534 



NATURE 



[August 24, 19 16 



from the zenith depends upon the qualities of the lens, 

 and no confident statement can be made until this has 

 been tested, but it is hoped that star trails perfectly 

 sharp for measurement will be secured up to an 

 angular distance of 3° from the centre. This gives us 

 as available for our purpose the stars over a belt 6° 

 wide down to the sixth, and possibly the seventh, 

 magnitude. The actual work of observing will be 

 very simple, and will only mean that the whole in- 

 strurnent is rotated through 180° at certain pre- 

 arranged times, and that the lens is open<xl after 

 twilight and covered before the dawn. It would be 

 possible for this to be done by mechanism controlled 

 by a clock. 



As the telescope hangs freely always in a vertical 

 position, we entirely get rid of one of the astronomer's 

 anxieties, the risk of error due 

 to flexure or bending of his 

 telescope, for though the tube 

 can be made apparently very 

 rigid, the excessively minute 

 degree of bending sufficient to 

 introduce appreciable errors is 

 difficult, if not impossible, to 

 avoid in a telescope which has 

 to be used in different posi- 

 tions. Then, again, the errors 

 due to changes of temperature 

 inside or close to the instru- 

 ment should almost disappear 

 in this form. First, no tem- 

 perature changes affect the 

 suspension ; so long as the 

 l)ody of the telescope remains 

 undistorted the position of the 

 true vertical in regard to 

 the optical axis remains 

 constant. Secondly, as the 

 whole hanging part of the 

 instrument is perfectly sym- 

 metrical about the vertical 

 axis, with the trifling excep- 

 tion that the plate-carrier and 

 photographic plate are not 

 circular, but rectangular, no 

 temperature change should 

 distort the axis. Any distor- 

 tion that can take place will, 

 in fact, be the very smal' 

 change of sc"!e that will re- 

 sult from the difference in th'^ 

 expansion of the glass plat ^ 

 and the brass rube. Thirdly, 

 it is possible, and in this in- 

 strument has been done, to 

 enclose the whole in an outer 

 case which can be made air- 

 tight and kept at a constant 

 temperature by a thermostat, 

 instrument in 



vane attached to a rod at the base is immersed in a 

 dash-pot or bath of glycerine. This rod must be 

 centred in prolongation of the vertical axis, otherwise 

 the capillarity between rod and liquid will introduce a 

 force deflecting the telescope from the true vertical. 

 While it would thus appear that in this form of instru. 

 ment most of the familiar sources of error are mini- 

 mised, it is interesting to note the introduction of one 

 possible cause of error, quite unfamiliar to astronomers, 

 namely, the deflection that might be due to the attrac- 

 tion of the earth's horizontal magnetic force upon the 

 hanging part. If the telescope-tube were, as is custo- 

 mary, made of iron or steel, this would reach a 

 serious magnitude, and even if a proportion only of 

 the suspended weight were of iron a perceptible devia- 

 tion might result. It would, in fact, not be safe to 



I'lG. 1. — Track of polar movement, 1900-3. 



Fig. 2. — Same track referred to axis rotating in the 

 earth with a fourteen-month period. 



Fig. 3.— Hodograph of Fig. 2. 



Fig. 4. — Hodograph referred back to axes fixed in 

 the earth, or torque diagram. 



In order to close the 

 front it is necessary to have a plane 

 parallel glass of slightly larger aperture than the lens. 

 As this glass has to be worked with the same refine- 

 ment as a lens, and as a" plane surface is more trouble- 

 some to work than a curved one, this is rather a 

 costlv addition. Whether, as a matter of fact, it is 

 worth while keeping the instrument at the same tem- 

 perature, or whether it will be better to reduce the 

 temperature change to a minimum by covering^ the 

 whole with non-conducting material, and then apply 

 the very small corrections necessary to the measure- 

 ments made on the plate, is a question for experience 

 to decide. 



As a heavv hanging mass would be liable :o long-, 

 continued vibrations when disturbed, a four-armed 



NO. 2443, VOL. 97] 



allow this proportion to exceed one-tenth of the whole 

 weight, and it therefore seemed better to exclude the 

 use of iron or steel altogether. There is accordingly 

 none, with the exception of the four thin flat pendulum 

 springs which form the gimbal suspension. 



In detaining you with these short descriptions of 

 recentlv devised instruments, I may appear to have 

 been wandering rather far from my subject, the 

 wanderings of the earth's pole. You will, however, 

 appreciate that in reality they follow very closely from 

 it, being instruments designed with the special object 

 of solving the particular problem we are discussing. 



We will now revert to the diagram of the observed 

 polar motion, and I will indicate how it is possible 

 to analvse this so as to separate the irregular move- 

 ments from the more orderly fourteen-month preces- 



