OF THE OBSERVATORY OF CAMBRIDGE BY GALVANIC SIGNALS. 489 



7. It is thought that accuracy is gained by covering the needle-dial leaving a slit 

 through which the needle is seen. Thus the start only of the needle is seen, not the end of 

 the swing. 



All the above rules were attended to in the transmission of the signals from the Cambridge 

 Telegraph Station, excepting the sixth and seventh. The former was neglected from 

 oversight, and the latter was found to be inconvenient, because, for reasons stated further on, 

 the signal-times were noted by two observers at the same time. 



After receiving the above scheme of operations, I waited on the Astronomer Royal for 

 the purpose of seeing the Greenwich arrangements, and I took the opportunity of proposing 

 to him that signals should be made on two successive nights, and that there should be an 

 interchange of observers, the Greenwich observer both of transits and signals going to 

 Cambridge, and the Cambridge observer of transits and signals going to Greenwich, after the 

 first night's observations. This proposal, which was made with the view of eliminating the 

 effect of any difference between the personal equations of the two observers, was assented to 

 by Mr Airy. I proposed also that in addition to the stars ordinarily used at the two 

 Observatories for the determination of clock-errors, a selected list of other stars should also 

 be observed, and in the instances in which the same star was observed at both Observatories, 

 that clock-errors should be deduced from the same values of the Apparent Right Ascensions 

 of those stars. It is clear that by this process, in which exact values of the Right Ascen- 

 sions of the stars are not essential, a larger number of stars are available for clock-errors, in 

 case the state of the sky be capricious. Mr Airy agreed to this arrangement, stipulating 

 only that the longitude deduced from the ordinary clock-stars should be calculated apart from 

 that deduced from corresponding stars. I consequently selected from the Greenwich Twelve- 

 year Catalogue a list of stars, whose meridian transits would occur shortly before and after the 

 time occupied at Cambridge in the observations of the signals, and in the conveyance of the 

 chronometers to and from the Station. 



At Greenwich, on May 17» the observations throughout the series, both of selected stars 

 and signals, were made by Mr Dunkin. The signals were observed by noting the time of deflec- 

 tion of the needle at the galvanometer attached to the case of the transit-clock, the time of each 

 signal being the instant of the first start of the needle. The contact was made without 

 exception at the Transit-circle, by the apparatus intended for use in the American method 

 of recording transits. In four batches the signals were made at the instants when a star 

 passed the Transit wires. The signals were given throughout the series by Mr Henderson. 

 The warning-signals were counted and a register kept of the number by Mr Criswick. 



At Cambridge, on the same day, Mr C. Todd observed the transits of stars for 

 clock-errors, and the signal-times. Mr Breen also observed signal-times. As at Greenwich, 

 the instant of the first start of the needle was noted. The Cambridge signals were all given 

 by myself, stationed at one corner of the room in which the observations were made, as far 

 from the observers (whose backs were turned towards me) as the limited dimensions of the 

 room admitted. Mr H. Todd counted the warnings and signals. Mr Sach, local super- 

 intendent of the Eastern Counties' Telegraph lines, was present to arrange the battery and 

 wires, and to perform the necessary changes of connexion during the observations. The 



63—2 



