502 



Transactions. 



classified as a long-period variable, but its clxanges take mucli less time than 

 the others just mentioned — apparently about thirteen weeks. This star has 

 a larger measure of irregularity in its behaviour. One peculiarity I have 

 noticed myself is that a week or two before the fall sets in, when the star 

 is usually at a fairly flat maximum, it seems to rise quite suddenly about 

 half a magnitude and then plunges rapidly down to its minimum. The 

 maximum on this curve is more like a cairn on the top of a hill than the 

 hill-top itself. 



Fig. 3 shows the curve of T. Gruis, in which the usual rapid rise and 

 slow fall is not to be found. There is even a suspicion that the fall is the 

 steeper part of the curve, and, if this is correct, it is a very unusual feature 

 in variable stars. 



The star R. Normae has the reputation of showing two maxima and 

 minima. In my own experience, however, the range of variation has been 

 so small that the curve is rather flat and shows no well-marked peaks of 

 maximum. 



All the stars I have mentioned so far are included in the second class 

 of variables — namely, the long-period variable stars. The others of the five 



>Ce "/ul 0^. JeJ^ OU- //^ _ ^ 



e^-^ 



jCf 2f f /(^2cf e /^26 7 // -2/ 7 7 -2^ / // 2/ / // i/ 



4 



i 1 1 — - 



i i —I— 



^ c 

 foo 

 II c 



Fig. 3.— Curve of T. Gruis in 1913. 



classes are — (1) Temporary stars; (3) irregular variables ; (4) short-period 

 stars ; (5) eclipsing stars. The fourth class has certain subdivisions, and 

 sometimes two other classes are added— cluster variable and suspected 



variable. 



The short-period stars are more difficult to observe, because they change 

 so rapidly that unless the observations are frequent they teach us nothing. 

 Of course, they require continuous good weather also. Three good speci- 

 mens of thei/ class are Kappa Pavonis, S. Trianguli, and S_. Normae. 

 I have watched these for several weeks while the weather was suitable, and 

 found them wonderfully regular both as to period and range of magnitude. 

 The period of S. Trianguli is six days; the other two take nine days. There 

 are a few stars whose periods cannot be classified as either long or short, 

 being about thirtv or forty days, Avhich is between the two classifications. 

 The onlv one I know among the southern stars is 1. Carinae, which is said 

 to vary between 3-3 and 5-5 in thirty-five days. I have not seen it brighter 

 than 3-8 or fainter than 4-5. and its period is apparently something just 

 over a month, and may be perfectly regular. 



The absolute regularity of the short-period stars gives us the first hint 

 .as to the reason of their fluctuations, for when we say their changes are as 



