452 On Refraction, 



each degree, there are given the logarithms of the refraction 

 in seconds and their differences : and in two auxiliary tables 

 are given the logarithms of the factors for correcting it for 

 the variations of height in the barometer and thermometer. 

 To make these corrections depending upon the temperature 

 always affirmative as far as the table extends, he has added 

 respectively to the logarithms of the two last tables their 

 greatest negative logarithm taken positively, and subtracted 

 from it the logarithms of the first table. 



To this succeeds a Table of mean refractions for true di- 

 stances from the zenith for every degree down to 80 , and 

 for every 10' thence to Ql°, with their differences, adapted 

 for 0" 76 metre of the barometer, and 12°\3 of the centigrade 

 thermometer*, with two auxiliary tables for reducing it to 

 any other state of the atmosphere. Directions are also 

 given for using them, together with the formulae from 

 which they were computed, reduced to a more simple 

 form. % 



In noticing the latest improvement made in this subject, 

 for which we are indebted to one of our own countrymen, 

 it is but fair to return him those acknowledgements to 

 which he is so justly entitled. When gentlemen of for- 

 tune give up the gay amusements of the world, and turn 

 aside from the pleasures of fashionable life, to cultivate 

 science in retirement, they deserve our warmest thanks : and 

 when we add to this the consideration that the science 

 they cultivate is not only one of the most interesting and 

 sublime, but of the utmost importance to a commercial na- 

 tion like Great Britain, our thanks certainly must be changed 

 into something more like gratitude. 



There are, indeed, very few who can afford to purchase 

 instruments of sufficient accuracy to make improvements 

 in a science so far advanced; and still fewer of those that 

 so amply possess the means of life, who would bestow that 

 time and attention which are requisite in acquiring the ne- 

 cessary knowledge forlhis purpose, and turning it to useful 

 account. 



Such is the present state of the navy of England, and 

 her maritime concerns, that the improvement of astronomy 

 is a subject which calls seriously for attention; more espe- 

 cially, as we have but one public institution for that pur- 

 pose. The commerce of France is nothing in comparison 

 of ours, yet the greatest encouragement is given in that 

 country to those who promote every science connected 



* Or, 29*92152 English inches, and 54° 5 of Fahrenheit's thermometer. 



with 



