METEOROLOGY IN ITS RELATION TO HORTICULTURE. 107 



The thermometers for use below the surface are specially made 

 instruments which can be lowered to the required depths in tubes sunk 

 in the ground ; and here, again, a perfectly open situation is indispensable 

 so that the sun's rays may have their full effect in warming the soil by 

 day, and terrestrial radiation may be unimpeded by night, and, indeed, 

 that the conditions may assimilate as closely as possible to those under 

 which plants live in the open air. 



The variations in the temperature of the air and of the soil are of 

 course primarily due to the radiation of heat from the sun, modified by 

 the effects of rain and wind ; but it is not alone the sun's thermal effect 

 which is of importance to the student of the conditions of plant life. The 

 molecular changes which occur in the processes of growth are powerfully 

 influenced by the sun's light as well as heat. And, indeed, light is 

 essential to the perfecting of all vegetable life. It is for this reason 

 important to know as much as possible respecting the duration and intensity 

 of the solar radiation, which comprises both light and heat. 



In the Chiswick observations to which I have referred the bulb of a 

 maximum thermometer was covered with black wool and exposed to the 

 sun's rays with the view of measuring their direct heating effect, and the 

 results obtained have been used by De Candolle and others in their 

 investigations. 



The corresponding observation is made to-day by inclosing a maximum 

 thermometer the bulb of which is coated with dull lampblack in a glass 

 jacket which has at one end a large bulb. The jacket is exhausted of air 

 and hermetically sealed, and the instrument is placed on a post, four feet 

 above the ground, in a horizontal position, so that the large vacuum bulb 

 is fully exposed to the sun. Possibly we do not get from this instrument 

 a perfect measure of the sun's total heating effect, but its indications are 

 probably sufficiently exact for the horticulturist ; and at any rate it has a 

 great advantage in the matter of simplicity over the more complex and 

 expensive apparatus occasionally employed as pyrheliometers. 



But an instrument which has now come into very general use, and 

 which yields data of great value to the botanical physicist as well as to 

 the sanitarian and to the meteorologist, is the sunshine-recorder. In its 

 most usual form this consists of a glass ball, or lens, which focusses the 

 sun's rays upon a strip of card placed in a frame. 



These focussed rays discolour the card or burn it completely through, 

 according to the intensity with which the sun may be shining ; and since 

 the trace passes along the card as the sun travels across tlie sky we get at 

 once a record of the duration as well as, to a certain extent, of the intensity 

 of the sunshine. 



In some forms of the instrument photography is employed for the 

 purpose of registration, but on the whole the photographic recorder is less 

 satisfactory for general use than the burning recorder which I have 

 described, and the Meteorological Office deals only with records from this, 

 form of instrument, which is known as the " Campbell- Stokes recorder," 

 from the two men to whom the instrument in its present form is due. 



So much then for the observations themselves ; now let me turn for 

 a few minutes to their application to horticulture and agriculture. 



At the outset some preliminary questions present themselves for 

 consideration. I said just now that the temperature of the air is observed 



