128 REPORT — 1844. 



Hygrometers. 



The wet-bulb thermometer of Masons hygrometer is mercurial : its scale 

 is divided to 0"1. The difference between this wet thermometer and the dry, 

 as set down in our observations, is derived from a comparison of it with our 

 standard thermometer. This hj'grometer has been placed outside of the 

 same window, near tlie standard thermometer, about an hour before every 

 observation. 



Tlie thermometer inclosed in one of tiie bulbs of the Daniell hygrometer 

 is also mercurial, and its scale divided into 0*1. The difference between the 

 dew-point and tiie dry tliermonieter, as set down in our tables, is also derived 

 from a comparison of this thermometer with our standard thermometer. We 

 found that the exterior tiiermometer varied from the standard sometimes 1^°. 

 This excellent iiygrometer is used at the same open window. 



The Saussure hygrometer is of the six-haired kind, made by Richer of 

 Paris. A system of levers is employed, by means of which the effect upon the 

 index is the mean result of all the expansions and contractions of tiie hairs. 

 It has the advantage of great strength, at least, l)ut is slow and is much less 

 to be depended upon than the Daniell. Before the observation it is exposed 

 for about an hour outside the same north window of the room B (fig. 1). 



Rain and Vapour Gauge. 

 This is, I believe, a new instrument. It indicates a mean result arising 

 from the quantity of water which may have fallen between any two given 

 periods, minus tlie quantity of vapour wliicli has risen in the same time (and, 

 of course, vice versa) on and from a circular plane of one foot diameter. 



A A (fig. 16), Plate XXXI. is a cylindrical vessel of zinc whose internal 

 diameter is one foot. 



B is anotlier cylindrical vessel attached to A, and communicating by a 

 little pipe 6 with it. C (vide dotted lines) is a glass vessel standing in B, and 

 having a small perforation near its foot. D D is a circular plate of brass 

 firmly screwed to a cap C, and d} d^ is a copper plate attached to the cap of 

 C also. E and F are cocks fixed upon D at a distance of about three-quarters 

 of an inch from each other. G is a pulley upon an arbour, which runs in 

 centres opposite to each other in the supports E and F ; the centres are 

 jewelled, and the carefully turned pivots of the arbour are of platinum. H is 

 an index carried by G, and 1 1 1 is the scale secured upon F. K is a silken 

 thread passing round a groove in G, descending through a hole in D, and 

 suspending a light copper covered dish L. JM is another silken thread pass- 

 ing in the contrary direction round another groove in G, and suspending a 

 weight N, which is somewhat lighter. Lastly, P is a glass shade placed upon 

 DD. 



This arrangement embraces a manifest application of the principle of the 

 wheel barometer. If a quantity of water is poured into A, exactly sufficient 

 to bring the index H to a given point, and if afterwards any addition to that 

 quantity of water should be made by rain, the index will point out the in- 

 crease upon the multiplying scale I; or if any diminution of that quantity 

 should be occasioned by evaporation, the loss will also be pointed out by 

 the motion of the index in the contrary direction. 



We have always therefore brought the imlex to zero by addition to or 

 subtraction from the water in A at sun-sct, and have observed at that hour 

 the mean results of deposition and evaporation for the preceding twenty-four 

 hours. A little reservoir is placed near it with a pipe and cock for supply- 

 ing water conveniently. This instrument is fixed upon a stand at about two 

 feet above the leaden roof (fig. 1), but M'ould be much more properly situated 



