Mr. Squire on the Occultation qfAldebaran on July 16; tyc. 24?5 



struments having plane sights, till after the time of Tycho, 

 when telescopic sights began to be introduced. 



Before the year 1700, Azout and Picard applied telescopes 

 to astronomical instruments, which have given such precision 

 to modern observations. Accordingly it has been found that 

 from this time observations began to possess an accuracy 

 formerly unknown ; and the differences between our theory 

 and the observations are diminished. 



By analyzing all the foregoing observations, and introdu- 

 cing expressions involving the squares and higher powers of 

 the time from the assumed epoch, a formula would be ob- 

 tained so as to represent all the observations with tolerable 

 correctness. It is evident, however, that although it might 

 represent the more early observations better, it would not re- 

 present the modern ones so well ; and as the former cannot 

 be allowed to possess great accuracy, it has been thought un- 

 necessary to attempt to investigate such a formula, as it can 

 be comparatively of little value when obtained. Besides, it is 

 known from the investigations of Laplace, that the obliquity 

 is a quantity which varies between certain limits not exceed- 

 ing 3°, and as the times of maximum and minimum when the 

 variation is nothing, are unknown, it is evident that the varia- 

 tion itself must be a variable quantity, and consequently it is 

 not the same now, in all probability, that it was some thou- 

 sands of years ago, or what it will be some thousands of 

 years hence ; nor have we any means of ascertaining its se- 

 cular change with any degree of precision. Astronomers must 

 therefore, in a great degree, be contented with the most ac- 

 curate annual variation within a moderate number of years 

 since the use of well-constructed instruments, and accurate 

 methods, of reduction, have been introduced ; reserving the 

 discussion of the formula of variation for distant periods, till 

 the lapse of ages and an accumulated mass of observations 

 afford the means of investigating this subject with advantage. 



I am, Gentlemen, yours, &c. 

 54, South Bridge, Edinburgh, WlLLIAM Galbraith. 

 August 10, 1830. 



XXXIX. On the Occultation of Aldebaran on July 16th, 1830; 

 and on the Accuracy of the computed Times of it given in cer- 

 tain Almanacks. By Mr. Thomas Squire. 



To the Editors of the Philosophical Magazine and Annals. 

 Gentlemen, Epping, Sept. 14, 1830. 



[" BELIEVE the occultation of Aldebaran by the moon on 

 ■*■ July the 1 6th of the present year, was not (on account of 

 clouds) very generally observed in this country. Nevertheless, 



I was 



