OP ARTS AND SCIENCES. 17 



the values of x and y. The value of y is approximately 15". 20 ; that 

 of X, 0".21 {t — 1814). Residuals are accordingly given in the last 

 two columns by subtracting the values of y and x thus obtained from 

 those observed. 



The residuals in the last two columns are evidently not due to acci- 

 dental errors, but whether they are caused by curvature of the path or 

 systematic errors of the observer is less evident. The first nine sets 

 are so discordant, that little dependence can be placed upon them. 

 The values of Ay show a very slight increase, followed by a diminu- 

 tion in the later values. A a: seems to diminish slowlj', the later values 

 of the Struves and of Dembowski being somewhat less than the earlier. 

 The curvature is so slight, that it has been thought to indicate an 

 hyperbolic orbit. The observations so far made will however be very 

 nearly satisfied by a large circular orbit seen obliquely, so that the 

 part described during the last century has been that near the end of 

 the minor axis of the apparent ellipse. 



If we take the mean of the residuals, we find the values for A re of 

 0".25 and for A y of 0''.15. As these include all kinds of systematic 

 errors, the deviations from a straight line can scarcely be regarded as 

 certain. 



II. Variable Stars of the Algol Type. 



Variable stars may be divided into several classes, according to the 

 nature of the fluctuations of their light. First, temporary stars, 

 which appear suddenly, and gradually fade away during the next few 

 months. The most famous star of this class is that observed in 1572, 

 by Tycho Brahe. The new stars in Corona Borealis in 18G6 and in 

 Cygnus in 1876, are recent exami)les of this class. Second, a large part 

 of the variable stars pass from their maximum to their minimum and 

 back again, in from six months to two years, the period and the bright- 

 ness at the maximum and minimum being somewhat variable. The 

 change in light is generally very great, amounting to several hundred, 

 or even thousand times. The most striking examples of this class are 

 o Ceti and -^ Cygni. Thirdly, we have the slight changes to which many 

 (or, according to Dr. Gould, most) stars are liable. These changes 

 seem to be irregular in many cases ; at least, their law is not j^et known. 

 Examples of this class are furnished in a Orioiiis and a Cassiopeice. 

 Fourthly, certain stars continually vary, going through a series of 

 changes in the course of a few days, which appears to be repeated ex- 

 actly. Two causes seem here to be superimposed, one producing one 



VOL. XVI. (n. S. VIII.) 2 



