164 Scientific Proceedings, Royal Dublin Society. 



rheotome is started, and continues making and breaking circuit on 

 wire II., moving the carriage and pricker at the same time longi- 

 tudinally in front of the drum. When, after a certain number of 

 contacts, the current returns by wire iii., a sinall electro-magnet 

 comes into action, which is so arranged as to throw the rheotome 

 out of gear, and simultaneously press the pricker forward on the 

 paper carrying the drum. The reading is now recorded on the 

 drum. The next movement of the drum to the succeeding contact 

 allows of the adjustment of the distant instrument, and also of the 

 carriage carrying the pricker. 



In conclusion, the following suggestions as to the forms which 

 might be given to some of the instruments may not be out of 

 place. 



In the case of the barometer, the siphon form of the instru- 

 ment- would be used, probably with wide vacuum chamber and 

 narrow air surface. The thermometer, wet bulb thermometer, 

 solar radiation thermometer, and terrestrial radiation thermometer, 

 are suitable, as already described, with such modifications as the 

 free and uninfluenced readings of the instruments in their several 

 positions demand. For the rain-gauge I would suggest the fol- 

 lowing arrangement : — 



The rain, collected in a suitable vessel, is conducted into a 

 vertical cylinder, which forms the longer branch of a U tube. 

 This tube contains mercury, the level of which in the shorter limb 

 is read in ascertaining the rainfall. The height to which the 

 mercury would be raised for a given fall of rain on a given 

 receiving surface is a question of the relative diameters of the two 

 branches. The receiving surface being eight inches in diameter, 

 it is easy to adopt such a diameter for the containing vessel that 

 the pressure due to one inch of rainfall, say, since last reading, 

 shall be represented by some three or four inches rise of the 

 mercury. For the purpose of emptying the water- vessel after each 

 reading, an electro-magnet is arranged to open a valve at the base 

 of the vessel. The current to this electro-magnet is interrupted 

 automatically when the water has fallen to a certain zero level. 

 This is effected by passing it through the mercury column in the 

 short limb of the tube, the fall of the mercury breaking the circuit 

 at the desired point. 



