412 



NATURE 



[June 19, 1913 



position, and of ensuring that the movable parts shall 

 move freely, yet without shake. 



"This we may do by attending to the well-known 

 fact in kinematics — ' .1 rigid body lias six degrees of 

 freedom.' " 



Designs in which this principle is carried out may 

 be called geometrical designs. A three-legged table is 

 a geometrical design, and a four-legged table is not. 

 A four-legged table either rocks on two legs, or 

 bends so that all legs touch the floor, and the amount 

 of bending and the pressure of each foot on the floor 

 depends on the stiffness of the table and the evenness 

 of the floor. Every time an ordinary chair is placed 

 in a new position, it takes a new shape. A surface 

 plate is a familiar example of the importance of three 

 supports, and nearly all scientific instruments rest on 

 three feet. Other examples of geometric design were 

 also given. 



Good Design and Bad Workmanship. 



A most important consideration in a good design 

 is that the instrument shall still work well when 

 the rubbing surfaces get worn or parts get bent, or if 

 the workmanship is not good. With perfect work- 

 manship and a bad design, you may get jamming in 

 the moving pieces and bending of parts which should 

 not bend, and the results obtained will be liable to 

 error and the working unsatisfactory. This considera- 

 tion brings out most forcibly the advantage of geo- 

 metrical designs, but also it' is a valuable test to all 

 designs. It is a long way from being the only test, 

 but it is always well worth while to consider separately 

 the effects of imperfect workmanship, or the bending 

 of each part and wearing of the rubbing surfaces. 

 Take the case of wear in a wheelbarrow. The axle 

 of the wheel usually consists of two round iron pins 

 running in holes in wooden rails forming the frame 

 of the wheelbarrow. Both the wood and the pins 

 wear; the pin gets smaller but keeps circular, and 

 wears its way into the wood and always fits it properly 

 on the side where pressure is taken.' The wheel will 

 work perfectly until either the holes break out of the 

 wood or the pin wears down very small and itself 

 gives way. But sometimes the axle is made differ- 

 ently; an iron rod is fixed to the two wooden rails 

 and passes through a hole bored along the centre of 

 the wheel. With use the iron rod wears on the under 

 side and does not remain circular, the hole in the 

 wheel gets larger ; the result is increased friction and 

 a loose and shaky bearing. 



The following test was applied to the Rocking Micro- 

 tome, which has been designed so far as possible on 

 the geometrical method. The iron castings of which 

 it is chiefly made were taken as they left the foundry, 

 were put together with as little work as possible, and 

 it at once cut good sections. This was a severe 

 ordeal, but sections as thin as 0003 mm. were cut, 

 proving that the instrument still worked with con- 

 siderable precision. 



This test for good design is not the only test, and it 

 may fail. Ball bearings are much used, and when 

 once used for any purpose they continue to be used 

 more and more ; this is the best test of a really good 

 mechanical device. All must admire their design, but 

 first-rate workmanship is essential ; in this must be 

 included the composition of the steel, the skill in 

 hardening, as well as the accuracv of the figure of 

 the working parts. A ball bearing, however, would 

 be a better thing even than it is at present if it did 

 not require such fine workmanship. It also requires 

 careful mounting, and it is interesting to notice that 

 the recent improvements in ball-bearing design are 

 in the direction of allowing it to work satisfactorily 

 on shafting which may be considerably' bent. 



NO. 2277, VOL. 91] 



The Advantage of Reversing the Parts of a Machine. 



An improvement in the design of a machine can 

 often be made by reversing the relative position of 

 two parts of it, or the part that moved can be fixed 

 and the part that was fixed can be made to move. 

 This reversal makes it possible to compare two or 

 more methods, and it is then easy to see which is best. 

 It is advantageous that "the survival of the fittest" 

 should take place early in the life of the machine, 

 and by this means, in fact, it takes place before the 

 design is completed. 



In the before-mentioned wheelbarrow it is easy to 

 see which is the best design, and if the designer had 

 deliberately considered whether the iron pins should 

 turn in the wooden rails or whether the iron bar 

 should be fixed, the bad design would never have been 

 made. It is surprising how often this reversal is 

 possible and advantageous, and how difficult it is to 

 realise that it is possible. We are so familiar with 

 a clock in which the frame remains at rest 

 and the hands move that it requires a considerable 

 mental wrench to realise that it is possible and in 

 some cases better that the clock itself should revolve 

 and the hour hand remain at rest. But in recording 

 apparatus it is usual to fix the clockwork in the 

 rotating drum carrying the paper, and to prevent 

 rotation of the hour-hand spindle. 



The lecturer concluded : — " I have spoken as a 

 manufacturer of scientific instruments, but my re- 

 marks applv equally or even more to the home-made 

 or rather laboratorv-made tvpe of instruments. And 

 it is with these that the greatest advances in know- 

 ledge have been made. If I could believe that what 

 I have said would be any help to the makers of the 

 wire, cork, and sealing-wax class of instruments, or 

 to the orthodox instrument-maker, I should be glad 

 to think I had done something to advance know- 

 ledge." 



THE STANDARDISATION OF 



HYDROMETERS. 

 ■\X,rE have reci ived from the director of the National 

 » * Physical Laboratory the following memorandum 

 for publication in Nature : — 



At the present time there appears to be consider- 

 able ambiguity as to the bases of standardisation 

 of hvdrometers graduated to read directly in specific 

 gravity. 



Three different methods have been brought to the 

 notice of the National Physical Laboratory, and it 

 seems desirable to determine which of these three- 

 should be considered as standard. 



The instruments are in all cases graduated for use 

 in a liquid at a definite temperature — we call this the 

 standard temperature of the instrument — and give the 

 specific gravity of this liquid at some definite tern 

 perature, which may or may not be the standard 

 temperature of the instrument, referred to water at the 

 same or at some other temperature. 



The following cases have arisen in practice : — 



I. (a) The liquid to be tested must be at the standard 

 temperature of the instrument. 



(6) The water to which the specific gravity is re- 

 ferred must also be at the standard temperature of 

 the instrument. Thus, if 85 F. be the standard 

 temperature of the instrument ' the liquid must be 

 at 85 F. when tested, and its specific gravity is 

 referred to water also at 85 F. 



II. (a) The liquid to be tested must be at the 

 standard temperature of the instrument. 



1 A more usual value for this temperature of the instrument would be 6o e F. 

 or 62 F. Thetemperature E5 F. is rhosen here as an example so as to 

 bring out the differences arising from the various methods of standardisation. 



