2 Proceedings of the Royal Irish Academy. 



situated on the sea-coast about eight miles to the south-east of Dublin, and 

 Prof. Conway's house, which is detached, lies quite close to the water's edge. 

 The situation is eminently suitable for making observations in pure air, inas- 

 much as the air in north-east, east, and south-east winds comes from over the 

 sea, and in south-west winds from over a country which is not very thickly 

 populated. West winds bring air from the direction of the town of Dalkey, 

 while north-west winds come from the direction of Dublin. This paper 

 contains the results of the Dalkey observations, which extended over a period 

 of about two months. 



Apparatus. 



The apparatus was designed so as to be more portable than that used in 

 Dublin. It consisted, as shown in fig. 1, of four equal cylindrical brass tubes 

 of radius 3T cms. and length 150 cms. mounted parallel to each other on a 

 wooden frame, with the ends A, B, C, D let into one face of a rectangular box 

 F. The section of the box was square, and the openings A, B, C, D were 

 symmetrically placed so that equal quantities of air were drawn through each 

 tube by means of a gasometer connected to the pipe E fixed at the centre of 

 the opposite face. Inside each of the tubes and insulated from it, a concentric 

 brass tube of radius 1'4 cms. was fixed as shown in fig. 2. N is a closely 

 fitting ebonite plug attached by means of a screw to the brass rod L, which 

 passed through the outer tube, being insulated from it by means of ebonite 

 collars M, and kept in position by nuts screwed on outside. By means of 

 such a support at each end, it was possible to make the tubes coaxial, and 

 keep them rigidly in that position. The inner tubes were connected to the 

 electrometer and the outer ones to a voltage sufficient to remove all the large 

 ions in the air passing through the annular space between the tubes. The 

 capacity of the system was "00055 microfarad, so that it was possible to deal 

 with much slower air-streams than with the apparatus used in Dublin. By 

 using four tubes instead of one it was possible to get a sufficiently large 

 volume of air passing through per see. without having too great a velocity, 

 and thereby necessitating the use of very high voltages to produce saturation. 



The apparatus was set up with the open ends of the tubes close to a 

 window opening towards the sea-front, so that, when the window was 

 opened, the air could be drawn in from outside and tested. The saturation- 

 current measures the total ionisation in the atmosphere, and from it, assuming 

 that each ion bears the electronic charge, the total number of ions per cc. may 

 be calculated. Such measurements give the sumiV+w of the number N of 

 large ions and the number n of small ions per cc. 



Some observations of the small ions were also made at Dalkey, so that the 



