CANADIAN LONGITUDES. 45& 



meridian by bisecting a star with the middle wire — as the correct 

 time can not be known by the chronometer, and procuring observations 

 for the correct local time would occupy much time — I recommend the 

 following way to get a transit instrument quickly into the meridian. 



First direct the instrument to a point about a degree and a half 

 from Polaris, in a line joining that star and the fifth star of the Great 

 Bear. This ought to place the transit instrument within half a degree 

 of the meridian ; then take from the nautical almanac two circum- 

 polar stars, that differ about twelve hours in their R. A. For instance, 

 51 Cepkeiand S Ursceminoris on March 1st, 1859, had a difference of 

 R. A. of 15m. 58*75s. — 51 Cephei passing sub. polo, first, and it will 

 come into the field of an inverting telescope on the right ; before it 

 has passed the second wire of the instrument (if it is not much out of 

 the meridian) by elevating the telescope, 8 UrscB minoris ought to be 

 in the field. If it is not, move the north end of the telescope to the 

 east until it is. When you have got these stars within range of the 

 field, and (as near as you can guess) 51 Cephei 15m. in advance, there 

 will then be sufficient time to adjust the level, which has of course 

 been put out by moving the instrument. Should you have a mean 

 solar chronometer, the difference of R.A. 15m. 58'75s. must be turned 

 into mean time. Now mark the time that 51 Cephei passes the middle 

 wire, and if the transit is in the meridian, 8 TIrsce minoris will come 

 to the middle wire in an opposite direction 15m. 58- 75s. after 51 

 Cephei. Should it not do so, mark its position at the moment that it 

 should be at the middle wire, and then by turning the azimuthal 

 screw, bring 8 Vrs. min. not quite half way towards the middle wire. 

 The distance that the star is to be moved is known by looking at de- 

 viation factors. This mode of proceeding is independent of the error 

 of the chronometer ; it only supposes that it goes with an even rate 

 for a quarter of an hour, and by one operation it should place the 

 instrument within one second of time of the meridian. You then take 

 observations for the errors of deviations, and proceed at once to get 

 local time. 



By looking at my report to Sir William Logan many causes of failure 

 may in future be avoided, and perhaps some trouble saved: — 



REPORT TO SIR W^. E. LOGAN, F. R. S. 



Quebec, 20th January, 1858. 

 Sir, — In the month of October, 1856, at your request I left Quebec 

 for Montreal, in order to determine by electric telegraph the longitude 



