METEOROLOGY. 



23 



book to the nearest degree. For example, if 

 nearer 56 degrees than 57 degrees, it should be 

 entered as 56 degrees. To set the instrument, 

 remove it from the screen and hold it bulb 

 downwards, at the same time shaking it gently 



Fig. 



-Maximum Thermometer. 



until there is a continuous thread of mercury 

 throughout its length. In this form of maxi- 

 mum the tube is contracted near the bulb in 

 such a way that when the mercury expands it 

 can flow past the narrowed passage, but cannot 

 return into the bulb until shaken down by the 

 observer when setting it. The minimum ther- 

 mometer (fig. 5) is a spirit thermometer. In 



Fig. 5.— Minimum Thermometer. 



the thread of spirit will be found a little metal 

 index, which will allow the spirit to flow past 

 it without moving when the temperature is 

 rising; but with a falling temperature the spirit 

 carries down the index with it as soon as the 

 end of the spirit reaches the upper end of the 

 index. In taking a reading, enter in the obser- 

 vation book the temperature indicated by the 

 end of the index farthest from the bulb to the 

 nearest degree. Then, in order to set the in- 

 strument, remove it from the screen and tilt it, 

 bulb upwards, so as to allow the index to run 

 down to the end of the spirit. When properly 

 set both thermometers should read alike, at all 

 events to within about a degree, and it is 

 always advisable before leaving them that the 

 observer should see that they do so. The tem- 

 perature of the air may at any time be ascer- 

 tained by reading the end of the spirit in this 

 thermometer. 



Spirit thermometers are unfortunately liable 

 to a serious defect. Some of the spirit occasion- 

 ally evaporates and becomes condensed in the 

 upper part of the tube. So that until the 

 evaporated spirit is returned to the main thread 

 the thermometer will continue to read too low. 

 It is owing to inattention to this source of error, 



or through not being aware of it, that the 

 temperature records of gardeners in times of 

 extreme cold are so often unreliable. As much 

 as 10 degrees of spirit have been known to 

 become lodged in the upper part of a thermo- 

 meter tube. An extreme case like this 

 only shows what erroneous readings may 

 be quoted, for, as will readily be under- 

 stood, 2 degrees of frost at once becomes 

 12 degrees of frost, 12 degrees of frost 

 22 degrees, and so on. And until set 

 right, each reading would continue to be 

 as much in error. If after both thermo- 

 meters have been set, the minimum thermometer 

 is found to differ from the maximum more than 

 about a degree, the end of the tube of the 

 former must be examined, and should any spirit 

 be detected there the instrument should be set 

 right. This can be done by holding the ther- 

 mometer securely in one hand by the end far- 

 thest from the bulb, and swinging it sharply 

 downwards at arm's-length, taking care of 

 course in doing so that it swings clear of 

 surrounding objects. This plan, with a 

 little patience, will compel the detached 

 portion of spirit to rejoin the main thread. 

 The thermometer should afterwards be 

 placed, bulb downwards, in a cool place 

 for an hour or so to complete the cure. 

 Spirit thermometers exposed to the direct rays 

 of the sun are very much more liable to get out 

 of order in this way than those sheltered at all 

 times from it. Hence the great advantage of 

 placing them on a screen such as has been pre- 

 viously recommended. 



The reading of the maximum thermometer 

 should be entered to the previous day, and that 

 of the minimum to the day on which the obser- 

 vation is made. Whenever the divisions on the 

 glass tube of either thermometer become in- 

 distinct, they can easily be renewed by rubbing 

 the point of a lead pencil along the tube when 

 dry. There are during the year certain marked 

 periods of unseasonably cold weather, which 

 occur with more or less regularity. Of these 

 the most noteworthy is a cold period between 

 the 9th and 14th of May, and another between 

 November 6th and 12th. 



Earth Thermometer. — A very simple and easy 

 method of ascertaining the temperature of the 

 soil is by means of one of Symons's earth ther- 

 mometers. 1 It consists of a stout iron pipe 

 drawn out at the bottom to a point, and driven 



* A Symons's earth thermometer for a depth of one foot can be 

 obtained of Mr. L. P. Casella, 147 Holborn Bars, London, E.C. 

 Price 21s. 



