TBANSACTIONS OF SECTION B. 719 



100 c.c. This number represents only the average of the brightest days, and larger 

 results have been obtained in summer. My average for the nineteen brightest 

 days in January of the present year is equal to 9-34 mgms. of iodine per 100 c.c. 

 per hour. Owing to the situation of the village with regard to the surrounding 

 mountains, the total amount of light per day is small compared with places which 

 are less shut in ; and, as will be seen from the accompanying table, the values 

 given are for the hours during which there was a bright sunlight. The actual 

 amount of possible sunlight on the days mentioned will be found in the meteoro- 

 logical records already referred to. It is interesting to note that in Manchester in 

 January 1892, with a day of 8-3 hours' light, or nearly half as long again as at 

 St. Moritz, the total liglit per week-day averaged only 4-5 mgms. iodine, or about 

 that obtained during half an hour's exposure at St. Moritz. Even on comparing 

 the Sunday values for the Manchester district, I find that the daily average is only 

 8-3 mgms., or less than the hour's average at St. Moritz. I believe that the com- 

 paratively large amount of sunlight per day experienced in the High Alps contri- 

 butes largely in determining the hygienic value of a sojourn in the«e mountain 

 health resorts. 



The maximum hour value was 13-5 mgms. per 100 c.c. on January 1, and the 

 lowest on January 24 of 5-7, and even this minimum was about 20 per cent, above 

 the average daily value in Manchester. 



6. On a Modified Form of Bunsen and Roscoe's Pendulum AcUnometer} 

 By Dr. Arthtje Richaedson and J. Quick. 



In Bunsen and Koscoe's pendulum actinometer the oscillations of a pendulum 

 cause a sliding shutter to pass backward and forward before sensitised paper, which 

 is thus exposed for a known time and again shaded from the light. 



In the present form an arrangement has been devised whereby the backward 

 and forward motion of the shutter is brought about by a movement in one direc- 

 tion only. 



This is done in the following manner : the shutter, which is made of a 

 fle.'cible material and in the form of an endless band, passes over the wooden 

 rollers, the adjacent surfaces being brought close together by means of two addi- 

 tional smaller rollers. Two slits of equal length are cut in the band, so that 

 when the latter rotates an aperture is uncovered when the slits overlap one 

 another, and which again close when the band has travelled round a certain 

 distance. 



Beneath this aperture the sensitised paper is placed, which is thus exposed for 

 definite times depending upon the length of the slits and the velocity of the band. 



In order to bring about the movement of the shutter one of the rollers is con- 

 nected with an eight-day clock, the escapement of which has been removed, the 

 alterations in the speed, usually occurring when a clock is running down under 

 such circumstances, being compensated by a fusee adjustment. 



Two advantages are claimed for this modification : — 



(1) It is portable, and measurements can be made when it is placed in any 

 position. 



(2) The time during which any portion of the slit is open (over the sensitised 

 paper) is directly proportional to that occupied in opening the entire slit ; since the 

 rate at which the shutter moves is constant, whereas in the pendulum apparatus 

 a series of calculations must be made to determine the length of time during 

 which the slit is open for each mm. of its entire length. 



7. On the Expansion of Chlorine Oas and Bromine Vapour under the 

 Influence of Light, By Dr. Arthur Richardson. 



It was first obser\'ed by Budde that when chlorine is exposed to the infiu- 

 ence of sunhght, an expansion of the gas occurs which is independent of the 

 ' Published in the Phil. Mag., xxxvi. (1893), pp. 459-163. 



