100 YEARS AGO

A Study of Recent Earthquakes. A subject attractive to the general reader... is an account of signs which have given warning of a coming earthquake. Underground sounds have been heard, springs have varied in their flow, horses, birds, dogs, and even human beings have been restless for some time before great earthquakes. In his reference to the Riviera earthquake in 1887, Mr. Davison remarks that as premonitions were noted at 130 different places within the central area, “there can be little doubt that they were caused by microseismic movements for the most part insensible to man.” In these days of psychical research we think that the author has lost an opportunity for romantic speculation... It is pointed out that the area over which earthquake sounds are heard is variable in different countries. One reason for this is that the limits of audibility vary with different races. From illustrations given it would appear that for certain sounds the Anglo-Saxon ear is more acute than that of the Neapolitan, and very much more than that of the Japanese.

From Nature 6 April 1905.

50 YEARS AGO

A new instrument called the ‘maser’ (microwave amplification by stimulated emission of radiation) has been invented by Prof. C. H. Townes, of the Physics Department, Columbia University, for which the claim is made that it enables time to be measured with an accuracy of one part in 1011. The ‘clock’ used is an ammonia molecule which radiates an electric dipole spectrum as a set of lines of about 6 mm. wavelength and, as used in the instrument, maintains its frequency to the above order of magnitude. This would be sufficient to enable it to measure variation in the rate of rotation of the earth... the instrument consists of a molecular beam of ammonia molecules which are excited in an electric field and then pass into a tuned cavity-resonator, where they induce each other to radiate by negative absorption... It seems that the method of use is to extract by means of wave-guides the radiation emitted by two ‘masers’, tuned to different but adjacent frequencies by the ammonia spectrum. When mixed, the beat frequency can then be counted electronically. The ‘maser’ is also claimed by its inventor to be very effective as an amplifier.

From Nature 9 April 1955.