CHAP. XIII.] OBSERVATIONS FOR ERROR 301 



meters are beating together, and it may be noted that an error 

 X)f 10 seconds, in estimating the instant at which it takes place, 

 will produce an error of only 0-025 second. It is, therefore, 

 an extremely accurate method to compare solar chronometers 

 through the intermediary of a sidereal chronometer, when one 

 is available. 



A chronograph has recently been fitted to a small chrono- Chrono- 

 meter, with specially adapted " quick- train " movement. This ^e^n the 

 beats too rapidly to allow time to be taken by counting the field, 

 beats, but it has the advantage of enabling the chronometer 

 to be carried with less liability to disturbance of rate than either 

 a pocket-clironometer or a box-chronometer beating half- 

 seconds. 



The current from a small electric battery being broken at 

 intervals of two seconds by an arrangement fitted to the 

 chronometer escapement, a lever is caused thereby to strike 

 an anvil over which a tape is unwound at a uniform rate by 

 means of clockwork mechanism. The observer causes a 

 similar lever to strike the anvil on completing the circuit of 

 another battery at the instant of observation. The fine points 

 with which the levers are armed are adjusted to strike at points 

 on the tape exactly opposite and close to each other. The 

 position of the perforations in the tape caused by the lever 

 actuated by the observer, relatively to the perforations at 

 two seconds' intervals made by the beating of the chronometer, 

 indicates the exact time at which the observation was made. 

 The time results are obtained from the tape by applying to it 

 a piece of glass, ruled with eleven straight lines converging to 

 a point. The ends of these lines on the base of the triangle 

 so formed are equidistant on the edge of the glass, so that 

 when the first and last lines are so placed as to coincide with 

 the perforations at the beginning and end of the two seconds' 

 interval, it is divided into ten equal parts. The base of the 

 triangle is always kept parallel with the line of the perforations 

 on the tape. 



Whilst the observations are in process the tape is watched by 

 an assistant, and the perforations marked at suitable intervals 

 to correspond with the time shown by the chronometer. 



The rate at which the tape is unwound can be varied within 

 certain limits, thus giving a larger or smaller time scale. 



