December 20, 1912] 



SCIENCE 



871 



TMs is too small a number of pairs to war- 

 rant a conclusion, but so far as tbey go, the 

 results sbow an advantage for regional com.- 

 pensation. Also in every instance the differ- 

 ence is plus on subtracting the anomaly for 

 the lower station from that for the higher, and 

 there is slight indication of a relation to the 

 difference in elevation. 'The plus difference, 

 if real, indicates an apparent excess of gravity 

 at the high station as compared with the low 

 station. This may be actual and due to some 

 condition of materials beneath the surface, or 

 it may result from a compensation correction 

 relatively too large being applied to the high 

 station, or from some other feature of the 

 reduction. The evidence given by these pairs 

 of stations is slight, but points to the possi- 

 bility of further interesting investigation, 

 which might be extended along similar lines 

 to a study of differences of deflections at 

 neighboring astronomic stations. 



The conclusions from the above are: 



1. From the general mass of results there 

 is practically no evidence showing whether 

 there is nearly complete local compensation 

 or only general regional compensation within 

 the areas considered, that is, within zones up 

 to 167 kilometers (104 miles) radius. 



2. The comparison of pairs of neighboring 

 stations differing considerably in elevation 

 shows an advantage for regional compensa- 

 tion, but the number of results is too small 

 for a definite conclusion. 



3. The comparison by pairs shows in each 

 case gravity at the higher station in excess as 



compared with gravity at the lower, which if 

 real, may be due to materials beneath the sur- 

 face, or to some conditions of the reduction. 

 The number of results, five, is, however, too 

 small for safe general conclusions. 



4. If practically identical results are ob- 

 tained with regional and with local compensa- 

 tion or if limited regional compensation is 

 nearer the truth, it may be possible to lessen 

 the labor of reduction of gravity observations 

 by computing the direct topographic effect 

 and the general compensation for a larger 

 zone about the station. 



The above discussion bears on one feature 

 only of the interesting reports to which refer- 

 ence is made. 



George R. Putnam 



Washington, D. C, 

 November 13, 1912 



SPECIAL ARTICLES 



THE VALUE OF THE CILIATE, DEJINIUM, IN THE 

 STUDY OF BIOLOGY 



Didinium appears only occasionally in ordi- 

 nary cultures for the protozoa usually studied 

 in the laboratory. Owing to this fact it is 

 not widely known and consequently its excep- 

 tional possibilities as laboratory material for 

 study in courses in biology have been quite 

 generally overlooked. I have had this animal 

 under almost continuous observation during 

 the past four years and have become fairly 

 well acquainted with it. The following notes 

 are based upon these observations. 



Didinia feed largely on paramecia. They 



