VI. HYGROMETERS. 699 



2852h. Self-registering Bain Gauge. Elliott Brothers. 



This instrument consists of an upright square metal case, with funnel or 

 receiver 10 inches square, with a set of counting wheels with dials registering 

 from yi- to 100 inches of rain fallen. The water falls into a trough with 

 a division in the centre, having a motion on an axis, and being put at an angle, 

 the rain overbalances it alternately, and is registered by the motion being 

 communicated to the set of wheels, and consequently there is no evaporation 

 to be deducted. 



Funnel, Bottle, Copper Fan, and Glass Measure for 

 Ham Gauge. F. Darton and Co. 



VI. HYGROMETERS. 



Mostra Umidaria (Hygrometer). An invention of Folli 

 da Poppi, perfected by the Accademia del Giniento ; the hygro- 

 scopical body is a band of paper. The Accademia del Cimento. 



Hygrometer, at condensation, invented by the Grand Duke 

 Ferdinand II. dei Medici. The Accademia del Cimento. 



Folli's hygrometer, made in 1664, consists of a band of paper fixed at both 

 ends, and having in the middle a corresponding weight, which in its rising or 

 falling (caused by the moisture shortening or lengthening the paper) trans- 

 mits its motion by means of a cord and pulley, to a hand which points out 

 the degrees of humidity on a quadrant. The members of the Accademia 

 del Cimento substituted parchment for paper, whilst Torricelli used oats as 

 an hygrometric substance. 



The hygrometer No. 23, that cf the Grand Duke, is founded on the prin- 

 ciple of the condensation of the aqueous vapour of the atmosphere, by means 

 of a cold substance. It consists of a truncated cone, made of a sheet of 

 tinned iron, covered on the inside with a layer of cork, and supported on 

 a tripod. Below the smaller aperture, turned downwards, there is suspended 

 a hollow glass cone, ending in a closed point likewise turned downwards, 

 and provided towards the upper part with an escape pipe. The upper cone is 

 then filled with snow or ice, which in melting runs into the glass cone, which 

 remains at the temperature ofO, whilst the excess of water runs through the 

 pipe into a separate vessel. The moisture of the air coming in contact with 

 the cold side, condenses itself and covers it with dew, which collecting by 

 degrees into drops, runs towards the point whence it falls into a graduated 

 vessel. If the time the experiment has lasted be taken into consideration, the 

 quantity of water thus collected will be found to be in proportion to the 

 humidity of the air. 



Experimenting in this manner, the members of the Accademia del Cimento, 

 found that the south winds are so charged with moisture, that in one minute 

 the hygrometer has given as much as 35, 50, and even 80 drops of water ; 

 whilst the north wind leaves the glass perfectly dry. 



2 8 53 a. Capt. Kater's Hygrometer, by Robinson. 



Royal Society. 



2853b. Jones' Hygrometer, with stem bent at an acute 

 angle. Royal Society. 



