Astronomical and Nautical Collections. 129 



tures of various climates, than to the occasional variations at 

 any one place. 



The steps of the computation, with a little variation from 

 those before enumerated, have been nearly these. First, to 

 find the mean apparent altitude for each star. Secondly, to 

 find the mean height of the exterior thermometer. Thirdly, to 

 reduce all the refractions to their mean state of pressure, with 

 the barometer at 30 inches, applying corrections, simply pro- 

 portional to the differences, in the usual manner. Fifthly, to 

 find the mean refraction, and the several differences from the 

 mean, and to add together such of them as are regular, that is, 

 such as agree with the differences of the thermometer in their 

 character, and to subtract the sum of such as are irregular. 

 Fifthly, to divide this result by the sum of the differences of the 

 thermometer, in order to obtain the experimental correction for 

 temperature ; which must, however, be increased by the varia- 

 tion of refraction corresponding to such a supposed change of 

 altitude. Sixthly, Mr. Groombridge having always reduced the 

 temperature of his interior thermometer to 55°, in registering 

 the height of the barometer, it will be necessary to make a slight 

 correction for the difference of this temperature from the mean 

 height of the exterior thermometer, in order to continue the 

 m.ode of computation before adopted. Seventhly, it is obvious 

 that if the tables and the observations were both perfect, the 

 correction for temperature, thus determined, ought also to re- 

 produce the observed refraction from the standard temperature 

 of the tables ; but this is seldom correctly true, and least of all 

 where the number of observations compared are few, although 

 they may not deserve to be wholly rejected. We may, therefore, 

 divide the difference of refraction from that of the standard tem- 

 perature, as exhibited in any tolerable tables, by the difference 

 of the mean temperature from the standard temperature of those 

 tables, for the joint result of the observations and tables. 



