to the first was repeated. The quantity recorded in the table is the excess of the second of these observations above the mean of the first and third. The photometer was a portable one with the usual glass screen, and was procured from Professor Leslie's own maker in Edinburgh. The observations with it were made in the following manner. The instrument was exposed so as already to have attained its maximum at apparent noon. The direct rays of the sun were then suddenly cut off, and the degrees lost during half a minute were numbered and recorded. This method of using the photometer was suggested by the principle on which the observations with the actinometer are made. The rain was received in a tin vessel having its aperture horizontal and of known area. The water was then measured, and it was easy to observe the quantity of it corresponding to the thousandth of an inch in depth. Merely a thread and feather were used to shew the direction of the wind, and its velocity was only estimated by the help of a scale given in the appendix to Dr. T. Young's Lectures. The velocity recorded is the mean of the separate estimated velocities. Professor Forbes[1] has recommended the adoption of Lambert's method of registering the direction of the wind. According to this the south was denoted by 0° S. W. by 45° W. by 90° and so round the whole horizon. But to this plan it is a weighty objection, though apparently overlooked, that if the wind be registered as varying between S. W. and S. E. or between W. and E. always going round by S. the result will erroneously represent the mean direction of the wind to have been from the N.
Remarks on the Tables.
In table I, under the head thermometer, is given the mean minimum and mean maximum temperature; also the mean at 9½ a. m. and 9½ p. m. as these two latter periods divided nearly equally the intervals between the periods of least and greatest temperature, the mean temperature of the month is deduced from the observations at all the four. The time of maximum temperature seemed to be between two and three p. m.; it was sometimes hastened by the early setting in of the sea breeze.
The hygrometric state of the air was determined by Leslie's method, the mean temperature of his dew-point and corresponding tension of vapour having been deduced from the mean temperature of the air, and mean temperature of the wet-bulb thermometer. I am aware that the tension of vapour corresponding to the mean temperature of the dew-point is a little less than the mean tension, but the difference can seldom exceed what is due to two or three tenths of a degree of Fahrenheit's thermometer. The column marked dampness, gives the
- ↑ Reports of the British Association, &c. vol. i. p. 849.
