ELIMINATION 



181 



of the mechanism without affecting the record. The speed of the clockwork 

 may be timed by the regulator under the stage. The same care must be given 

 to this as to any other instrument of precision. The balls must not be left 

 under conditions permitting them to rust. It is not necessary that the instru- 

 ment be set immediately beside the scale-pan; it can be removed to any 

 desired distance provided it is above the balance, the balls being made to run 

 by gravity from one to the other through a glass tube. A special balance, 

 having a mercury-contact circuit closer, designed for use with this instru- 

 ment is among my normal apparatus and has been mentioned earlier (page 



175)- 



Meteorological Instruments. Exact studies in the relations of physi- 

 ological phenomena to atmospheric conditions are greatly facilitated by the 

 existence of accurate instruments developed by meteorologists. So far as 

 concerns our present subject, these relate to the measurement of temperature, 

 humidity, light, and barometric pressure. 



Thermometers and Thermographs. The thermometer is too well known 

 to require comment. Very important is the registering thermometer, or 

 thermograph. The Richard fom (Fig. 46) has outside the case a laterally 



Fig. 46. — The Richard thermograph; X5. 

 From the Price-list of Richard Freres, Paris. 



flattened but lengthwise-curved thin metal tube, which is straightened by 

 the swelling of the contained alcohol as temperature rises, and curves elas- 

 tically as the alcohol shrinks with falling temperature. These movements 

 are made to raise and lower an arm carrying a pen which writes upon a record 

 paper ruled in degrees and hours and turned by an eight-day clockwork. 

 Thus a continuous temperature record for a week results. The Draper 

 form is shaped more like an ordinary clock, with the bulb inside and a 

 revolving record paper in place of the face. These instruments are made 

 also by other firms, and that of Lander and Smith of Canterbury, Eng- 

 land, has the advantage, for some purposes, of recording not upon curved 

 but upon straight lines. 



Hygrometers and Hygrographs. The humidity measurer, or hygrometer, 

 is constructed upon either of two principles. First, advantage is taken of 

 the fact that certain substances, hair, horn, some plant tissues, readily absorb 



