266 SCIENCE OF GARDENING, PART II. 



riments hare taught us, that for every eighty feet of nearly perpendicular height that the 

 barometer is placed above the level of the sea, the quicksilver sinks one tenth of an inch : 

 observations alone, therefore, must determine the heights of the quicksilver, which in each 

 place denotes either fair or foul weather. 



1285. The hygrometer is of various sorts, but cord, fiddle-string, and most of the siA- 

 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. 



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. 



Oil of vitriol is found to grow sensibly lighter or heavier in proportion to the lesser 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 drams 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. 



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 equilibria in fair weather; the 

 other end of the steel-yard, rising or falling, and pointing to a graduated index, will 

 show the changes. 



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 be- 

 come fair. 



The ivhalebone hygrometer, originally invented by De Luc, is esteemed one of the best 

 now in use. 



128G. The rain-gauge, pluviometer, or hyetometer is a machine for measuring the quan- 

 tity of rain that falls. 



A hollow cylinder forms one of the best-constructed rain gauges : it has 

 within it a cork ball attached to a wooden stem (fig. 76.), which passes through 

 a small opening at the top, on which is placed a large funnel. When this in- 

 strument 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 observ- 

 ation the cylinder must be emptied. 



A coj)per funnel forms another very simple rain-gauge : the area of the opening must 

 be exactly ten square inches. Let this funnel be fixed in a bottle, and the quantity of 

 rain caught is ascertained by multiplying the weight in ounces by '173, which gives the 

 depth in inches and parts of an inch. 



In firing these gauges, care must be taken that the rain may have free access to 

 them ; hence the tops of buildings are usually the best places, though some 

 conceive that the nearer the rain-gauge is placed to the ground the more rain it will 

 collect. 



In order to compare the quantities of rain collected in pluviometers at different places, 

 the instruments should be fixed at the same heights above the ground in all such places ; 

 because, at different heights, the quantities are always different, even at the same 

 place. 



1287. Thermometer. As the weight of the atmosphere is measured by the barometer, so 

 the thermometer shows the variations in the temperature of the weather ; for every change 

 of the weather is attended with a change in the temperature of the air, which a thermo- 

 meter placed in the open air will point out, sometimes before any alteration is perceived 

 in the barometer. 



The scales of different thermometers are as follow. In Fahrenheit's the freezing point is 32 degrees, 

 and the boiling point 212 degrees. In Reaumur's the freezing point is 0, and the boiling point 80 degrees. 

 In the centigrade thermometer, which is generally used in France, and is the same as that of Celsius, 

 which is the thermometer of Sweden, the freezing point is 0, and the boiling point 100 degrees. As a rule 

 for comparing or reducing these scales, it may be stated, that 1 degree of Reaumur's scale contains 2| degrees 

 of Fahrenheit, and to convert the degrees of the one to the other, the rule is to multiply by 9, divide by 

 34, and add 32. One degree of the centigrade scale is equal to one degree and eight-tenths of Fahrenheit ; 

 and the rule here is to multiply by 9, divide by 5, and add 32. Any of these thermometers may be proved 

 by immersing it in pounded ice for the freezing point, and in boiling water for the boiling point, and if 

 the space between these points is equally divided, the thermometer is correct. 



