663 



" All that part of the paper which is made to pass over the slit in 

 the screen, by the motion of the clock, becomes now therefore suc- 

 cessively exposed to a strong light, and is consequently brought into 

 a state which fits it to receive a dark colour on being again washed 

 with the usual solution, excepting those small portions upon which 

 dark images of the lower parts of the pendulums of the electrometer 

 are projected through the slit. These small portions of course re- 

 tain the light colour of the paper; and from the long curved lines 

 or bands, whose distances form each other, at any given part of the 

 photograph, i. e. at any given time indicate the electric tension of 

 the atmosphere at that time. 



" By certain additions to the instrument above described, the kind 

 as well as the tension of electrical charge is capable of being re- 

 gistered ; and by the employment also of a horizontal thermometer, 

 &c., it is adapted to the purposes of a Thermograph, as well as 

 Photo-harometrograph and Magnetographr 



January 28, 1847. 

 LEONARD HORNER, Esq., Vice-President, in the Chair. 



" On the Lunar Atmospheric Tide at St. Helena." By Lieut.- 

 Colonel Edward Sabine, R.A,, For. Sec. R.S. 



The results of the observations made by Captain Lefroy, of the 

 Royal Artillery, Director of the Magnetical and Meteorological Ob- 

 servatory at St. Helena, are here given ; from which it appears, on 

 the examination of the barometrical changes during seventeen months, 

 that a maximum of pressure corresponds to the moon's passage over 

 both the inferior and superior meridians, being slightly greater in 

 the latter case, and that a minimum corresponds nearly to the rising 

 and setting, or to six hours before and after the former periods. The 

 average atmospheric pressures are 28*2714< inches in the first case, 

 and 28-2675 in the last; the difference being 0'0039 inch. The 

 height of the cistern of the barometer above the sea is 1 764? feet ; 

 and the latitude of the Observatory 15° 57' S. These results were 

 still further confirmed by those of a series of observations during 

 two years. These observations also establish the conclusion that 

 the moon exerts a greater influence on the amount of atmospheric 

 pressure at the periods of her perigee than at those of her apogee. 



February 11, 1847. 

 The MARQUIS OF NORTHAMPTON, President, in the Chair. 

 The following paper was read : — 



" On the Amount of the Radiation of Heat, at night, from the 



