354 



NATURE 



[February 9, 1893 



of the alley. The object of this secondary railway is to carry the 

 smaller carriages, on which are mounted the actual experimental 

 apparatus of different kinds ; so that these may be adjusted on 

 this railway to any desired position fore and aft on the main 

 carriage. The carriage is driven by means of wire rope from a 

 stationary lo" Tower spherical engine, a high power being 

 required so as to start the truck quickly for high speed experi- 

 ments. The ordinary speeds range between lOO and 500 feet 

 per minute ; for some classes of models experiments are occa- 

 sionally made up to about 850 feet per minute or nine and a 

 half miles per hour. The truck has been run at over 1200 feet 

 per minute, or about fourteen miles per hour. The governor, by 

 which the speed of the engine is regulated, is a very interesting 

 and ingenious piece of mechanism, which has been modified 

 from the design of that which was used on the engine at Torquay. 

 There are two symmetrical bell-cranks carrying weights, and 

 attached to each other by links, having slotted holes so as to 

 allow the bell-cranks to have a very small range of freedom of 

 angular motion. When a given speed of rotation is reached, the 

 centrifugal force of the weights overcomes the tension of a spiral 

 spring, provided for the purpose, and the governing action is 

 brought into play in the following manner : — There is a 

 hooked rod, by means of which the increase in the 

 angular altitude of the weights (due to centrifugal force) 

 brings a friction disc break into play, which in turn 

 has the effect of extending a spiral spring connected with 

 the engine throttle valve, which is thus closed so as to shut off 

 steam. It will be easily seen how much more delicate an ad- 

 justment this device gives than the old Watt governor with the 

 balls acting directly on the valve. The extension of the spring, 

 and the consequent distance of departure of the throttle 

 valve from its full open position are proportional to the fric- 

 tional turning movement applied to the stationary wheel, which 

 movement is itself proportional to the pressure brought to bear 

 upon it by the bell-cranks ; in other words it is proportional to 

 the excess of the speed above that at which the centrifugal force 

 of the weights just equals the tension of the spiral spring. To 

 give greater sensitiveness of action the bell-cranks are not hung 

 on pin joints but on flat springs after the fashion of a clock pen- 

 dulum, safeguards being provided in case of the springs breaking. 

 With the Torquay governor, which was similar in principle to 

 that described, although differing in appearance, the adjustment 

 was so delicate that a variation of speed in the running of the 

 carriage of half a foot per minute was seldom exceeded even at 

 the highest speeds. The value of working against the 

 resistance of spiral springs will be noticed in this 

 mechanism, their steadying action being especially valuable. 

 It would be impossible for us to attempt to describe the 

 mechanism constituting the copying apparatus of the model 

 shaping machine, and we can only hope to give a mere outline 

 of the general principles. A rough hollow model of the ship to 

 be constructed is cast in paraffine wax, a material which is found 

 to lend itself most perfectly to the necessities of the experiments. 

 The drawing from which the operator has to work is stretched 

 on a table, and the grooves representing the water lines are 

 copied from the drawing by means of the mechanism. These 

 grooves are formed by a pair of revolving cutters, the fore and 

 aft motion being communicated to the model whilst the cutters 

 move laterally. One cutter is on each side— for of course full 

 models are required — and they approach or recede symmetrically 

 in such accordance with the longitudinal travel of the model as 

 to trace in plan upon it the intended horizontal section. This 

 due accordance of the lateral motion of the cutters with the 

 longitudinal motion of the model is accomplished by the operator 

 so regulating the cutter motion as to maintain a tracer in contact 

 with the corresponding water-line on the drawing. By suitable 

 mechanism the drawing itself is made to imitate the longitudinal 

 travel of the model, while the tracer imitates the lateral travel of 

 the cutters. In the Torquay machine the tracer was guided by 

 an adjustable template set to the curve of each water-line, but 

 afterwards the tracer was made to follow the line on the drawing 

 by the operator. In the present machine the cutters are raised 

 or lowered to get the different water-lines. The cutters run at 

 27CX3 revolutions per minute. The grooves having been cut, the 

 surplus material is removed by hand. 



We shall not follow Mr. Froude in his description of the 

 further details of the mechanism, as it would be unintelligible 

 without the drawings by which he illustrated his description. 

 The various arrangements are, however, fully worthy of study 

 by all who are interested in ingenious mechanical devices ; but 



NO. 



215. VOL. 47] 



we must refer our readers to the printed transactions of the 

 institution, in which the diagrams will appear when the volume 

 is published. There is also a weighing machine, which is 

 necessary to obtain the actual dead weight of the model, so that 

 the amount of ballast required to get the necessary displacement 

 corresponding to trial draught may be determined. This machine 

 will weigh up to 1000 lbs. with great accuracy, and is similar in 

 principle to an ordinary chemical balance, except that it is a 

 steel yard, having one arm 6 inches and the other 5 feet in 

 length. 



The discussion on this paper was opened by Mr. W. H. 

 White, the Director of Naval Construction, who is the official 

 head of the Admiralty department, of which the Haslar 

 establishment forms a branch. Mr. White spoke of the 

 advantage these model experiments had been to the navy, 

 saying that the great advance in the speed of ships which bad 

 been obtained of late years would not have been reached to the 

 full extent had it not been for the model experiments carried out 

 at Torquay and Haslar by the late Mr. Froude and his son, the 

 author of the paper. Mr. J. I. Thornycroft also pointed out 

 the great economy that had been made in expenditure upon 

 navy ships by finding out beforehand what the proposed vessel 

 would do, and what was required in the way of power to reach 

 that performance. Mr. Thornycroft made especial reference to 

 the ingenuity of the device whereby a line on the drawing, 

 which might not be quite accurate, would be' made to give the 

 desired result in the model, and this without an expensively- 

 constructed apparatus. Various other speakeis having been 

 heard, and Mr. Froude having briefly replied, so far as there was 

 anything to reply to, the meeting adjourned until the next 

 evening. 



On the members assembling on Friday evening, the 3rd 

 inst., the president, Dr. Anderson, again occupied the chair, and 

 Mr. Matthews's paper on the Southampton waterworks was read. 

 This contribution is interesting, as it describes what we under- 

 stand is the largest water-softening plant yet installed. The 

 quantity of water that can be satisfactorily dealt with is 

 from 7.\ to 2\ million gallons per day of 24 hours. Of 

 course the principle of softening hard water by lime is very far 

 from new, but it has made slow progress, in spite of the vast 

 quantities of hard water, otherwise unobjectionable, that there are 

 in the chalky southern half of our island. This limited applica- 

 tion of a means whereby a bad water in one respect can be made 

 a good one in all respects does not appear, to judge by the pro- 

 ceedings of last Friday, to spring from any inherent defect in 

 the system — beyond that which would arise from the disposal of 

 the refuse lime in crowded cities — but rather from the careless- 

 ness of public authorities and water-supplying companies to 

 the wants and comforts of the people at large. 



The meeting terminated with the usual votes of thanks, the 

 President aonouncing that the summer meeting would be held 

 this year at Middlesborough on Tuesday, August i, and the 

 following days. 



THE SEVEN IMAGES OF THE HUMAN EYE. 

 TT is well known that in the human eye, besides the refracted 

 image, which serves the purposes of vision, there are formed 

 three reflected images known under the name of " Purkinje's 

 images." M. Tcherning has discovered three additional ones, 

 so that the total number is brought up to seven. ^ 



In its passage into the interior of the eye each ray of light has 

 to pass through the cornea, the aqueous humour, the crystalline 

 lens, and the vitreous humour before finally arriving at the 

 retina. At the surface of each of these constituents the ray is 

 liable to be partially reflected, thus giving rise to four reflected 

 images. These were all seen and described by Purkinje At the 

 beginning of the century, but only three were observed by 

 Helmholtz and others. These three can be easily observed by 

 two persons on holding a lighted match between their eyes, and 

 moving it about so that the reflections seem to come from the 

 pupil. One of them, that reflected by the front of the cornea, 

 is much brighter than the two others, which are formed by the 

 front surfaces of the crystalline and the vitreous humour respec- 

 tively. The fourth image is due to reflection from the posterior 

 surface of the cornea. It may be discovered by careful observa- 

 tion of the brightest image by means of a magnifying glass. As 

 I See Seances de la Societc Franfahe dc Physique, Avril-Noverabre, 



