498 ON FINDING THE TIME ACCURATELY 



justified in this conclusion, by the results of many hundred trials, 

 verified by calculating as simple altitudes those from which I 

 had deduced the time as equal altitudes. Indeed I am rather 

 disposed to give the preference to the simple altitudes for ac- 

 curacy, if a considerable change of temperature has taken place 

 between the morning and evening observations, as the conse- 

 quent change of refraction is seldom taken into account in 

 the calculation of the equations to equal altitudes, though it 

 may produce a sensible effect on the determination of the 

 time. To overcome these difficulties, has been my sole object, 

 and my only wish, in making this communication, is to enable 

 every observer to put this method to the test, in the hope that 

 he may derive the same satisfaction from it that I have done. 



In the morning, when the sun has nearly 10° of altitude, 

 and farther from noon than two hours, if far advanced in the 

 season, but otherwise, the nearer the prime-vertical the bet- 

 ter, I observe eleven successive altitudes of the sun's lower 

 limb, reflected from quicksilver, where the situation will admit 

 of it ; where it did not, I have employed with equal success 

 pure limpid oil covered by one of Troughton's plate-glass an- 

 gular roofs ; but in most situations it may be used in a room, 

 without any cover ; always taking care, in the morning obser- 

 vations, to set the index, with the utmost accuracy, to an even 

 10' or 20' orreater than the sun's altitude, and then to wait the 

 contact. If I use a chronometer, I have an assistant, who 

 counts the seconds aloud, which I direct him to note, with the 

 fractions, &c. at each observation. In the afternoon I pro- 

 ceed in the same way, only setting the sextant 10' or 20' less 

 than the sun's altitude, and carefully noting the barometer and 

 thermometer, for each series of observations, in order to cor- 

 rect the mean refraction. 



I 



