18 Journal of the Mitchell Society [August 



bulky, is easily manipulated and, in the hands of a novice, yields bet- 

 ter results than he can usually obtain with a regular photomicro- 

 graphic camera. 



Rainfall Characteristics of North Carolina. Thorndike Saville and 

 R. J. Morton. 



A map of North Carolina has been prepared showing isohyetal 

 lines (lines of equal annual rainfall), compiled from station averages 

 to 1921. The last such map appeared in 1890. A map of the state 

 has also been prepared showing lines of equal coefficients of variation 

 from the mean annual rainfall. Curves of progressive means of an- 

 nual rainfall for selected stations in the state have been prepared to 

 investigate periodic changes in rainfall. No definite rainfall cycle 

 has been indicated at any station. At Hatteras a distinct climatic 

 change seems indicated, involving a decrease in annual rainfall. It 

 is believed that progressive means may be used to prognosticate the 

 probable trend in rainfall for a year or two in advance. The studies 

 reported upon are being continued and will be published. 



McCrudden's Volumetric Method Compared With the Shohl and Ped- 

 ley Method for Determivation of Calcium. J. 0. Halverson and 

 L. M. Nixon. 



1. McCrudden's method as modified by Halverson and Schulz 

 (Jr. Ind. & Eng. Chem. 12: 77. 1920) gives accurate results in the 

 determination of Ca in urine if the urine is oxidized with ammonium 

 persulphate before proceeding with analysis. 



2. Results obtained by this method average 4.5 per cent more ac- 

 curate and are more uniform than those obtained by Shohl and Ped- 

 ley's method. (Jr. Biol. Chem. 50: 537. 1922). This is important 

 where only small amounts of Ca are present, as in agricultural and 

 biological products. 



3. There is little difference in the time necessary for carrying 

 out the two methods. 



On the Curvature of Manifolds. J. W. Lasley, Jr. 



A surface may be regarded as a two-parameter family of points. 

 Its linear element is then given by a quadratic differential form in 

 two variables. We regard surfaces thus characterized as manifolds 



