532 A UNIVERSAL METEOROGRAPH, 



During this revolution the galvanic current is employed only in trans- 

 mitting the indications of the instruments in the way before described; 

 but, as soon as the revolution is finished, the minute hand is caught again 

 and the current is made to set in motion a train of clockwork to raise 

 the weight or wind up the spring, until the hour is ended and the minute 

 hand begins another revolution. 



In some preliminary trials with the apparatus I have encountered a 

 difficulty which has obliged me to make a slight addition to it. The in- 

 dex of the auoroid barometer can overcome but a feeble resistance, even 

 a slight touch being sufficient to move it from its position, and the same 

 is the case with the hair hygrometer; the metallic thermometer, however, 

 is much less sensitive in this respect, particularly that of Dr. Krecke. 

 The indexes of these instruments, as has been said before, should not 

 touch the surface of the divided arc, but should move freely a slight 

 distance above it; since however on the other hand, the point attached 

 to I he minute hand should not be too fine if we wish to maintain electrical 

 connection with the metallic rim for several months without cleaning the 

 surfaces, it would be apt while passing over the tips of the barometer and 

 hygrometer indexes to carry them with it and thus give a false record of 

 the indications of these instruments. To avoid this source of trouble the 

 same mechanism which produces the escape of the minute hand presses 

 down a finely grooved ring, s s s, Figs. IV and IV &, so as to hold the in- 

 dexes against the divided circle during the short period of the revolution 

 of the hand that the gold point may run over their ebonite tips without 

 risk of displacing them. While the minute hand is at rest the ring will 

 be raised and the indexes again left free to move. The stoppage of the 

 motion of the indexes for a minute or two would be of no consequence, 

 since in so short a time the variations in the instruments would be 

 extremely slight. In the case of the indexes which mark the direction 

 of the wind and the amount of rainfall such an arrangement would be 

 unnecessary, as the motive force is very great and the ebonite points, as 

 said before, may press on the plate. 



The problem of registering the indications of meteorological instru- 

 ments placed at a considerable distance from the observer or at a great 

 height above him is then completely solved. As regards, however, the 

 hygrometric condition of the air I believe that while August's, Daniel's. 

 and the hair or catgut hygrometers furnish close approximations to the 

 truth, a determination of the weight of water contained in a known 

 quantity of air is the only way to obtain exact results. My hygrometric 

 areometer* will make this determination in a very simple way and also 

 permit of automat ic registry. There is nothing to prevent our obtaining 

 tile air from a distant point by means of an aspirator and a rubber tube 

 and passing it through the areometer at the principal station. As the 

 detached observatory is connected with this station by a telegraph wire 

 the latter may be placed inside the rubber tube, which will then serve as 



"Archives Neerlandaises T vi, p. 419. Poggendorff'a .inn. Bd. xcm, p. 34:3. 



