July 20, 191 1] 



NATURE 



THE GYROSTATIC COMPASS.' 



MESSRS. ELLIOT BROTHERS, the English 

 makers of the Anschiitz gyro compass, are to 

 DC congratulated on the publication of an admirable 

 account of the principle, construction, and practical 

 use of this, the newest and most marvellous of all 

 applications of the gyrostatic principle to a useful pur- 

 pose. This book is chiefly translated from the Ger- 

 man publication by Anschiitz and Co., but the more 

 mathematical part, due originally to Herr M. Schuler, 

 has been modified by Mr. Harold Crabtree and Mr. 

 Alfred Lodge so as to be more in accord with English 

 mathematical usage. Commander Chetwynd and 

 Commander Marston have also assisted in making 

 those parts which deal with practice as useful as pos- 

 sible. With such skilled collaboration and with so 

 admirable an. original to work upon, it is not sur- 

 prising that the result is so charming and instructive. 



A gyrostat may be supported so that it has three 

 (angular) degrees of freedom, or so that it has only 

 two, one angular motion being 

 constrained. With three degrees 

 of freedom and really perfect 

 balance the axis of rotation 

 would maintain itself indefinitely 

 in space and so the axis of rota- 

 tion, if it were set parallel to 

 the earth's axis, for instance, at 

 any time and place would, rota- 

 tion being maintained, retain 

 that parallelism indefinitely, and 

 show both latitude and the ship's 

 course. The trouble is partly 

 the difficulty of obtaining suffi- 

 cient gyrostatic stability, but 

 chiefly the impossibility of 

 obtaining a really perfect 

 balance, and so, however small 

 the turning couple due to want 

 of balance may be, precession 

 will be set up and a cumulative 

 error will grow and make the 

 indications useless. If, however, 

 the axis of a gyrostat is sub- 

 jected to an elastic constraint 

 tending to keep it level, but 

 allowing it to tilt in spite of the 

 constraint, and if, further, the 

 gyrostat is supported so that no 

 couple acts upon it in azimuth 

 as resulting from the support, 

 then with sufficient gyrostatic 

 capacity the whole combination cannot rest in 

 any position except that in which the axis is point- 

 ing north and south very nearly, and the direction of 

 rotation is in the same direction as that of the earth. 

 For if the axis should be in any other direction the 

 rotation of the earth will, through the elastic con- 

 straint, tend to elevate one end of the axis and depress 

 the other, and under this constraint the axis will 

 precess towards the north and south direction. When, 

 however, this direction is reached, the axis will be 

 neither exactly level nor at rest, and the motion 

 would, like a pendulum, continue as far to the other 

 side if some system of damping were not introduced. 



In the Anschiitz gyro compass the gyro wheel is 

 supported by a hollow steel ring immersed in mercurv 

 and with the centre of flotation a little above the 

 centre of gravity, thus giving the necessary elastic 

 constraint to the axis. The whole floating svstem 

 (Fig. 1) is located by the compound piece, S, T, the 



1 *The Anschiitz Gyro Compass. History, Description, Theory. Prac- 

 tical Use." Pp. ii + roq. (London: Elliott Brothers, ioic.) 



two parts of which serve as two electric poles com- 

 municating with a three-phase motor within the gyro 

 wheel A in the casing B. The compass card, R, is 

 carried by the casing B. The third electric pole is 

 supplied by the mercury, Q, and floating hollow ring, 

 S, of steel. The whole case for the mercury, K, is 

 carried on gymbal rings, like an ordinary compass. 

 The gyro wheel, A, is run at a speed of 333 turns a 

 second, and is subject to a bursting stress of 

 10 tons per square inch. Its axle, like that of a de 

 Laval turbine, is sufficiently flexible to enable the 

 wheel to establish its own dynamical axis, and it is 

 supported on ball bearings of the highest perfection. 



Beautiful as the invention is, closely as the material 

 and construction approach perfection, perhaps no 

 detail affords such a pleasing surprise as the system 

 by which the damping of the motion, to which refer- 

 ence has been made, is effected. Side holes are pro- 

 vided in the casing, A, for the admission of air, and 

 at the bottom there is a tangential opening through 

 which a blast of hot air escapes. As something like 



Fig. 1. — Vertical sect 



NO. 2177, VOL. 87] 



half a horsepower is being absorbed within the casing, 

 this air circulation affords the necessary cooling. The 

 orifice, a, b (Fig. 2), for the escape of air is partly 

 covered by a shutter, u, carried by a pendulum, so 

 that when the gyro axle is horizontal the escape of 

 air is symmetrical on either side of the shutter, but 

 when it is inclined the escape of air is greater on that 

 side on which the axle is elevated. The reaction due 

 to the increased escape of air on one side tends to 

 turn the casing round opposing the precession, and 

 so bringing the axle towards the horizontal position. 

 It therefore comes about that each excursion of the 

 axle is only about one-fifth of the previous one in the 

 opposite direction, and in about three hours from 

 starting with axle level and 45 east or west, it has 

 settled down steadily to its position of rest almost 

 exactly due north and south. 



Almost exactly, but not exactly ; not that there is 

 anv vagueness as to the point of rest, but because 

 there are certain corrections, a latitude correction, 

 with which the little weight, f. has to do, and which 



