48 



Abstract of Bight Months 9 Meteorological 



[Jun- 



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 Edin- 

 burgh. The observations with it were made in the following manner. 

 The instrument was exposed so as already to have attained its maxi- 

 mum at apparent noon. The direct rays of the sun were then suddenly 

 cut off, and the degrees lost during half & 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 ob- 

 serve 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 re- 

 corded is the mean of the separate estimated velocities. Professor 

 Forbes* has recommended the adoption of Lambert's method of regis- 

 tering the direction of the wind. According to this the south was de- 

 noted 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 erroneous- 

 ly 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 9§ a. m. and 9^ 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 ain 

 and mean temperature of the wet-bulb thermometer. I am aware that 

 the tension of vapour corresponding to the mean temperature of the 

 dew 7 -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 Assoeialion. &c. vol. i. p. 249, 



