August 2, 19 17] 



NATURE 



449 



There is another problem <rf no less conse- 

 quence growing- out of tiie same conditions, 

 which will, with equal insistence, demand the 

 attention of Prof. F. G. Cheshire and his col- 

 leagues. ^^'e have referred to the multiplicity 

 of the varieties of optical glass to which the 

 manufacturers of optical instruments are accus- 

 tomed. This is, on the face of it, a great point 

 in their favour ; but when more narrowly con- 

 sidered it will be found to handicap them 

 appreciably. Let the matter be considered from 

 this point of view. A manufacturer develops by 

 trial and error a very successful optical instru- 

 ment — 2l field-glass, for example. He uses a 

 particular combination of optical glasses in its 

 construction. His whole design is built upon 

 the special properties in respect of refraction and 

 dispersion of these particular glasses. So long 

 as he can procure a supply of them upon the 

 open market he does not need to modify any 

 feature of his manufacture. It is a question of 

 repetition merely, and he closely guards the 

 secret of his success. But the time may come 

 when the particular variety- of optical glass upK>n 

 which he relies is no longer available, or not 

 available in the required quantities. What can 

 he do then? Speaking in general terms, he can 

 then do nothing. The practical optician cannot 

 tell him how to substitute staple glasses for the 

 special varieties which he has been accustomed 

 to employ, and hence he insistently demands 

 the accustomed supplies. Thus the lavish 

 variety of optical glasses available to the manu- 

 facturing industry has actually tended to restrict 

 growth of output and to reduce adaptabilitv. 



Here is one of the practical questions await- 

 ing solution by means of laboratory research. If 

 the optical glass industry is to be well estab- 

 lished in this country, it must be an industry 

 which will satisfy- the requirements of the manu- 

 facturing trade: if it is to be established with a 

 minimum of effort, those re<juirements must be 

 reduced to the narrowest limits which can suf- 

 fice. To work out the principle of equivalence 

 between varieties in the combination of glasses, 

 and so to concentrate the glass-maker's labour, 

 so far as is practically possible, upon the produc- 

 tion of a few staple varieties, will be a very im- 

 portant object of practically applied research. 



These instances may serve to illustrate some 

 of the less generally recognised fields for the 

 activitA- of the new department. We have not 

 adverted to the more obviously important 

 objects of training teachers, instructing work- 

 men, educating experts, and advising manufac- 

 turers. These may be left to speak for them- 

 selves, since the limits of our space preclude any 

 adequate exposition of them in the present article. 



With one word of congratulation we may close, 

 and that upon the choice of a director for the 

 new undertaking. Prof. Cheshire brings to his 

 ask a mind not only well stored with the 

 technical knowledge of his subject, but also 

 TOstructed by a wide experience of its practical 

 side. We have ever\- confidence that In his 

 "hands the machinery of the new department at the 

 NO. 2492, VOL. 99] 



Imperial College of Science and Technology will 

 be directed to practically important exxis, and 

 while reserving our congratulation of him per- 

 sonally, upon the sound principle which forbids 

 premature compliments to the warrior who is 

 girding on his armour, we congratulate the 

 authorities who, b\' their choice of him for the 

 important post which he is about to 611, have 

 sho^wn how larg« practical considerations, such 

 as those to which we have here adverted, bulk 

 in their i^iew. That, at least, is as it should be. 



THE AEROPLANE BOMBER'S PROBLEM. 

 nr'HE problem which the bomber on board an 



-^ aeroplane has to solve is more difficult than 

 the corresponding problem of the bomber on board 

 an airship, since the aeroplane must move with 

 respect to the air to support itself, while the air- 

 ship may be brought to rest over the object to 

 be bombed. The bomb on release has a horizontal 

 speed equal to that of the aeroplane, and if the 

 air were at rest and offered no resistance to the 

 motion of the bomb through it, the path of the 

 bomb would be a parabola with its axis vertical 

 and its vertex at the point of release. The re- 

 sistance of the air prolongs the time of fall of 

 the bomb to an extent which depends on its size 

 and weight, and may be 50 per cent, if the over- 

 all density of the bomb is small. Any horizontal 

 motion of the air causes a drift of the bomb do^iTi 

 the wind which depends on the speeds of the 

 various layers of air through which it passes dur- 

 ing its fall, and on the resistance the air offers to 

 the sideways motion of the bomb through it. 



The dynamical equations which express the 

 above facts have not y^et been rigidly solved, but 

 they are so closely related to the equations for high- 

 angle fire in gunoer}- that the appa-oximate 

 methods of Col. Siacci or of Capt. Ingalls, 

 or the recently published graphical method of 

 Prof. Dalby, are all appdicable, when the weight, 

 altitude, and initial horizontal speed of the bomb, 

 the mean speed and direction of the wind, and the 

 resistances to downward and sideways rootioe of 

 the bomb through the air are known. The alti- 

 tude is shown on the aneroid of the aeroplane. 

 The speeds of the aeroplane with respect to the 

 groiind and of the wind at the aeroplane are found 

 by passing over an object wdth and against the 

 wind respectively, and noting how long it takes 

 the aeroplane to pass from the verticallv over- 

 head position to one, say, jo° or 20° up or dow« 

 the wind from it. From the speed of the wind 

 at the aeroplane thus found an estimate of the 

 mean speed of the wind in the layers of air through 

 which the bomb has to pass must be made, and 

 this estimate can only be a rough one. With 

 these data the bomber consults tables or curves 

 previously prepared for the bombs to be used, 

 which give him the bearing from the machine of 

 the spot at which a bomb released at the moment 

 should fall if the conditions remain unchanged. 

 Thus the bombing aeroplane requires to be 

 equipped with apparatus for measuring horizontal 

 and vertical angles and times. 



