Dec. 5, 1884.] 



♦ KNOWLEDGE 



465 



Then if the gyroscope were not rotating the median plane 

 of the disc would still remain uorthand-south. Thus, sup- 

 posing t A }), the tangent northand-south line to the earth's 

 surface at A, meeting the earth's axis, produced, at p, the 

 median plane of the gyroscopic disc which had coincided 

 with < A ;> when the gyroscope was at A, would coincide 

 with t' B/) the northand-south tangent line through B, 

 when the gyroscope was at B. This would correspond with 

 the direction n C « in Fig. 6, a direction inclined to (or dis- 

 placed from) N S. But in the actual case of a heavy 

 rotating disc, there would be no such displacement, unless 

 the attachments of the axial supports were rigid, in which 

 case of course the rotating disc would be forced irresistibly 

 by the overwhelming power of the earth's motion to retain 

 its north and-south position. In the case of the delicately- 

 poised gyroscope of Foucault, the median line N S would 

 only be affected by such portion of the earth's action as was 

 conveyed through the suspending line, and at the knife- 

 edges supporting the internal ring, and this action — in 

 accordance with the principle we have considered — instead 

 of shifting X C S to the position u C *■ (Fig. 4), can only 

 slightly shift the uppermost point C towards c (the rotation 

 being in the direction indicated by the arrows), leaving the 

 points ?and k (corresponding to N and S) on a line parallel 

 to N S. Thus if a horizontal telescope is directed to the 

 median line of the disc, at X or at S, this line will appear 

 to move in a direction opposed to that of the earth's rota- 

 tion, the northern side moving towards the east as at I, 

 the southern side moving towards the west as at k. 



Thus, when observed under telescopic power, the gyro- 

 scope is seen steadily travelling against the direction of the 

 earth's rotation, at the precise rate which calculation shows 

 to correspond to the latitude of the place of observation. 

 For the time being, the gyroscope is, so to speak, like an 

 independent planetary body vyith its oirn proper polar axis 

 directed constantly to the same point of the celestial 

 sphere. 



We are led naturally to the proof of the earth's rotation 

 derived from the precession of the equinoxes. The earth's 

 mass in rotation is a species of gyroscope ; and were the 

 earth a perfect sphere it would rotate for ever with its 

 polar axis in a constant position. But the earth being 

 oblately spheroidal, external bodies have a sort of pull 

 upon it, in a manner precisely resembling that which we 

 have conceived as operating upon our experimental gyro- 

 scope as above illustrated. The proof of the earth's rota- 

 tion derived from the consequent slow change in the 

 position of the earth's axis is not, however, founded on 

 the accordance of the observed effects, with dynamical 

 theories — though this accordance is a very strong argu- 

 ment per se. It is the fact that by the precessional 

 motion of the earth's axis, and the similar but smaller 

 motion called nutation, all the stars in the heavens, and 

 the planets also, seem to be affected with a species of tremor, 

 which afibrds the most convincing proofs of the earth's 

 rotation. It is obviously unreasonable to suppose that the 

 stars and planets which we have already seen to be disasso- 

 ciated from each other as respects their principal move- 

 ments, should only resemble each other in the matter of 

 these strange, tremulous motions. When we see, on the 

 other hand, that a simple dynamical theory, the results of 

 which we can readily confirm by experiment, accounts for 

 all these observed tremors, as due to a peculiarity of the 

 earth's rotation, we can atford to smile at the philosophers 

 who hope to simplify astronomical conceptions by setting 

 the earth at rest in the centre of the universe. Indeed, 

 the proof of the earth's rotation thus derived is as effective 

 as the proof of the earth's revolution derived from the 

 aberration of the fixed stars ; it is not, however, so readily 



rendered convincing to those unfamiliar with the abstruse 

 branches of mathematics. 



Lastly, there remains to be mentioned the proof of the 

 earth's rotation derived from the observed rotation of the 

 primary planets. It has been established by the most 

 satisfactory modes of measurement, that the planets are 

 globes like our earth, some inferior to her in magnitude, 

 but others far surpasbing her. These bodies are seen to 

 be in rapid rotation upon their axis, those of them which 

 most closely resemble the earth performing a complete 

 rotation in about twenty-four hours. The argument deduced 

 from this circumstance is powerful in itself; but its force 

 will be more clearly seen when we proceed, as we are now 

 to do, to consider the proofs of the earth's revolution round 

 the sun. 



(To be continued.) 



GORHAM'S PUPIL PHOTOMETER. 



THIS beautiful and most ingenious instrument, to which 

 our attention was first called by Mr. Tindall, in a 

 letter (1500) which appeared on p. 391, is as simple as it 

 appears to be effective. It consists of a piece of bronzed 

 tubing about 1-9 in. long, and 1.5 in. in diameter, with one 

 end closed by a disc, in which are a series of pairs of 

 minute holes pierctd on radii of the circle, and drilled with 

 absolute accuracy at distances varying from •O.'i in. to '28 in. 

 apart. A cap with a radial slot in it i-o narrow as only to 

 leave one of these pairs of holes visible at a time rotates 

 over the brass disc ; and round the cylinder just below the 

 edge of the cap the linear distance of each pair of holes is 

 engraved exactly opposite to it. Our first figure represent* 



Fig. 1. 



Fig. 2. 



the cap as seen when looking down on to it. The pair of 

 holes -10 in. apart are shown as a isible through the slot. 

 Fig. 2 exhib its the tube as seen sideways, to show how the 

 figures are engraved upon it. The instrument seems to 

 have been originally devised by Mr. Gorham for measuring 

 the diameter of the pupil of the eye under the stimulus of 

 light of varying intensities. It is used by looking through 

 the open end of the tube (the bottom in Fig. 2), when two 

 discs of light will be seen, like a double-star. The cap is 

 turned until a pair of these discs appear which are precisely 

 tangent, or jast touch one another at their edges. Then 

 may the diameter of the pupil be read off on the scale 

 by mere inspection to -01 in. But this is by no means 

 the only — or, in fact, the chief — use of Dr. Gotham's in- 

 strument,' inasmuch as the magnitudes of the pupil really 

 furnish the measures of the intensities of the lights under 

 whose influence its diameter varies. Hence it may be 

 used as a photometer; and for this purpose we proceed as 



