30 



KNOWLEDGE 



[February 1, 1895. 



\ ^ 



7 





and 14 balance eacli other, each having a ball at its outer 

 end. The bars 15 limit the play of the pendulum to (say) 

 half an inch in each direction. But even this limited play 

 of the pendulum may be further reduced by means of 

 electric contact appliances. 16 is a screw-threaded rod, 

 fixed to the bars 15, and passing through a slot in the 

 pendulum rod. 17, 17, are two nuts for regulating two 

 light springs, 18, which keep the pendulum in a vertical 

 position only when the vessel is truly horizontal, the 

 springs immediately giving way when the vessel departs 

 from it. 



Supposing that the vessel is travelling from left to right 

 on an even keel, and the shaft 8 is turning in the direction 

 of the bent arrow ; the pinion 10 rotates freely between 

 the teeth of the rack, and the rack remains stationary. 



Now suppose 

 the head of the 

 vessel rises 

 two or three 

 degrees, the 

 pendulum im- 

 mediately falls 

 ^ against the 

 after bar 1 5 , 

 raises the 

 pinion 10, and 



thereby drives the rack 7 from left to right. This operation 

 lowers the tiller 4, and, with it, the steering planes. The 

 effect of this will be to slightly raise the stern and restore 

 the vessel to the horizontal position. 



In order to stop the vessel gradually, and bring it 

 gently down to terra firma, you have only to push the 

 pendulum forward by hand, when the rack 7 will be driven 

 from right to left by the pinion, the tiller and steering 

 planes will be inclined upwards, thus bringing the stern 

 downwards. The increased angle of inclination at once 

 reduces the vessel's speed, and it approaches the ground 

 slowly and lands safely. 



In case of an accidental stoppage of the rotating shaft 8, 

 the pendulum may be fixed, and the rack may be operated 

 by hand, by means of a lever attached to one of the guide 

 bars 20 ; or the shaft 8 may be turned by manual power. 



The mechanical detaUs may be varied to a very great 

 extent. 



Note. — Since the above communication was forwarded 

 to Knowledge, I have read Mr. Maxim's letter in the 

 January number. Mr. Maxim has credited me with a 

 number of theories which 1 never entertained in my life, 

 and, therefore, they need no reply. My illustration of a 

 boat with a rope attached to a fixed object, or to a floating 

 anchor, has been conjured by Mr. Maxim, by a process 

 best known to himself, into a somethinn ^hich he multiplies 

 by 13-1 ! Then Mr. Maxim (after misquoting what I said 

 about Henson's machine) says : " I know nothing about 

 Henson's machine except what I have heard." Henson's 

 patent is dated 29th September, 1842, No. 9478. and is 

 well worth studying. It anticipates all the essential 

 features of Maxim's machine. 



My article is unanswered and unanswerable as to the 

 loss by slip, and the entire neglect of this mode of 

 propulsion by millions of aeronauts, from the tiny house- 

 fly to the Australian crane, all witness to the correctness 

 of my article, and I have one more witness in Mr. Maxim 

 himself. In his paper read at the Society of Arts meeting 

 on the 28th November, 1894, he says (page 35 of the 

 report) : " The slip, while running at forty miles an hour, 

 is almost exactly eighteen miles an hour." So he owns to 

 forty-five per cent, loss by slip. — T. M, 



THE HESSIAN FLY. 



By E. A. BuTLEB, B.A., B.Sc. 



ABOUT eight years ago, some excitement and con- 

 sternation were produced by the announcement 

 that the destructive "Hessian tly " had been 

 discovered in England. When, therefore, this 

 discovery was made, it seemed as though we 

 might look for a most noxious and troublesome, and 

 possibly permanent, addition to that part of the insect 

 fauna of the British Islands by which so much of the 

 agriculturist's toil is neutralized. But whatever fears might 

 have been entertained on this score, they have not yet been 

 realized to any serious extent, and although it is quite true 

 that the Hessian fly is to be regarded as fairly included in 

 the British fauna, yet we do not seem to be at present 

 much the worse for the fact. The area of the distribution 

 of this pest is a wide one, for it has been met with through- 

 out the greater part of the eastern districts of both England 

 and Scotland, especially near the sea-coast. It has occurred 

 in other parts as well — e.g., Mr. F. Enock reports having 

 found large numbers in barley and wheat fields near 

 Stroud, in Gloucestershire ; but the eastern counties have 

 been hitherto its head-quarters. Since its first appearance 

 in 1886, the damage done by it in Great Britain has been 

 estimated at a loss of from one to twelve bushels of grain 

 per acre. 



The Hessian fly may be taken as another and most 

 striking illustration of the principle we have alluded to on 

 other occasions, that the worst pests, so far from being 

 conspicuous and easily recognizable species, are usually 

 obscure and often very minute. The Hessian fly is, in 

 fact, an insect of such insignificant appearance that none 

 but a trained entomologist would be likely to suspect, on 

 seeing it, that it was capable of becoming a serious plague. 

 It is merely a minute two-winged fly, belonging to the 

 family of gall-gnats, some species of which we described 

 in our articles on " Galls and their Occupants." It is a 

 fragile insect, not more than one-sixth of an inch in length 

 of body, and its fully-expanded wings stretch only about 

 twice that distance. It is difficult to realize that a 

 creature so minute and so delicate cau possibly be the 

 author of any serious amount of damage. And yet, that 

 it is one of the most formidable foes the growers of cereal 

 crops have to encounter is, unfortunately, only too true. 

 The scene of its greatest destructiveness has hitherto 

 been America, it having spread from west to east and 

 from north to south over the greater part of the United 

 States. 



The family to which the Hessian fly belongs is called 

 Cecidoniyida, and its own name, Cecidomyia destructor, very 

 significantly proclaims its evil reputation. Id is destruc- 

 tive to both wheat and barley, but does not seem to attack 

 oats. The damage done is of an indirect nature ; the 

 larva absorbs the juices of the plant from a point near the 

 base of the stem, just above a joint. In consequence of 

 this cutting off of supplies from the upper part of the plant, 

 the stalk becomes weak, and when the ear is developed, 

 also in a dwarfed condition, the weakened stem can no 

 longer support the weight of the ear, and bends over at a 

 spot just above the joint where the insect is situated. The 

 ear is thus laid prostrate, just as if it had been blown over 

 by the wind, and as it lies on the ground, it not only 

 becomes spoilt with damp and dirt, but also falls an easy 

 prey to field-mice. 



There can be little doubt that this insect was in the 

 country at least some few years before it was discovered, 

 though very dift'erent views are held as to the exact date 



