_Method of Determining the Amount of Ozone in the Atmosphere._--At the meeting of the American Association, an instrument for determining the relative quantity of ozone in the air was presented by Professor Horsford. It consisted of a tube, containing at one end a plug of asbestus, moistened with a solution of iodide of potassium and starch. This plug within the tube, attached to an aspirator, would, as air passed over it, become blue. If much water flowed from the aspirator, and of course much air flowed over the asbestus before it became blue, the quantity of ozone indicated would be small. If but little water flowed (and this could be measured), the quantity of ozone indicated would be greater. The quantities of ozone would be inversely as the volumes of air passing through the tube before blueness is produced.--_Annual of Scientific Discovery_, p. 219. HYDROGRAPHY. 3. _On the Phenomena of the Rise and Fall of the Waters of the Northern Lakes of America._--At a meeting of the American Academy, February 1849, Mr Foster, of the United States Mineral Survey in the North-west Territory, presented the result of some observations, undertaken with a view of determining whether the waters of the northern lakes are subject to any movements corresponding to tidal action. The result of these observations had convinced him that these waters do not rise and fall at stated periods, corresponding to the ebb and flow of the tide, but are subject to extraordinary risings, which are independent of the influence of the sun and moon. These risings attracted the attention of the earliest _voyageurs_ in these regions. Charlevoix, who traversed the lakes nearly a century ago, says, in reference to Lake Ontario:--"I observed that in this lake there is a sort of reflux and flux almost instantaneous; the rocks near the banks being covered with water, and uncovered again several times in the space of a quarter of an hour, even if the surface of the lake was very calm, with scarce a breath of air. After reflecting some time on this appearance, _I imagined it was owing to springs at the bottom of the lake, and to the shock of their currents with those of the rivers which fall into them from all sides, and thus produce those intermitting motions_." The same movements were noticed by Mackenzie in 1789; by an expedition under Colonel Bradstreet in 1764; on Lake Erie in 1823, and at various later periods. In the summer of 1834, an extraordinary retrocession of the waters of Lake Superior took place at the outlet of Sault St Marie. The river at this place is nearly a mile wide, and in the distance of a mile falls 18·5 feet. The phenomena occurred about noon. The day was calm, but cloudy. The water retired suddenly, leaving the bed of the river bare, except for a distance of thirty rods, and remained so for nearly an hour. Persons went out and caught fish in pools formed in the depressions of the rocks.