March 12, 1908] 



NA TURE 



449 



upon the size of the ship. Probably, too, large ships may 

 be regarded as being steady enough already. 



If we apply the Schlick method to Mr. Brennan's car, 

 as 11 of (4) (putting a = ol is negative, there is instability 

 of motion whether there Is or is not friction. We may. 



Fig. 



however, do as the exhibitor of the model (at the Physical 

 Society meeting) has done — make h also negative. That 

 is, make the gyrostat frame unstable by having the centre 

 of gravitv of the frame E.\B above the axis DG. In 

 this case, if there is absolutely no friction either of the 

 / or K kind, there will be steady vibrations about a mean 

 position, but any friction will cause the swings to get 

 larger and larger. It is to be noticed that even without 

 friction there will be instability if «!, the moment of 

 momentum of the fly-wheel, is less than a certain amount. 

 Mr. Brennan's method of working is quite different. 

 Fig. 2 shows his model car (about 6 feet long). It is 

 driven bv electric accumulators carried by the car. His 

 gvrostat 'wheels are driven by electromotors, not shown 

 in Fig. 3 ; as they are revolving in nearly vacuous spaces 

 they consume but little power, and even if the current 

 were stopped they would continue running at a sufficiently 

 high speed to be effective for a length of time. 



It will be found that energy is wasted by friction, and 

 also work has to be done in bringing the car to a new 

 position of equilibrium, and all this is supplied by the 

 electromotors. Should the gyrostat really stop or reach 

 a certain low speed,, two supports are automatically 

 dropped, one on either side of the car; each of them drops 

 until it reaches the ground, one of them dropping perhaps 

 much farther than the other. 



The real full-size car which Mr. Brennan is now con- 

 structing may be pulled with other cars by any kind of 

 locomotive, using electricity or steam or petrol, or each 

 of its wheels may be a driving wheel. He would prefer 

 to generate electric power on his train, and to drive every 

 wheel with an electromotor. His wheels are so indepen- 

 dent of one another that they can take very sharp curves 

 and vertical inequalities of the rail. The rail is fastened 

 to sleepers lying on ground that may have sidelong slope. 

 The model car runs on an iron gas-pipe ; the ground is 

 nowhere levelled or cut, and at one place the rail is a 

 steel wire rope spanning a gorge (Fig. 2). It is interest- 

 ing to stop the car in the middle of this rope and to 

 swing the rope sidewise, watching the perfect automatic 

 balancing. The car may with confidence be left here for 

 hours, balancing itself with nobody in charge. If the 

 load on the car — great lead weights — be dumped about 

 into new positions, the car effects balance with no apparent 

 effort. But if, the car not running but merely balancing 

 itself, a person standing on the ground pushes against 

 it, the car will push in opposition, and by pushing 

 judiciously a person can really disturb the car's vertical 

 position considerably ; it is as if an indignant animal were 

 resisting the push. Left to itself now, the car quickly 

 rights itself. 



The car is supported by a mono-rail bogie at each end ; 

 each bogie has two wheels pivoted vei'tically and hori- 

 zontally, so that curves may be very sharp and the ground 

 may be uneven. 



diagrammatic representation of Mr. 

 Brennan's pair ol gyrostats in sec- 

 tional elevation and plan. The cases 

 G and G', inside which the wheels 

 F and F' are rotating in vacuo at 

 the same speed and in opposite direc- 

 tions (driven by electromotors not 

 shown in the figure), are pivoted 

 about vertical axes EJ and F'J'. 

 They are connected by spur-toothed 

 segments JJ and J'J', so that their 

 precessional motions are equal and 

 opposite. The whole system is 

 pivoted about C, a longitudinal axis. 

 Thus when precessing so that H 

 comes out of the paper, so will H', 

 and when H goes into the paper, so 

 does H'. 



When the car is in equilibrium the 

 axes KH and K'H' are in line NN' 

 across the car in the plane of the 

 paper. They are also in a nearly 

 horizontal line which is at right angles 

 to the total resultant force on the car. 

 I will call this the mid-position. 

 II! be the moment of momentum of either wheel. 

 Let us suppose the car to tilt so that the shell D comes 

 up against H, the spinning axis (or a roller driven by 



Let 





Fig. 3. 



the spinning axis) of the gyrostat. H begins to roll away 

 from me, and if no slipping occurred (but there is always 

 slipping, and, indeed, slipping is a necessary condition) 

 it would roll, that is, the gyrostats would precess with a 



NO 2002 VOL. 77] 



