apparatus which it is desired to maintain as nearly as possible in a fixed relation to the true vertical. Attached to a double pivot symmetrical compass, they would eliminate the turning errors of that instrument, but at the cost of undue complication ; attached to a bomb-sight they would eliminate the largest errors of bombdropping, as was about to be practically demonstrated on a large scale just at the close of the war. They also contribute an essential element to the successful application of aerial photography to accurate topographic mapping. It is difficult to state the accuracy with which they will operate, as this depends entirely upon the conditions of use, but it is safe to say that in a carefully piloted plane the best of them would certainly maintain the true vertical to I o or less.
There remains to be considered only one class of instrument, all of which are still in the stage.of a dream rather than a realization, that is, absolute position indicators which would indicate at any time, without any outside observations, the absolute position (latitude and longitude) of a plane. So far as known, experiments and suggestions have been along the following lines:
I. The use of the direction of total magnetic force.
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The gyroscopic maintenance of a fixed direction, compensated for the earth's rotation.
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The double integration of acceleration to give velocity and then distance travelled (really automatic dead reckoning).
All of these would involve, among other things, a satisfactory stabilizing device, but the additional difficulties are so great that they have so far proved insurmountable. I feel that this problem of a position indicator is a sufficiently difficult one to leave to the readers of the JOURNAL, and worthy their best efforts.
The High Frequency Spectra of Lead Isotopes. C.D. CooK-SEY and D. CoOKSEV.--Aronberg, corroborated by Merton, has found a difference of .0043 Angstrom between the wave-lengths of a certain line of ordinary lead and of lead of radioactive origin. Others have investigated the X-ray spectra of lead and have stated that the spectra of ordinary and of isotopic lead differ in wave-length by only a few ten-thousandths of an Angstrom. The authors confine their work to a single line of the L-series and conclude that the two varieties of lead give lines which differ in wave-length by less than .005 per cent.
G. F. S.