RECENT ADVANCES IN SCIENCE 9 



limited, and it therefore appears that the dissipation of energy 

 in the tides is of the right order of magnitude to account for 

 the unexplained part of the moon's secular acceleration. 



Dr. Jeffreys, in the paper referred to above, proceeds 

 further and shows that, on a tidal friction theory, it is pos- 

 sible, when a value is assumed for the lunar secular accelera- 

 tion, to calculate the value of the secular acceleration of the 

 sun. If the value so calculated is found to agree with the 

 value derived from observation, it would afford further sup- 

 port to the theory. He considers various types of tidal fric- 

 tion. Tidal friction might occur in the body of the earth, being 

 due to imperfection of elasticity, which latter may be regarded 

 as a combination of plasticity and afterworking. Plasticity 

 in the outer crust of the earth would damp out the 14-monthly 

 variation of latitude, whilst afterworking would render the 

 crust impermeable to earthquake waves. It is possible, 

 however, that there may be sufficient plasticity near the centre 

 to account for the phenomenon, though it is concluded that 

 the balance of evidence is against an explanation involving 

 bodily friction. It is shown also that atmospheric phenomena 

 and tides in mid-ocean are not capable of accounting for 

 the secular acceleration. The theory of dissipation in regions 

 of strong tidal currents is discussed mathematically, and it is 

 shown that, to account for the observed lunar acceleration, a 

 secular acceleration of the sun of o"78 per century is neces- 

 sary. This value does not differ greatly from the most prob- 

 able value of about i" arrived at by Fotheringham. It is 

 further proved that this theory does not give rise to difficulties 

 due to the existence of the 14-monthly period of latitude- 

 variation. 



It would therefore appear that Taylor's theory gives the 

 first satisfactory explanation of the lunar secular acceleration. 



Studies of the Nebulce. — Vol. xiii of the Publications of 

 the Lick Observatory contains a detailed account of the ex- 

 tensive and important studies of the nebulae which have 

 been carried out by the Lick observers during the past several 

 years. The volume is in the main a record of the observa- 

 tional material which has been gathered together. Although 

 much of this material cannot yet be fitted into a theoretical 

 framework, an attempt has been made to do so wherever 

 possible. Most readers will probably agree, however, that the 

 volume raises more problems than it solves. It therefore pos- 

 sesses great interest both for the practical and for the theoretical 

 astronomer. 



Part I contains descriptions of 762 nebulae and clusters 

 which have been photographed with the Crossley reflector, by 

 H. D. Curtis. In connection with this work a count has been 



