APPENDIX. 593 



Rain is most plentiful, and thunder most frequent, when the quicksilver 

 fluctuates a little below the average. If it descend rapidly and considerably 

 below this point, storms of wind are indicated, while, in proportion as it rises 

 above the same point the probability of fair weather increases. Settled weather 

 must not be expected while the column of mercury is below its mean height 

 for the month. Sudden and considerable changes in either direction are com- 

 monly followed by fair or foul weather equally transient; while a steady rise 

 from day to day of about a tenth of an inch in twenty-four hours, or a prolonged 

 fall in the same proportion (either of them passing the mean point), may gene- 

 rally he trusted as prognostics of continued fair weather or rain. 



The Hygrometer, by which the state of the atmosphere, with regard to 

 moisture, is ascertained, is a contrivance denoting the degree of temperature at 

 which moisture begins to be deposited upon a cold body ; as we see in summer 

 in the familiar instances of a bottle of wine brought from a cellar, or a decanter 

 of water fresh filled from a well. This degree is called the dew-point ; and 

 from it the degree of dryness may he accurately calculated. Tables of the mean 

 averages of temperature for a large circle round London have heen calculated, 

 and are referred to in the Almanac. 



The Thermometer in general use throughout this country is that known as 

 Fahrenheit's; but the one commonly employed on the Continent is that formed 

 by Reaumur, and constructed upon a different scale. Though the degrees of 

 heat in the atmosphere are not universally attended to, yet in many farming 

 operations the precise state of temperature should be known, and no well- 

 regulated farm-house should be without the means of ascertaining it. The 

 temperatures of the boiling and freezing points are, in the above thermometers, 

 thus stated : — 



Boiling . . Fahrenheit, 212° ; Reaumur 80°. 

 Freezing . . Ditto 32°; Ditto 0°. 



Each degree of Fahrenheit's is therefore equal to | of that of Reaumur. 



It is well known to the agriculturist and gardener that, without the direct 

 influence of the sun, whatever may be the temperature of the air, the fruits of 

 the earth seldom come to perfection. The different duration of the day, and the 

 different height of the sun, are the causes of those variations of natural heat 

 which diversify the seasons of the year ; it is therefore of importance to know the 

 force of this important agent, and the modifications to which it is exposed. 



The best thermometers for this purpose are those upon Rutherford's con- 

 struction : one of which, formed of mercury, pushes a small pin of iron wire 

 before it, which it leaves at the highest point ; and the other, of spirit, draws 

 after it a small index of glass, which remains at the lowest. In addition to the 

 mean temperature, and the highest and lowest points, or range of the thermo- 

 meter in the shade, the indications are given of a register thermometer covered 

 with black wool, and placed in the sun, and of another placed at night upon a 

 plat of short grass, and exposed to the full aspect of the heavens. The black 

 thermometer registers this efl'ect, and shows both the mean and extreme differ- 

 ence between such a thermometer and another placed in the shade. 



Of not less consequence is it to know the cold produced at night from the 

 radiation of heat from the surface of the earth, in situations where its passage is 

 uninterrupted to the clear sky. This in calm unclouded weather always re- 

 duces the temperature of filamentous substances, such as grass, considerably 

 below that of the air. It is this depression of temperature which is the cause of 

 dew. The surfaces of vegetables become cooled by this radiation of heat into 

 space, below the dew-point, and the vapour of the atmosphere is deposited upon 

 them. Dew is never formed upon grass on cloudy nights, or in sheltered 

 situations. 



It will be found that vegetation is liable to be affected at night from the in- 

 fluence of radiation, by a temperature below the freezing point of water, ten 

 months in the year: and that even in the two months July and August, the 

 only exceptions, the radiant thermometer sometimes falls to 35°. 



The formation of dew is one of the circumstances which modify and check the 

 refrigerating effect of radiation ; for as the vapour is condensed, it gives out the 



VOL. II. 2 Q 



