xxviii President'' s Address. [^ug. 29^ 



points. Hansen's lunar tables, published in 1860, were an immense 

 advance on all previous tables : they represented the Moon's motion 

 with great accuracy over the whole range of reliable observation 

 from 1750 to 1864, and they also gave a fair representation of the 

 somewhat vague accounts of lotal eclipses in ancient times. It was 

 known from Prof. Adams' researches that Hansen's value of the 

 secular acceleration of the Moon's mean motion was nearly double 

 its theoretical value, but a possible and plausible explanation of this 

 discrepancy was found in the action of the tides on the time of 

 revolution of the earth. But from 1865 to the present time the Moon 

 has been gradually running away from the tables more and more every 

 year, so much so that in the year 1887 the errors of the tables 

 amounted to as much as one second of time. Naturally many attempts 

 have been made to discover the cause of this unpleasant disagreement 

 between theory and observation. Mr. Neville, Director of the Durban 

 Observatory, is at present engaged on a revision of the lunar theory. 

 Sir G. Airy has published a Numerical theory, founded on Delaunay'g 

 work, which however has not turned out to be all its distinguished' 

 author could have wished. Prof. Newcomb, while engaged in pre- 

 paring accurate places of ihe Moon for use in the Transit of Venus 

 expedition, detected several small errors in Hansen's work but wa& 

 obliged, in order to satisfy the observations discussed, to introduce 

 further empirical corrections. The introduction of such corrections 

 really amounts to determining the Moon's moan motion from recent 

 observations only, and there is no reason to hope that they will hold 

 good for more than a few years : in fact evidence is not wanting to 

 shew that they are beginning to fail already. Mr. Stone has brought 

 forward an explanation, which however has not met with acceptance 

 among astronomers : nevertheless, on his hypothesis, Hansen's tables 

 continue to represent the Moon's place as well now as they did 

 formerly. Several other distinguished mathematicians are also at 

 work on the subject, and it is to be hoped their labours will let more 

 light on this difficult theory ; but at present the Moon fully deserves 

 its title of " the refractory satellite." 



A most valuable series of observations on lunar heat has been made 

 by Prof. Langley by means of the bolometer, an instrument of his • 

 own invention. The rationale of this instrument is briefly as follows : 

 an excessively thin strip of platinum wire is inserted in two arms 

 of an apparatus which is practically a Wlieatstone's bridge and a 

 current of electricity is passed through it ; rays from the object- whose^ 



