January 27, 1910] 



NA TURE 



379 



imaginal and larval anatomy of Scolytid beetles as those 

 in the technical memoir have ever been issued before, 

 while in the systematic portion are to be found, not only 

 full structural accounts of the various species, but figures 

 showing the characteristic form of the brood and larval 

 galleries in each case. The accompanying figures, slightly 

 reduced from the original, give some idea of Dr. Hopkms s 

 excellent illustrations. 



Most Scolytid beetles— the well-known Hylurgus (or 

 Myelophilus) piniperda, for example— lay their eggs in 

 (lying or unhealthy trees or in felled trunks, the vigorous 

 How of sap and secretion of resin in healthy growing trees 

 being unfavourable for the development of the larva;. 

 .Members of the genus Dendroctonus, however, prefer, as 

 a rule, healthy trees for breeding purposes ; hence the 

 destruction wrought by the insects may become exceed- 

 ingly serious (see the photograph reproduced), and it is 

 not possible to exterminate large numbers of the beetles 

 and larvae by " trap-trunks " or " trap-logs," according to 

 the practice of German foresters with Hylurgus and 

 similar bark-beetles. As is usual in American economic 

 work, attention has been paid to the natural enemies of 

 the destructive beetlrs, and experiments have been made 



Fig. 2.— Yellow Pines killed by the Western Pine Beetle (Dcnihvc/miiis 

 drcvicornis), Yosemite National Park. 



with imported specimens of the handsome European beetle 

 Clertis formicarius, which drags bark-beetles and larvae 

 from their burrows and ruthlessly devours them. 



The genus Dendroctonus has a remarkable distribution. 

 Twenty-three species are found in North .'\merica, the 

 genus spreading northwards to Labrador and Alaska and 

 southwards through the Mexican highlands into Guate- 

 mala. In the " Old World " a single species only is 

 known — D. micaits, which inhabits Russia, Germany, 

 Denmark, and southern Scandinavia. While " the species 

 of this genus of beetles are the most destructive enemies 

 of the coniferous forest trees of North .'\merica." they 

 are hardly known in Europe except to special students of 

 the Scolytida;. The absence of Dendroctonus from northern 

 Scandinavia, from our own islands, and from western 

 Europe generally, suggests that the former geographical 

 connection between the outlying European D. niicans and 

 the numerous American members of the genus was by 

 way of Siberia and Alaska. G. H. C. 



NO. 2100, VOL. S2] 



ATMOSPHERIC ELECTRICITY IN EGYPT. 

 T N Paper No. lo of the Survey Department of Egypt 

 -'■ Mr. H. E. Hurst discusses two years' results of atmo- 

 spheric electric potential obtained from a Kelvin water- 

 dropper electrograph at Helwan from March, 1906, to 

 February, 1908. From observations made with a Kelvin 

 portable electrometer, and experiments on the disturbing 

 effect due to the presence of instrument and observer, a^ 

 factor was obtained, multiplication by which transfers curve 

 readings to potential gradient in the open (volts per metre 

 of height). The mean value found for the potential 

 gradient from the two years was 113, a value lower than 

 is usually encountered in Europe. In the second year, 

 however, owing to the more open scale employed, there 

 was at times considerable loss of trace, and an allowance 

 which Mr. Hurst makes for this would bring up the value 

 of the potential gradient for that year from 119 to 129, and 

 the mean for the two ye^rs from 113 to 118. The curves 

 were not smoothed, and were measured only at the even 

 hours, and there is rather excessive irregularity in the 

 diurnal inequality curves which are given for individual 

 months of the year. All show a prominent minimum in 

 tlie earlv morning from 4 a.m. to 6 a.m., and some a 

 srcondar'y minimum in the early afternoon, but successive 

 months differ in this respect rather widely. In the mean 

 diurnal inequality for the year there is little variation in 

 the potential from 10 a.m. to 10 p.m. ; the value at 8 p.m. 

 is the absolutely largest in both years. One very excep- 

 tional phenomenon is that the potential is decidedly highest 

 in summer. The mean potential gradient from the four 

 months June to September was 136, while that from the 

 four months November to February was only qS, no allow- 

 ance being made in either case for loss of trace. Curiously, 

 however, the mean range of the diurnal inequality was 81 

 for the four mid-winter months as compared to 50 for the 

 four months June to September. 



The mean diurnal inequality for the year was analysed 

 in a Fourier series. The amplitude of the 24-hour term 

 was nearly double that of the 12-hour term. The latter 

 term was found, as at Kew, to be almost exactly in phase 

 with the 12-hour Fourier " wave " for barometric pressure. 

 No connection could be found between potential gradient 

 and temperature, sunshine, or any other meteorological 

 element except wind direction and possibly vapour pressure. 

 Other observers have associated sudden changes of potential 

 with sunrise and sunset, but no such connection seems_ to 

 exist at Helwan. During sandstorms negative potentials 

 are sometimes encountered, but not always. 



The present publication is welcome evidence of the 

 scientific activity of the Helwan Observatory, and contains 

 several results of much interest. It is to be hoped, how- 

 ever, that a more complete analvsis will be made some 

 vears hence, when sufficient data have accumulated to give 

 fairly smooth results for individual months of the year, and 

 that the curves will then be measured at all hours of the 

 day, and not merely at the even hours. 



C. ClIREF.. 



N 



AMERICAN HYDROLOGY.^ 



EGLECTING the quantity disappearing through 

 evaporation as relatively insignificant, the rainfall 

 over any area either finds its way on or near the surface 

 into streams or percolates into the ground to form sub- 

 terranean reservoirs, which are tapped in many cases, 

 naturally by springs and artificially by wells. Each of 

 these processes has a distinct and valuable bearing upon 

 the industrial and hygienic resources of a country, and in 

 countries where there is no separate hydrological service 

 the scientific investigation of the national water supply 

 comes within the purview of the geological department, as 

 is the ca.se in the United States. The two papers which 

 form the subject of this brief notice illustrate in a very 



1 Surface Water Supply of the United Sta'es, 1907-8. Part ii.. South 

 Atlantic Coast and Eastern Gulf of Mexico. Prepared under the direction 

 of M. O. Leighton by M. R. Hall and R. H. Bolster. Water Supply Paper 

 No. 142 Pp. 2!6. „ „ 



Underground Water Resources of Connecticut. By Herbert F. rireeory, 

 wiih a Study of the Occurrenceof Water in Crystalline Rocks, by E. E.Ellis. 

 Water Supply Paper No. 232. Pp. 200. (Washington ; Government Print- 

 ing Office, 1909 ) 



