MANUAL OF NATUEAL HISTORY. 691 



mercury. The most convenient register -thermo- 

 meter is that of Sykes, in which the extremes of 

 heat and cold are recorded by a single instrument. 



3. Hygrometer. One of the most delicate is the 

 instrument usually known as the " Wet and dry 

 bulb thermometer," which, being rather fragile, must 

 be carried in a soft padded, carefully adapted case. 



4 A good Compass. 



Other apparatus, &c. may be added according to op- 

 portunity or inclination, such as the "Anemometer/' 

 for calculating the force of the wind ; the " Actino- 

 meter/' for studying solar radiation; a "Rain-guage," 

 for measuring the fall of rain ; a " Cyanometer/' for 

 estimating the depth of blue colour of the skies; or, 

 where circumstances permit, and previous acquaint- 

 ance qualify, "Magnetical Instruments." 



The most important point to be attended to is 

 daily general observation, carefully entering in the 

 columns of a register particulars of the following 

 items, viz. : Pressure, Temperature, Moisture, Wind 

 (direction and force), Weather, Clouds, Rain, and 

 other observations as may be necessary. The proper 

 hours for daily observation are 3 A.M,, 9 A.M., 3 P.M., 

 and 9 P.M. ; and, where it can be managed, two addi- 

 tional hours should be added, namely, 6 A.M. and 

 6 P.M. Occasional hourly observations should be 

 made, for which purpose Sir J. Herschell recom- 

 mends the 21st of each month to be appropriated, or 

 at least in the months of March, June, September, 

 and December. The great things, however, to be 

 kept in view, are regularity and accuracy. Occa- 



