36$ SCIENCE OF AGRICULTURE. Part II. 



curial column, but are not necessarily connected with fluctuations of pressure. The 

 words deserve to be particularly noticed when the mercury removes from " changeable " 

 upwards; as those on the lower part should be adverted to, when the mercury 

 falls from "changeable" downwards. In other cases, they are of no use: for, as 

 its rising in any part forebodes a tendency to fair, and its falling to foul, weather, it fol- 

 lows that, though it descend in the tube from settled to fair, it may nevertheless 

 be attended with a little rain, and when it rises from the words " much rain " to " rain " 

 it shows only an inclination to become fair, though the wet weather may still continue in 

 a less considerable degree than it was when the mercury began to rise. But if the mer- 

 cury, after having fallen to " much rain," should ascend to " changeable," it foretells fair 

 weather, though of a shorter continuance than if the mercury had risen still higher ; 

 and so, on the contrary, if the mercury stood at " fair " and descends to " change- 

 able," it announces foul weather, though not of so long continuance as if it had fallen 

 lower, 



2417. Concavity of the surface of the mercury. Persons who have occasion to travel 

 much in the winter, and who are doubtful whether it will rain or not, may easily ascer- 

 tain this point by the following observation : A few hours before he departs, let the 

 traveller notice the mercury in the upper part of the tube of the barometer ; if rain 

 is about to fall, it will be indented, or concave ; if otherwise, convex or protuberant. 



2418. 13 arorneter in spring. Towards the end of March, or more generally in the 

 beginning of April, the barometer sinks very low with bad weather ; after which it 

 seldom falls lower than 29 degrees 5 minutes till the latter end of September or October, 

 when tlie quicksilver falls again low with stormy winds, for then the winter constitution 

 of the air takes place. From October to April, the great falls of the barometer are from 

 29 degrees 5 minutes to 28 degrees 5 minutes, and sometimes lower ; whereas, during 

 the summer constitution of the air, the quicksilver seldom falls lower than 29 degrees 5 

 minutes. It therefore follows that a fall of one tenth of an inch, during the summer, 

 is as sure an indication of rain, as a fall of between two and three tenths is in the 

 winter. 



2419. The hygrometer is of various sorts, but cord, fiddle-string, and most of the sub- 

 stances commonly used, become sensibly less and less accurate, so as at length not to 

 undergo any visible alteration from the different states of the air, in regard to dryness or 

 moisture. The most common of all barometers is that formed of the beard of the wild 

 oat, v^vena fatua. 



2420. A sponge makes a good hygrometer on this account, as being less liable to be changed by use than 

 cord. To prepare the sponge, first wash it in water, and when dry wash it again in water wherein sal 

 ammoniac or salt of tartar has been dissolved ; and let it dry again. Now, if the air becomes moist, the 

 sponge will grow heavier; and if dry, it will become lighter. 



2421. Oil of vitriol is found to grow sensibly lighter or heavier in proportion to the less or greater 

 quantity of moisture it imbibes from the air. The alteration is so great, that it has been known to change 

 its weight from three drachms to nine. The other acid oils, or, as they are usually called, spirits, or oil 

 of tartar per deliquium, may be substituted for the oil of vitriol. 



2422. Steel-yard hygrometer. In order to make a hygrometer with those bodies which acquire or lose 

 weight in the air, place such a substance in a scale on the end of a steel-yard, with a counterpoise which 

 shall keep it in equilibrio in fair weather; the other end of the steel- yard, rising or falling, and ix)inting 

 to a graduated index, will show the changes. 



2423. Line and plummet. If a line be made of good well dried whipcord, and a plummet be fixed to 

 the end of it, and the whole be hung against a wainscot, and a line be drawn under it, exactly where the 

 plummet reaches, in very moderate weather it will be found to rise above such line, and to sink below it 

 when the weather is likely to become lair. 208 



2424. The hair hygrometer of Saussure, and the whaletyone hygrometer, originally ^ ^.^^ 

 invented by De Luc, are esteemed two of the best now in use. '^-'n_^7^ 



2425. The best and, indeed, only perfect hygrometer is that of professor Leslie. It con- 

 sists of a siphon tube, with a ball blown at each end {Jig. 208.), and filled with air. 

 A coloured liquid fills one leg of the siphon; the ball on the opposite limb, smoothly 

 coated with tissue paper, is the evaporating surface ; this is kept perpetually moist by 

 means of a thread passing from a jar with water as high as the instrument to the 

 covered ball. The cold produced by evaporation causes the air in that ball to contract, 

 and the coloured liquid is forced into that stem by the elasticity of the air included 

 in the naked ball. This rise is exactly proportional to the dryness of the air. {T.) 



2426. The rain-gauge, pluviometer, or hyetometer, is a machine for measuring the 

 quantity of rain that falls. 



2427. A hollow cylinder forms one of the best-constructed rain-gauges ; it 

 has within it a cork ball attached to a wooden stem (fg. 209.), which passes 

 through a small opening at the top, on which is placed a large funnel. When 

 this instrument is placed in the open air in a free place, the rain that falls within 

 the circumference of the funnel will run down into the tube and cause the cork 

 to float ; and the quantity of water in the tube may be seen by the height to 

 which the stem of the float is raised. The stem of the float is so graduated as to 

 show by its divisions the number of perpendicular inches of water which fell on 

 the surface of the earth since the last observation. After every observation the 



209 cylinder must be emptied. 



