64 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



Devonport. Time-balls are dropped on Mount Wise at 1^ 

 and at 1^' 5'^ Greenwich mean time. 



Greenwich. A time-ball is dropped daily at 1 P.M. ; a 

 ball is automatically dropped at Deal. Time-balls are pro- 

 posed for Portsmouth and Start-Point. Hourly currents 

 are sent from the observatory to the Post-office, from which 

 they are distributed to subscribers. Currents are sent au- 

 tomatically at 10 A.M. and 1 P.M., and w^idely distributed; 

 and almost daily signals are sent directly to about 600 places, 

 including railway termini, and from these places they are 

 repeated to the lines radiating therefrom. The system is 

 practically complete. 



VISIBILITY OF STAKS IN THE DAY-TIME. 



Dr. Rudolf Wolf communicates to the Philosophical Society 

 of Zurich (20th year, page 179) a note on the visibility of stars 

 in the day-time from the bottom of deep wells. It appears 

 that Mr. F. Carpentier, of Zurich, tried the experiment of 

 looking for stars in the day-time from the bottom of a deep 

 well (ninety feet), and that he was successful. It was done 

 when the experimenter was but a lad, and although there is 

 no doubt as to the fact, the names of the stars seen and the 

 particulars of the observation can not be recovered. 



MEASURE OF TIME BY HOUR-GLASSES. 



Dr. Wolf, of Zurich, having occasion to use an hour-glass 

 which was intended to mark half-hours, endeavored to deter- 

 mine the probable error of a measure made by means of this 

 instrument. The ending: of the flow of sand could be ob- 

 served quite exactly, ^. e., to within one second of time ; the 

 mean of sixty trials gave him for the time of flow 28 29.95^ 

 d=5.7^ The probable error of a single observation was no 

 less than 43.9 seconds. It thus appears that this instrument, 

 which was extensively used by the ancients, is not a trust- 

 worthy one for exact determinations. 



