244 WRITINGS OF JOSEPH HENRY. [1855- 



fibres of wood, and the principal expansion takes place 

 in a direction at right angles to these fibres; it is also 

 absorbed laterally between them, though in a less quan- 

 tity. The warping of furniture is simply due to the ex- 

 halation of the water in the form of vapor from the pores 

 of the wood and the consequent shrinking of the part from 

 which the exhalation has taken place, while the other parts 

 retain their original bulk. To prevent this it is necessary 

 to imprison the vapor by a coating of an impervious sub- 

 stance, such as varnish or paint, or what is still better to 

 expel the moisture by baking the wood and subsequently 

 filling its pores with some resinous substance. It is impor- 

 tant however to observe that when a substance is to be pro- 

 tected from moisture by a covering of paint or varnish, care 

 should be taken to cover every part with the impervious 

 mixture, for the moisture may be drawn in through even a 

 nail hole and pervade the whole interior capacity of the 

 wood. 



Various instruments for indicating the moisture of the 

 atmosphere without accurately measuring its changes have 

 been constructed upon the principle of the absorption and 

 consequent change of dimensions of different substances. 

 An instrument, which has lately been very widely described 

 in the newspapers under the erroneous name of a simple 

 barometer, is composed of two shavings of light wood glued 

 together so as to make a ribbon of double thickness; the 

 fibres of one layer being at right angles to those of the other. 

 The absorption of the moisture into the shaving in which 

 the fibres are lengthwise tends merely to increase the width 

 and not the length of the compressed ribbon, while the 

 absorption of moisture into the shaving of which the fibres 

 are transverse tends to increase the length of the ribbon and 

 thus causes it to curl. The foregoing instrument belongs to 

 the class denominated hygroscopes, intended simply to in- 

 dicate the changes which take place in the vapor in the 

 atmosphere without furnishing the means of measuring its 

 precise amount. For this purpose various substances are 

 employed, such as a stretched cord, a human hair deprived 



