May 24, 1895.] 



SCIENCE. 



571 



Hann's dynamical theory, instead of Ferri'ls 

 convectional tlieory of extra-tropical cy- 

 clones, being adopted. All those regions 

 whose precipitation is in large part de- 

 pendent on extra-tropical cyclonic storms 

 would under these conditions have an in- 

 creased annual rainfall ; and the lakes of 

 interior basins in temperate latitudes would 

 consequently increase in volume. The 

 winter rains of subtropical belts, such as the 

 northern Sahara, would extend further to- 

 wards the equator, for the equatorward mi- 

 gration of the tropical belt of high pressure 

 in winter is essentiallj- a result of the in- 

 creased vigor of the circumpolar circulation 

 at such times; thus the formerly greater 

 rainfall indicated by the desert wadies might 

 be explained. The coincidence of greater 

 precipitation during the same epochs of time 

 over the glaciated, the lacustrine and the 

 desert areas is, however, not yet independ- 

 ently proved. W. M. Davis. 

 Harvard University. 



GRA VITY MEASUREMENTS* 

 Relative measurements of the force of 

 gravity were made in 1894 by the U. S. 

 Coast and Geodetic Survej' at twenty-six 

 stations, mostly located along the thirty- 

 ninth parallel from the Atlantic coast to 

 Utah. Points were included on the Atlan- 

 tic coast, Appalachian mountains, central 

 plains, Rocky mountains (incluiling the 

 summit of Pike's Peak, 14,085 feet in alti- 

 tude), western plateaus, and the eroded 

 valleys of the Green and Grand rivers. 



* ' Results of a Transcontinental Series ot Gravity 

 Measurements, ' \^y O. R. Putiuan, read February '2, 

 1895, Philosophhical Society of Washinffton, BuUitin 

 Vol. xiii. ; preliminary results were presente<l l)ef(ire 

 the National Academy of Sciences by Dr. Menden- 

 hall, November, 1H94. Mr. G. K. Gilbert, of the U. 

 8. Geological Survey coiiperated in this work by mak- 

 ing a geological examination of the stations. His 

 conclusions and a discussion of the results in connec- 

 tion with the theory of isostnsy are published in the 

 same Bulletin. 



The half second pendulum apparatus de- 

 signed by Dr. T. C. Mendenhall was used, 

 with methods not before employed with 

 short pendulums. They were swung at a 

 low air pressure (60 mm.), each swing last- 

 ing eight hoiu-s, and the successive swings 

 covering the entire interval between the first 

 and last time observations, usually forty- 

 eight hours. The two chronometers used 

 were rated by star observations made with 

 a portable transit in the meridian. The 

 flexure of the support was measured and 

 correction applied. The results indicate 

 the entire elimination of errors due to diur- 

 nal irregularities of rate, and show that 

 there was practicall}- no wear of the agate 

 knife-edge. Determinations made at the 

 base station (Washington) several times 

 during the year show a range of only 

 .000,004 second in the mean period of the 

 three pendulums, indicating a high perma- 

 nency of period, and throwing some light 

 on the invariability of gravity. The aver- 

 age time required per station was .slightly 

 over five days. 



Values of gravitj' for Washington de- 

 rived relativeh' from absolute determina- 

 tions made in various parts of theworld show 

 a considerable discordance, the range being 

 from 980.047 to 980.285 djTies. The re- 

 sults of the past season are based on a pro- 

 visional value adopted for Washington. As 

 the}' were carried out with the Siime instru- 

 ments and uniform methods, it is probable 

 that their relative accuracy is much higher 

 than that of many of the absolute measures. 



The results are discussed principally in 

 connection with tiie question of reduction 

 to sea level, the distriliution of the stations 

 with resjiect to an unusual variety of conti- 

 nental conditions rendering the series valu- 

 able in this connection. This is an impor- 

 tant question in the application of pendu- 

 lum observations to the geodetic problem 

 of the earth's figure, and involves the vari- 

 ous theories as to the condition of the 



