TOI-. XLV.] PHILOSOPHICAL TRANSACTIONS. 4igt 



hensive that I may incur the censure of some persons, for having spent so much 

 time in the pursuit of such a seeming trifle : but the candid lovers of science 

 will, I hope, make due allowance for that natural ardour with which the mind is 

 urged on towards the discovery of truths, in themselves perhaps of small mo- 

 ment, were it not that they tend to illustrate others of greater use. 



The apparent motions of the heavenly bodies are so complicated, and affected 

 by such a variety of causes, that in many cases it is extremely difficult to assign 

 to each its due share of influence, or distinctly to point out what part of the 

 motion is the effect of one cause, and what of another : and while the joint ef- 

 fects of all are only attended to, great irregularities and seeming inconsistencies 

 frequently occur ; whereas, when we are able to allot to each particular cause its 

 proper effect, harmony and uniformity usually ensue. 



Such seeming irregularities being also blended with the unavoidable errors 

 which astronomical observations must be always liable to, as well from the im- 

 perfection of our senses, as of the instruments that we make use of, have often 

 very much perplexed those who have attempted to solve the phenomena: and till 

 means are discovered by which we can separate and distinguish the particular , 

 part of the whole motion, that is owing to each respective cause, it will be im- 

 possible to be well assured of the truth of any solution. For these reasons, we 

 generally find, that the more exact the instruments are that we make use of, and 

 the more regular the series of observations is, that we take ; the sooner we are 

 enabled to discover the cause of any new phenomenon. For when we can ba 

 well assured of the limits, wherein the errors of the observations are contained, 

 and have reduced them within as narrow bounds as possible, by the perfection 

 of the instruments which we employ ; we need not hesitate to ascribe such ap- 

 parent changes, as manifestly exceed those limits, to some other causes. On 

 these accounts it is incumbent on the practical astronomer, to set out at first 

 with the examination of the correctness of his instruments ; and to be assured 

 that they are sufficiently exact for the use he intends to make of them : or at 

 least he shovild know within what limits their errors are confined. 



The lovers of this science in general, cannot but acknowledge their obligations 

 to lord Macclesfield on this account : but I find myself more particularly bound 

 to do it; since, by means of his lordship's most accurate observations, I have 

 been enabled to settle some principal elements, which I could not at present 

 otherwise have done, for want of an instrument at the royal observatory, proper 

 for that purpose : for the large mural quadrant, which is there fixed to observe 

 objects lying southward of the zenith, however perfect an instrument it may be 

 in itself, is not alone sufficient to determine, with proper exactness, either the 

 latitude of the observatory, or the quantity of refraction corresponding to differ- 

 ent altitudes : for it being too heavy to be conveniently removed, and the room 



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