JUNB 29, 1S83.] 



SCIENCE. 



597 



whole apparatus is covered with a wooden box, 

 removed in the ilkistration. This cover turns 

 on the hinges at c, and, when closed, rests in 

 the grooves /. The tubes r and r' are furnished 

 with two cloth-lined metal collars, which can be 

 pressed up against the box where the tubes pass 

 through it. The outer end of r is closed with 

 a plate in which there are three roiind holes side 



it forms the driving-weight. The downward 

 velocity is about 80 mm. per hour. This is 

 sufficient to allow of changes from minute to 

 minute being easily distinguished. For the 

 purpose of allowing different rates of speed, 

 it is proposed to put another rack on the back 

 of S, which, by a sliding motion, maj' be made 

 to catch on a second pinion of different size. 



by side in a horizontal line. Before this plate is 

 the diaphragm d, which can be turned on a ver- 

 tical axis, and through which there is one hole. 

 With this diaphragm the central opening in the 

 end of r may be alone left open. In front is 

 placed a kerosene lamp. From the flame of 

 this lamp a fine pencil of raj's passes through 

 the hole in d, along the tubes r and r', and is 

 reflected b}- a total reflecting-prism, p, which 

 throws it on the mirror, G, of the galvanometer, 

 which is connected in circuit with the line by 

 the wires z. From the mirror G the light is 

 reflected back through the lens I, which brings 

 the raj-s to a focus on the photographic plate. 

 This plate is put in a holder, 7c, in the slide S, 

 before the beginning of the observation . There 

 are spring clamps on S, so that, when the cover 

 is drawn from in front of the plate, the holder 

 will remain in S. In order that it may be pos- 

 sible to expose the plate after the box-cover is 

 put down, there is a slit covered with rubber 

 cloth in the box, through which the fingers maj- 

 reach the top of the plate-holder and pull out 

 the sliding front. The slide S travels on guides 

 F, and on one side is fui'nished with two roll- 

 ers, and on the other with one ; so that the 

 movement maj' be as straight as the guide against 

 which the two rollers press. In the front side of 

 F there is a horizontal slit at the height of the 

 focus of the rays. The back side of S carries 

 a rack which fits a pinion on the driving-axis 

 of the clock (7. The downward movement of 

 <S is therefore regulated bj- this clock, of which 



For lesser changes the pendulum maj^ be varied 

 in length. 



The wires leading to the galvanometer are 

 connected with a commutator. When the 

 needle is in its position of rest, a straight line 

 will be marked on the plate by an upward 

 movement of the slide. From this line the 

 deflections caused when the earth-currents pass 

 are measured. Time-signals ma}' be made by 

 turning back the diaphragm d, when marks will 

 be made on each side of the neutral line. From 

 time to time, currents of known strength may 

 be sent through the apparatus, and will pro- 

 duce spots, as 6. 



Fig. 2 shows one of the diagrams obtained. 

 The abscissa line was drawn through the por- 

 tions a, which were marked bj- the light. The 

 portions a are broken, and at these points 

 occur the dots 6, the result of the known cur- 

 rents, c, c are the time-signals. 



A NEW CONDENSING-HYGROMETER. 



Every one who has had occasion to use the com- 

 mon form of condensing-hygrometer for the deter- 

 mination of the dew-point of the air, as devised by 

 Eegnault, has found great difiiciilty in obtaining 

 sali.-factory results, especially if the air is in rapid 

 motion, and there is a great difference between the 

 dew-point and the air-temperature. 



Professor Crova of Montpellier, France, recogniz- 

 ing these defects, has devised a ne.vf form of this ap- 

 paratus which obviates many of the difficulties, and 

 goes far toward making this justly important instru- 

 ment one of precision. 



