Mr. Galbraith on the Reduction of North Polar Distances. 335 



water at the same instant. It is difficult, however, to determine 

 the form of these lines, which are generally, no doubt, per- 

 pendicular to the direction of the current, from the want of 

 means to determine the time of high water at sea. The times 

 in the map are intended to be Greenwich time; the extent, how- 

 ever, included by the map, and the scale, are so small, that the 

 differences between the Greenwich time and the time at the 

 place, in the present state of our information is of little con- 

 sequence. 



LI 1 1. On the Reduction of the North Polar Distances of Stars 

 observed at Greenwich, and corrected by Brad ley's Refrac- 

 tions^ to Distances according with Ivory's Refractions. By 

 W. GALBRAITH, Esq. A.M.* 



TN the construction of the following Table, it is supposed that 

 * the observations are made when the celestial objects are 

 on the upper meridian ; consequently, the zenith distances are 

 North as far as 38 nearly, and South as far as 85, where the 

 Table of Bradley terminates in the first volume of Mr. Pond's 

 Greenwich Observations, page v.f 



The error of the refractions of Bradley at the pole is 

 0"*53; that is, 0" g 53 must be subtracted from polar di- 

 stances, as those of the Greenwich Catalogue by Bradley's re- 

 fractions, to obtain those by Ivory's. The French refractions, 

 as well as those of Brinkley and Bessel, differ but slightly from 

 those of Ivory at moderate zenith distances ; consequently the 

 Table will also give the results from these tables very nearly. 



As the corrections were computed when the barometer is 

 supposed at 30 inches, and the thermometer at 50 Fahrenheit, 

 there will be a slight difference if the barometer and thermo- 

 meter deviate greatly from these, though in all ordinary cases 

 the error from this cause will be very slight, since the cor- 

 rection is only a small relative quantity. Such a table as this 

 is alluded to by Mr. Pond in page 237, but has not, so far as 

 I am aware, been any where published. The difference, in- 

 deed, never much exceeds l"-J till about 83 zenith distance, 

 and at 85 it is not less than 6"J, though, no doubt, observa- 

 tions when great accuracy is required will not be made at such 

 low altitudes. 



From the peculiar mode of observing by the mural circle 

 as used at Greenwich, some other consequences would be 



* Communicated by the Author. 



f It may be remarked here, that the logarithms of the factor for the 

 English barometer in page xi. to reduce the French table for pressure, are 

 all slightly erroneous by supposing 0"'-76 = 29 m< 94 instead of 29-922. 



elicited; 



