RESOURCES OF AMERICA, SILK, ETC. 423 



to their silk-worms, wholly by their own sense of feeling a very 

 uncertain mode. With them the use of the thermometer for meas- 

 uring heat and cold, is as little known as is the watch for measur- 

 ing time. 



THERMOSTAT. The thermostat serves to control and to modify the 

 temperature. Letters patent were granted to Dr. Andrew Ure, of 

 England, for an instrument of this kind, in 1831. It is formed by 

 firmly uniting, face to face, two long, thin slips of metal, of different 

 expansibilities ; one formed of steel, and the other of hard, hammered 

 brass, both firmly riveted or soldered face to face, is found to answer 

 well. Such a compound bar is found to warp or bend more or less, 

 according to the degrees of heat or of cold, and may be made to op- 

 erate in various ways. When formed in the shape of a ring or hoop, 

 but not closed, by the unequal expansion or contraction on either 

 side, it opens or closes more or less by cold and heat; and, finally, 

 being employed as an agent for the opening of valves or stove regis- 

 ters, or air ventilators, &c., it serves not merely to indicate, but to 

 control and regulate the temperature of the apartment. 



Mr. Loudon states, that instruments for this purpose have been in 

 use, latterly, in England, for regulating the temperature of hot- 

 houses ; and as the temperature can never remain long perfectly 

 stationary, those registers or ventilators are found to open or shut 

 continually, almost every moment of time. 



THE HYGROMETER is a very simple instrument, to ascertain the 

 moisture of the air. A piece of sponge will make a good hygrome- 

 ter. Let the sponge be washed in pure water, then dried, and then 

 let it be washed in a solution of sal ammoniac, or salt of tartar and 

 water, and again dried. If the air becomes moist, the sponge will 

 grow heavier ; but if dry, the sponge grows lighter. It may be at- 

 tached to a beam like a steelyard, with an index, which will rise or 

 fall with every change. Saussure's hygrometer is deemed one of 

 the most perfect 



HURDLES are usually formed either of thin boards or of wicker, 

 but those formed of twine netting, with meshes five eighths of an 

 inch asunder, have been much approved. In the first ages, finer 

 hurdles of brass wire have been used and preferred. These hurdles 

 are placed an inch above others, formed of boards or of laths covered 

 with stout paper. When the leaves are placed on the netting, the 

 worms ascend, and the litter falls through. These hurdles slide, 

 and are arranged in stages one above another, about fourteen inches 

 asunder. The chrysalides in the cocoons are destroyed by exposing 

 them on sheets to the noonday sun for a few hours, or in a confined 

 room of glass, exposed to the sun ; or they may be stifled by the 

 fumes of burning charcoal, or by other modes. But the very best way 

 is, if possible, to reel the cocoons as soon as formed. The reel most 

 highly approved is the reel of Piedmont. The operation by this ma- 

 chine is the most perfect ; the threads having a lateral motion, no 

 two being laid in the same place, otherwise the gum with which 

 they abound would cause adhesion, and spoil the silk. In the reel 

 of Piedmont, the threads are wound spirally along the reel the 

 thread comes not in exactly the same place until after more than 

 2,000 revolutions of the reel. Its operation and construction are 

 simple, but of the most perfect kind. 



