GARDEN TOOLS. 



65 



extreme power of the sun by day, but there are others to indicate 

 the highest and lowest temperature of the air. .These are placed 

 under protection of a little shed (fig. 

 78), so arranged that neither sun nor 

 rain can affect the instruments. 



My scientific instruments were made 

 by Messrs. Thornthwaite of Newgate 

 Street. 



Visitors are requested never to 

 touch these instruments ; but, as 

 I well know the tendency of all 

 Englishmen to see with their fingers, two or three common thermo- 

 meters are placed on the stand to draw away their attention from 

 the real instruments of observation. Some years ago I designed a 

 thermometer to use with an electric current to determine at a distance 

 any deviation of limit of temperature in houses ; thus a gardener may 

 have, in his bed-room, an instrument to show the temperature of every 

 plant-house. This was perfectly successful ; but it has not been 

 employed at my garden. 



The drying power of the air is estimated by the difference of 

 temperature shown between a dry and a wet bulb thermometer ; as the 

 greater the difference, the greater the dryness of the atmosphere. The 

 bulb is kept wet by a covering of silk, the end of which dips in dis- 

 tilled water. This contrivance is called Mason's hygrometer, and i, 

 shown in the centre of our meteorological observatory (fig. 78). 



It has always appeared to me advisable to determine the amount 

 of evaporation which takes place in a definite time, as that has an 

 important bearing upon vegetation. For this purpose I employ 

 a tube graduated to one-tenth of an inch (fig. 79), which shows 

 how much water evaporates per week. These evaporations are 

 calculated to lead to much valuable information, and are nearly 

 as important as thermometers. To determine the amount of 

 rain which falls week by week, we use a rain-gauge, the result of f.g. 79. 

 the observations of which is given in the calendar of my garden. 



F 



