THE portable dust-counting apparatus, with which the observations given in the paper were taken, was shown to the meeting. The apparatus, which was described in a previous communication to the Society, is small and light. It is carried in a small sling-case measuring 8 x 5 x 3 inches. The stand on which it is supported when in use packs up, and forms, when capped with india-rubber ends, a handy walking stick, 1¼ inch in diameter and 3 feet long. No alterations have been made in the original design, and the silver mirrors which at first gave trouble and required frequent polishings, have been used every day for two or three weeks without requiring to be polished, when working in fairly pure country air.