Encyclopaedia of Gardening 303 



converted into each other as follows: (a) Centigrade into Fahren- 

 heit: (i) if the temperature be above freezing (o C.) multiply by 9^. 

 divide by 5, and add 32; (2) if below o C., bia-t above 1 8 C., 

 multiply by 9, divide by 5, and subtract the result from 32; (3) if 

 below 18 C., multiply by 9, divide by 5, and subtract 32 from the- 

 result, (b) Fahrenheit into Centigrade: (i) if above 32 F., sub- 

 tract 32, multiply by 5, and divide by 9; (2) if below 32 F., but 

 above o F., add the temperature to 32, multiply by 5, and divide- 

 by 9. (c) Reaumur's scale is converted in the same way, 4 being 

 used instead of 5. Thermometers should be fixed in a central' 

 position in a house, and shaded from the sun. A " plunging "" 

 thermometer, with a long perforated tube, is made for testing the- 

 heat of beds. 



Thinning. A practice which becomes the more necessary the- 

 thicker the plants come up. Crowded plants are rarely good (see 

 Annuals, Vegetables, Fruit, etc.). Thin sowing saves much labour 

 in thinning. Thinning is best done when the soil is moist. 



Thistle. See Carduus and Cnicus. 



Thistle, Globe. See Echinops. 



Thistle, Melon. See Cactus Melocactus. 



Thomas's Phosphate Powder. See Manures Basic slag. 



Thorn. See Crataegus. 



Thorn Apple, Datura Stramonium. 



Thrift, Armeria maritima. 



Thrips. A small, lively insect (Heliothrips Adonidum) whichi 

 infests both the foliage and flowers of many plants, indoors and out. 

 Vigorous syringing and vaporising with cones at fortnightly intervals- 

 from April to October under glass will keep it in subjection. Out 

 of doors syringing with a paraffin emulsion is good (see Paraffin). 

 Peas sometimes succumb to thrips when languishing in poor or dry- 

 soil. With liquid manure and mulching they make a better fight.. 



Thuja (Thujopsis). See Thuya. 



Thunbergia (thunber-gia, after C. P. Thunberg. Ord. Acantha- 

 ceae). Beautiful evergreen warm-house climbers. The most 

 popular is alata, which bears yellow flowers in summer (see the- 

 Botanical Magazine, t. 2591); alba, white; and aurantiaca, orange,, 

 are varieties of it. Although they are perennials they are best 

 treated as annuals; coccinea, scarlet; erecta (syn. Meyenia erecta. 

 Bot. Mag., t. 5013), orange and blue; fragrans, white, sweet; andi 

 grandiflora, blue (Bot. Mag., t. 2366), are other good species. They 

 may be grown against low pillars. Alata is good for hanging 

 baskets, and may be grown in a cool house. Propagation is by 

 seeds sown in a warm house or frame in spring, or by cuttings in a 

 warm case in summer. Loam, with a third of peat, and a little* 

 decayed manure or leaf mould, suits. 



Thunia. Now classed with Phaius, which see. 



Thuya, Arbor Vitae (thu-ya, from thyon, sacrifice; the resin was- 

 used as incense in sacrifice. Ord. Conifer ae). Hardy evergreen^ 



