18 SCIENCE PROGRESS 



Lagrange discussed the stability of three bodies whose relative 

 positions are the vertices of an equilateral triangle and showed 

 that the motion was stable when the orbits were coplanar. 

 The problem remained of theoretical interest until the dis- 

 covery of certain asteroids which with the sun and Jupiter 

 formed an approximately equilateral triangle. Such asteroids 

 ' are known as the Trojan planets. In this paper the theory of 

 their librations is worked out. 



Petter, G. B., The Determination of a Cometary Orbit, 

 Mem., B.A.A., 21, pt. ii. 191 8. This paper gives a demonstra- 

 tion of Leuschner's short method of determining orbits from 

 three observations, with examples, suited to the reader with 

 but slight mathematical knowledge. 



Glancy, A. Estelle, On the Orbit of (132) ^Ethra, Ast, 

 Journ. 31, No. 3, 191 8. 



Neubauer, F. T., Orbit and Perturbations of (716) 

 Berkeley, Lick Obs. Bull. 301, 191 7. 



Mundt, C. S., On the Orbit of (718) Erida, Lick Obs. 

 Bull. 302, 191 7. 



Pickering, E. C, Asteroids bright in 191 8, Harv. Coll. 

 Obs. Circular, No. 203, 191 8. The Harvard Observatory pub- 

 lishes every year a useful abstract of ephemeroids of asteroids 

 attaining a magnitude of 10 m. and brighter during the year. 



Fischer- Petersen,. J., Die auf der Kopenhagener Stern- 

 warte ausgefuhrten numerischen Untersuch&ngen iiber das 

 Dreikorperproblem (Probleme restreint), Pub. o. g. Medd. 

 fra Kobenhavns Obs. No. 27. This is a useful and fairly ele- 

 mentary account of the valuable work done at Copenhagen in 

 investigating periodic and retrograde orbits in the special case 

 of the problem of three bodies in which one has a very small 

 mass in comparison with the other two. 



Variable and Binary Stars. — Turner, H. H., On the Classi- 

 fication of Long-period Variable Stars, M.N., R.A.S., 78, 92, 



1917. 



Phillips, Rev. T. E. R., Note on the Light Curves of 

 Short-period Variables, M.N., R.A.S., 78, 185, 191 8. 



Wilson, R. E. (a) The Orbit of the Spectroscopic Binary 

 7 Phoenicis, (b) The Orbit of the Spectroscopic Binary o- 

 Puppis, Lick Obs. Bull., No. 303, 191 7. 7 Phoenicis is of 

 special interest as being the only late type star at present 

 known to be moving in an essentially circular orbit. 



