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A transportable ac voltage source is described, in which sinusoidal signals are digitally-synthesized in the audio-frequency range. The rms value of the output waveform may be calculated by measuring the dc level of the individual steps used to generate the waveform. The uncertainty of this calculation is typically f10 ppm from 20 Hz -10 M z at the 7 V level.

Achieving accurate optical CT 3D dosimetry without the use of viscous refractive index (RI) matching fluids would greatly increase convenience. Software has been developed to simulate optical CT 3D dosimetry for a range of scanning configurations including parallel-beam, point and converging light sources. For each configuration the efficacy of 3 refractive media were investigated: air, water, and a fluid closely matched to Presage (RI = 1.00, 1.33 and 1.49 respectively). The results revealed that the useable radius of the dosimeter (i.e. where data was within 2% of truth) reduced to 68% for water-matching, and 31% for dry-scanning in air. Point source incident ray geometry produced slightly more favourable results, although variation between the three geometries was relatively small. The required detector size however, increased by a factor six for dry-scanning, introducing cost penalties. For applications where dose information is not required in the periphery, some dry and low-viscous matching configurations may be feasible.

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The following communication has been delayed in being brought before the Geological Society in the expectation of receiving further facts from Dr. Oldham, or that he would publish a substantive account of his own results after his examination of the earthquake-region. The latest communications received, however,by the writer from Dr. Oldham render it doubtful whether he will be able, consistently with the pressure of his official duties, to pursue the subject further; and as the facts which he has collected and well explained, of the production of enormous earth-fissures as effects of this earthquake, are of great importance, the writer has deemed it best to place them on record now, and to add to Dr. Oldham's account some remarks of his own as to the very striking secondary effects of this earthquake. Cachar, the site of the earthquake of the 10th January, 1869, is a British Province of Eastern India, about 110 miles in length from north to south, by 60 or 70 miles in width. Its boundaries on the north and south are Assam and Tipperah (an independent state), and on the east and west Munnepoor and Silhet respectively. It is hill-surrounded, except on the south and west, and consists chiefly of extensive plains, through which many large and small rivers wind and wander through ever changing courses in deep muddy beds, in which they rise from 25 to 35 feet in the rains. The chief river is the Barak, which is navigable for great distances, the Juroo and