[STEWART] WIRELESS AND LONGITUDE DETERMINATIONS 45 



equally well. As a matter of fact there is a systematic discordance 

 of approximately a tenth of a second between Arlington signals and 

 our standard time; the longitudes deduced by the above method are 

 nevertheless based on Ottawa. 



The probable error of a comparison between Arlington and the 

 intermediary chronometer is ± -01 sec. for a single coincidence; that 

 of the complete comparison between the sidereal chronometers at the 

 two stations may therefore be taken as from ± -01 sec. to ± -02 sec, 

 and is of the same order as for an exchange by telegraph ; the time of 

 transmission of the signals is practically negligible, since they travel 

 with the velocity of light. It has been found, however, that there is 

 a small personal equation in the receipt of signals which must be 

 measured and taken into account; the values obtained (difference 

 between observers) for the seasons of 1914, 1915 and 1916 were -013 

 sec, -008 sec. and -010 sec. respectively. It may be worth noting 

 here that a method has now been devised^ by which even this small 

 personal equation is almost w^holly eliminated. It has been tested 

 at the Observatory and found to have decided advantages, coupled 

 also, however, with some slight disadvantages. 



The field aerials used have been from four to five hundred feet 

 in length and fifty to seventy feet in height; two wires are used, 

 carried on spreaders of eight to ten feet in length ; these are supported 

 on trees, which have always been found available, though sometimes 

 with difificulty; it has consequently never been necessary to erect 

 artificial masts; wherever possible, a ground connection has been 

 made in a small lake or stream; failing this, an iron rod driven into 

 moist earth will usually suffice. The strength of signals naturally 

 varies considerably with the nature of the country and with the 

 efficiency of the aerial it has been found possible to erect; usually, 

 however, unless on exceptional nights, it has been possible to make 

 satisfactory comparisons; one of the favorable features is that signals 

 are usually strongest at night. 



The instrument used for the transit observations is of a small, 

 compact and very portable type; this is important, since most of the 

 stations occupied have been accessible only by canoe. Fourteen 

 stations have been occupied by this method up to the present; it 

 seems probable that it will find increasing application in the future. 



The average precision of the longitudes determined has been very 

 little inferior to that of those determined by wire; considering the 

 fact that the telescope is considerably smaller and that the observers 



1 Monthly Notices, March 1917. 



