances, that it is preferable to use 

 smaller screws, which operate on smaller quantities of 

 water, but secure a more economical expenditure of 

 power for a given speed, or enable higher speeds to be 

 attained with a given horse-power. In setting aside the 

 commonly received view, and making trial of a system 

 wherein the mean velocity of the outflowing jets is ex- 

 tremely great, while the quantity of water operated on is 

 small, Dr. Fleischer has made an experiment of the 

 greatest interest to all concerned with steam propulsion. 

 If his figures are accepted it is obvious that his system 

 involves much less waste work than the Ruthven system, 

 between the power indicated in the cylinders and the 

 power accounted for in the outflowing jets. On the other 

 hand, as we have endeavoured to explain, this economy 

 of the Fleischer system does not represent the compara- 

 tive efficiency of the propelling apparatus : because the 

 high and variable velocity of outflow must involve a con- 

 siderable amount of waste work in the race. A complete 

 comparison could only be made if in the same vessel, or 

 in two vessels of identical form and with identical boiler- 

 power, there were fitted, first, the Fleischer hydromotor ; 

 and secondly, the Ruthven arrangement. Then with the 

 same steam-producing power a careful series of trials 

 would settle the matter conclusively. The Swedes did 

 something of this kind in order to compare the efficiencies 

 of twin-screws and water-jets, with the result that the 

 latter were shown to be greatly inferior. Of course it 

 cannot be expected that Dr. Fleischer would undertake 

 such trials unaided ; on the other hand, if his system is 

 put forward for adoption in preference to the Ruthven 

 system, it must, at least, be shown to be more efficient, 

 not only in certain intermediate stages in the operations 

 of giving momentum to the jets, but as a whole. This 

 result does not appear to have been attained as yet, so far 

 as can be judged from the published results of trials. The 

 information which is accessible is not complete, and some 

 of the proposed standards of comparison are open to 

 doubt. It is to be hoped, however, that the zeal and 

 ability which have been displayed already by Dr. Fleischer 

 will be still further illustrated in the continued investiga- 

 tion of the capabilities of his novel system of propulsion. 



W. H. White 



A RAPID-VIEW INSTRUMENT FOR MOMEN- 

 TARY ATTITUDES 



'"FHE wonderful photographs by Muybridge of the 



- 1 horse in motion and those by Maret of the bird on 



the wing induced me to attempt the construction of 



apparatus by which the otherwise unassisted eye could 

 verify their results and catch other transient phases of 

 rapid gesture. Its execution has proved unexpectedly 

 easy, and the result is that even the rudest of the instru- 

 ments I have used is sufficient for the former purpose ; 

 it will even show the wheel of a bicycle at full speed as a 

 well-defined and apparently stationary object. This little 

 apparatus may prove to be an important instrument of 

 research in the hands of observers of beasts, birds and 

 insects, and of physicists who investigate such subjects 

 as the behaviour of fluids in motion. 



My object was (1) to transmit a brief glimpse of a 

 moving body, (2) to transmit two or more such glimpses 

 separated by very short intervals, and to cause the suc- 

 cessive images to appear as simultaneous pictures in 

 separate compartments in the same field of view. 



The power of the eye to be impressed by a glimpse of 

 very brief duration has not, I think, been duly recognized. 

 Its sensitivity is vastly superior to that of a so-called 

 "instanteous" photographic plate when exposed in a 

 camera, but it is of a different quality, because the im- 

 pression induced at each instant of time upon the eye 

 lasts barely for the tenth of a second, whereas that upon 

 a photographic plate is accumulative. There is a con- 

 tinual and rapid leakage of the effect of light upon the 

 eye that wastes the continual supply of stimulus, so that 

 the brightness of the sensorial image at any moment 

 is no more than the sum of a series of infinitesimally 

 short impressions received during the past (say) tenth of 

 a second, of which the most recent is the brightest, the 

 earliest is the faintest, and the intermediate ones have 

 intermediate degrees of strength according to some law, 

 which an apparatus I shall describe gives us means of 

 investigating. After the lapse of one-tenth of a second 

 the capacity of the eye to receive a stronger impression 

 has become saturated, and though the gaze may be in- 

 definitely prolonged the image will become no brighter 

 unless the illumination is increased. 



This being premised, let us compare the sensitivity of 

 the eye with that of the rapid plate in the photographic 

 camera under conditions in which the eye is just capable 

 of obtaining a clear view, let us say during an overcast 

 day in a sitting room whose window does not occupy 

 more than one-thirtieth of the total area of wall and 

 ceiling, which is the light under which most of us habitu- 

 ally write and read. A glimpse under these circumstances 

 of one-tenth of a second in duration, suffices, as we have 

 just seen, to give a clear view, but the sensitive photo- 

 graphic plates sold in the shops as "instantaneous" will 

 not give a portrait in that light under thirty seconds 

 exposure. In other words, the sensitivity of the eye is 

 fully 300 times as great as that of the plate. Of course I 

 am aware that more sensitive plates than these have been 

 made, and I have seen a rapidly revolving wheel photo- 

 graped under the momentary illumination of an electric 

 spark, but I have never heard of that being done when 

 at the same time the revolving wheel was not perfectly 

 distinct to the eye. 



The range of ordinary illumination is very great. The 

 photographer who requires thirty seconds in a dim 

 window-light, would photograph clouds in some minute 

 fraction of a second, showing that the illumination of 

 the latter is fully one thousand-fold greater. If then 

 the eye has been shaded and adapted to a dim light, 

 an object in bright sunshine may require no more than 

 the thousandth part of the tenth of a second to be 

 visible, and in saying this, I am confident that I am 

 underestimating what could be done. Consider what 

 even this means : a cannon ball of ten inches diameter 

 in its mid career travels with a velocity of little more 

 than 1,000 feet in a second ; in one ten thousandth of a 

 second it would shift its place through only one tenth of 

 its diameter, and would present to the eye, if it could be 

 viewed under the above-mentioned conditions, the ap- 



