APRIL 4, 1921 pittier: notes on swartzia 155 



Thus the recovery curve can be constructed by making a tracing of 

 the drift curve, inverting it and shifting it along the time axis by an 

 amount ^o- The algebraic sum of the ordinates of these two curves 

 forms the desired curv^e for z sls a. function of /'. This method has 

 been applied with fair success to data furnished by- Dr. C. E. Van 

 Orstrand for the slow stretch in a steel tape over a period of four 

 months, when suspended under tension. The greatest difference be- 

 tween the calculated and observed after-effect at any time during this 

 long period was less than 9 per cent of the initial after-effect. 



A similar check on equation (19) should be possible by reference to 

 Michelson's data for torsion. His recovery curve formula may be 

 written^ 



z = KRe-'^^^' (1 - e-"^^o) (20) 



Substitution from (9) into (19) gives 



z = KRe-"^^ [1 - e.-«(Vtr+l^- VP)] (21) 



This, for small values of ^', reduces to Michelson's experimental for- 

 mula (20), but for large values it diverges. It is not possible to make 

 an exact test of equation (19) without reference to the original observa- 

 tions, w^hich were not given in the paper above cited. 



These examples may suffice to make clear the general purpose of 

 the superposition theory. It is not a molecular theory, neither does 

 it aim to deduce a priori anything about the form of the drift function. 

 It presupposes the availability of just such data as Michelson's drift 

 curve (9) and then proceeds to develop the necessary interconnections 

 between the drift and the remaining irreversible effects. 



* 



BOTANY. Notes on the genus Swartzia in Panama and Guatemala.'^ 

 Henry Pittier. 



The genus Swartzia of the Caesalpiniaceae contains a large number 

 of species, many of which are closely related, and the genus as a whole 

 is badly in need of revision. In the present paper an attempt is made 

 to systematize the representatives of the genus now known from 

 Panama. A little known species of the genus from Guatemala is also 

 described. ^^' 



6 Op. cit. See also Journ. Geol. 28: . 1920. /K 



1 Received February 11, 1921. / 



