July 1, 1892.] 



KNOWLEDGE 



133 



upon the age of tbe system. For a more complete discus- 

 sion of these questions I must refer Mr. Mouck to my paper 

 on the '• Evohition of the Double Star Systems," which 

 will appear in November, and will probably remove any 

 doubt that may remain as to the correctness of the theory 

 of double star evolution. Numerical calculation shows that 

 tidal friction is an amply sufficient cause to account for 

 the great eccentricities, and also that the eccentricity 

 developed under difierent conditions would be very difi'erent. 

 It is not, therefore, surprising that eccentricities of every 

 kind exist, as this could have been predicted from theory. 



Very faithfully yours, 

 Zimmer Straza, Berlin. T. -J. J. See.] 



M 



A MEAN-TIME SUNDIAL. 



■ OST people know that an ordinary sundial does 

 not give clock time — sometimes the dial time is 

 fast and sometimes it is slow as compared with 

 clock time : for sundial daj's are not, like 

 ordinary days, all of equal length. ^Yhen the 

 earth is in the part of its orbit which lies nearest to the 

 sun, it moves faster, and describes a greater angle about 

 the sun in twenty-four hours than when it is in the part 

 of its orbit furthest from the sun ; consequently, when 

 the earth is in perihelion the solar day exceeds the sidereal 

 day by more than tbe average amount, and the shadow of 

 the gnomon comes roimd again to twelve o'clock a little 

 later than when the earth is in aphelion. 



The mean time shown by ordinary clocks is based upon 

 tbe division of a mean day, which corresponds to. the mean 

 or average length of the day as measured by the sundial 

 shadow at different parts of the year. The clock time 

 corresponds to the dial time which would be shown by a 

 "fictitious" or "mean" sun moving uniformly in the 

 equator at the same average rate as that of the real sun in 

 the ecliptic. 



The " equation of time " corresponds to the difference 

 of time which would be shoT^Ti on a dial by the nnJ sun 

 and the iiudn sim. It is reckoned as plus when the 

 sundial is slower than the clock, and minus when it is 

 faster. It is the correction which must be applied to the 

 ordinary dial time in order to obtain mean time, and it 

 sometimes amounts to more than sixteen minutes. 

 Although a great deal of ingenuity and thought has for 

 centuries been expended upon the construction of simdials, 

 I am not aware tliat any one of the old dial makers ever 

 succeeded in contriving a dial to show mean time. The 

 difficulty has, however, at last been overcome in a very 

 simple manner by ilajor-Cleueral .J. R. Oliver, who has 

 devised a gnomon which practically makes its own 

 correction for the equation of time. 



The peculiarity of the instrument is that the time is 

 indicated, not by the shadow of a straight edge, as in the 

 old sundials, but by the point where an equatorial circular 

 line is cut by the edge of the shadow of a curved surface, 

 the curvature of which is so arranged with respect to the 

 sun's distance above or below the equator as to compensate 

 for the " equation of time." 



The instrument, says Major-General Oliver, is a universal 

 one, and consists of a meridianal semi-circle, the diameter 

 of which is an axis carrying the curved gnomon, and an 

 equatorial cii'cular arc. The latter has engraved upon its 

 concave surface a graduated line, on which are marked the 

 hours and their subdivisions. There is a screw for clamp- 

 ing the meridianal arc at the proper position for any given 

 latitude, and another clamp for adjusting the equatorial arc. 

 The dial not only indicates local mean time, but by a 



very simple adjustment may be set so as to show any 

 required standard time. Thus it might be set at Plymouth 

 to indicate Greenwich time. 



Strictly speaking, there ought to be two gnomons, one 

 to be used from .June to December, and the other from 

 December to June ; but by adopting a gnomon of mean 

 contour tbe gi-eatest error introduced at any time is only 

 about one minute, an amount not more than the unavoid- 

 able one due to the softness of tbe edge of the shadow. 



Four times a year the equation of time vanishes, and the 

 gnomon would then intersect its own axis. To allow for 

 the necessary thickness of the latter, a slight adjustment of 

 the hour circle is necessary at these times. 



Although tbe invention is little more than a scientific 

 toy so far as England is concerned, it is believed that it 

 would be of great use in countries where sunshine is 

 plentiful and means of regulating the time are scarce. 

 South Africa, South America, Australia, and India may be 

 adduced as instances. In India, for example, there are 

 numerous up-country stations at a distance from any rail- 

 way, where there are practically no means of correcting 

 clocks and watches. 



The figure shows the form of the dial devised by Major- 

 Cleneral Oliver. A comparatively cheap mean-time sun- 

 dial might, I would suggest, be easily made by amateurs, 

 with a glass globe such as is used for keeping gold-fish. A 

 divided circle would need to be etched or painted round 

 its greatest girth, and the gnomon might be made of a 

 knitting needle centrally placed and carrying a piece of 

 wood or gun-metal turned to scale from a curve which 

 may be laid down from a table (such as one finds in 

 Whittaker) giving the equation of time for each day of the 

 year. The knitting needle should be passed centrally 

 through a cork or bung in the mouth of the globe, and 

 into a block fixed by marine glue at the bottom of the 

 globe ; and the globe must then be tipped on one side 

 and supported in a stand so that the knitting needle is 

 parallel to the earth's axis. From December 'ioth to April 

 1.5th, when according to the almanacs the sim is after the 

 clock, one reads the time from the following edge of tbe 

 shadow. When the equation of time has vanished and the 



