240 On the Construction of Meteorological Instruments. 



b, bj Seven valves, four of which are shut, and three open. Thej^ 

 exclude rain, snow, &c. and confine the heat in seven short 

 tubes, which receive the bulbs of th^ instruments, when these 

 are in their horizontal position. 



Cf A glass frame or screen for protecting the bulbs of the instru- 

 ments when in the vertical position. The wooden frame of 

 the apparatus is varnished, so as to be completely water-proof. 



d, A lamp giving out no smoke or soot. 



e, A chimney. 



The temperature of the instruments, when they have received 

 * their horizontal position, may be kept above that of the at- 



^ * mosphere in various ways ; as by causing water, or ardent 

 spirits and sulphuric acid, to distil slowly from separate re- 

 servoirs, and through capillary tubes, on the bulbs of the in- 

 struments. An index may even be fixed to its place at any 

 given instant by means of a magnet, and an artificial supply 

 of heat be thus dispensed with ; but, in general, a lamp will 

 be found most convenient. On many occasions, however, even 

 a lamp is unnecessary, provided the air of at moderately warm 

 room has free access into the vapour chamber, A, fig. 2. ; par- 

 ticularly if the communication be such as to favour a constant 

 circulation of air. 



f, An index for marking the hour or time at which the regis-* 

 tering commences after each inspection. The outer circle is 

 divided into seven equal parts, and is fixed to the wooden 

 frame. The inner circle slides on the outer, and is divided into 

 twenty-four parts, corresponding with the twelve hours of the 

 day and night. If, for example, it is wished, at 9 a. m., to as- 

 certain the temperature each hour, for seven hours in succes- 

 sion, the index is made to point to 7 on the outer circle, and 

 10 A. M. on the inner circle is brought opposite to 1 on the 

 outer circle. 



The axis of the index projects so as to admit of its being connect- 

 ed with a small time-piece or table-clock, by means of a small 

 rod which may be removed at pleasure. When the appara- 

 tus is fixed on the outside of a window, the small rod passes 

 through a perforation in the frame of the window, the time- 

 piece being placed on a table within the room, and the two 

 may be thus adjusted in less than half a minute. 

 Fig. 2. represents the interior mechanism of the apparatus. 



a, A cylinder, around which, at equal distances, and in two spi- 

 ral lines, are seven projecting pins. The index ^ fig. 1. is 



