386 



NATURE 



\Mar. 19, 1874 



vation with the instrument itself may be more readily 

 employed for th-s purpose. Ascertain the true solar time 

 by means of a good watch and a time equation table, set 

 the hand of the polar clock to correspond thereto, and 

 turn the vertical pillar on its axis until the colours of the 

 selenite star entirely disappear. The instrument then 

 will be properly adjusted. 



" The advantages a polar clock possesses over a sun- 

 dial are : — ist. The polar clock being constantly directed 

 to the same point of the sky, there is no locality in which 



it cannot be employed, whereas, in order that the indica- 

 tions of a sun-dial should be observed during the whole 

 day, no obstacle must e.xist at any time between the dial 

 and the places of the sun, and it therefore cannot be 

 applied in any confined situation. The polar clock is 

 consequently applicable in places where a sun-dial would 

 be of no avail ; on the north side of a mountain or of a 



lofty building for instance. 2ndly. It will continue to 

 indicate the time after sunset and before sunrise ; in fact, 

 so long as any portion of the rays of the sun are reflected 

 from the atmosphere. 3rdly. It will also indicate the 

 time, but with less accuracy, when the sky is overcast, if 

 the clouds do not e.\ceed a certain density. 



" The plane of polarisation of the north pole of the 

 sky moves in the opposite direction to that of the hand of 

 a watch ; it is more convenient therefore to have the 

 hours graduated on the lower semicircle, for the figures 



will then be read in their direct order, whereas they 

 would be read backwards on an upper semicircle. In 

 the southern hemisphere the upper semicircle should be 

 employed, for the plane of polarisation of the south pole 

 of the sky changes in the same direction as the hand of a 

 watch. If both the upper and lower semicircles be gra- 



sL 





Fig. 19.— Wheatstone's Polar Clock. 



duated, the same instrument will serve equally for both 

 hemispheres." 



The following is a description of one among several other 

 forms of the polar clock which have been devised. This 

 (Fig. 20) though much less accurate in its indications than 

 the preceding, beautifully illustrates the principle. 



" On a plate of glass twenty-five films of selenite of 

 equal thickness are arranged at equal distances radially in 

 a semicircle ; they are so placed that the line bisecting the 

 principal sections of the films shall correspond with the 

 radii respectively, and figures corresponding to the hours 

 are painted above each film in regular order. This pLite 

 of glass is fixed in a frame so that its plane is inclined to 

 the horizon 38° 32', the complement of the polar eleva- 

 tion ; the light passing perpendicularly through this plate 

 falls at the polarising angle 56' 45' on a refiectorlof black 

 glass, which is inclined iS" 13' to the horizon. This ap- 



FiG. 20.— Polar Clock. 



paratus being properly adjusted, that is so that the glass 

 dial-plate shall be perpendicular to the polar axis of the 

 earth, the following will be the effects when presented 

 towards an unclouded sky. At all times of the day the 

 radii will appear of various shades of two complementary 

 colours, which we will assume to be red and green, and 

 the hour is indicated by the figure placed opposite the 

 radius which contains the most red ; the half-hour is indi- 

 cated by the equality of two adjacent tints." 



W. Spottiswoode 

 {To be continued.) 



