TRANSACTIONS OF SECTION G. 743 



only useless, but is apt to bo positively misleading, as giving an impression of an 

 accuracy which has no real existence. 



The relative value of accuracy in different sets of observations is iu itself a 

 matter in which a sense of proportion is wanted, and often very badly wanted. 

 Where one has to measure half a dozen things of which two are very easily 

 measured and the remaining four are only measurable with great difficulty, it is 

 only human nature that we should spend our energies on getting extremely 

 accurate results with the first two and roughly do our best with the others. It 

 is very difficult under such circumstances to remember that the accuracy of the 

 whole is not the accuracy of the best part of our work, but of the worst. 



The extraordinary eftiact of a want of sense of proportion is nowhere better shown 

 than in the absurd statements which are constantly made as to technical matters in 

 public prospectuses, and the still more absurd statements made in those very 

 numerous documents of a similar kind of which some of us see a great many, but 

 which do not finally emerge into public view. Fortunes are apparently to be made 

 by inventions which, although doubtles? ingenious, yet only concern one way of 

 doing a thing which could be done equally well in half a dozen other ways. Every 

 one is expected to run after a piece of apparatus which is to save 50 per cent, of 

 something, the total cost of that something, however, being so very small that 

 nobody cares to save in it at all. I need hardly mention the all too common case 

 where" a contemplated saving of 10 per cent, in the cost of a material works out 

 yearly to an amount much more than equal to the whole cost of the original article. 



Ibelieve that experimental work in an engineering laboratory can educate this 

 critical sense of proportion very admirably in a number of ways In the first place, 

 it directs quantitative work into very varied channels, and not along one particular 

 line. Secondly, it compels the observer to combine a number of measurements in 

 such a way that the relative importance of accuracy in each can be seen. In the 

 case of an engine trial, for instance, the combined results are affected by the 

 accuracy of measurements of the dimensions of the machine, by the apparatus and 

 methods used for measuring the water, by the indicator, and by its springs, by the 

 speed counter, by the thermometers, and so on. An error of 1 per cent, in counting 

 the revolutions is just as important as an error of 1 per cent, in measuring tho 

 water, or in measuring the mean pressure. I am afraid that one could point to a 

 good many cases in which this has been more or less forgotten. Then, by making 

 a series of measurements all in absolute quantities, the relative importance of each 

 quantity to the desired total result can be seen. Thus it will be found that changes 

 in certain quantities affect the total result to a A-ery small extent, while changes in 

 others affect it very largely, .so that not only is the accuracy with which different 

 quantities can be determined very different, but also the same degree of accuracy 

 is of very different importance according to the particular quantity to which it 

 refers. Once it is found that a final result is exceedingly little affected by a par- 

 ticular set of changes, it ceases to be of importance to measure or observe those 

 changes in any extremely minute way, and of course the reverse holds equally good. 

 Finally, and this perhaps is the most 'important matter of all, measurements m such 

 a laboratory are made to a great extent under the complicated conditions under 

 which the actual final result has to be obtained in practical work. They are not made 

 under the conditions which insure the greatest individual accuracy of each result. 



It will be seen that throughout, but particularly in the two last points which I 

 have mentioned, the work of an engineering laboratory is in intention and in 

 essence different from that of a physical laboratory. The aim of the latter is to 

 make its problems as simple as possible, to eliminate all disturbing elements or 

 influences, and to obtain finally a result which possesses the highest degree of abso- 

 lute accuracy. In most physical investigations the result aimed at is one in which 

 practically absolute accuracy is attainable, although attainable only if infinite pains 

 be taken to get it. It is the business of the physicist to control and modify his 

 conditions, and to use only those which permit of the desired degree of accuracy 

 being reached. In such investigations it sometimes becomes almost immoral to 

 think of one condition as less important than another. Every disturbing condition 

 must be either eliminated or completely allowed for. That method of making the 



