ALLISON ON THE PROGRESS OF WEATHER KNOWLEDGE. 317 



easy to throw a portion of mercury into an ordinary glass tube and 

 attach the latter to a scale graduated carelessly, thereby manufac- 

 turing an interesting but pernicious toy — while it is comparatively 

 difficult to expend the necessary time, skill, and money upon a true 

 Thermometer, that we can scarcely wonder at the very wonderful 

 pieces of mechanism which are popularly received as measures of 

 Heat. I have several times found so-called Thermometers differing 

 from the standard full seven degrees, while scarcely two Thermo- 

 meters taken haphazard from a large stock will agree exactly. 

 Our climate is quite liable enough to extraordinary changes of 

 Temperature without our calling in art to assist nature in that 

 respect. At our first class stations we use no Thermometers whose 

 readings have not been compared with a verified standard. A 

 large bulb in free air, narrow tube, porcelain scale, without back- 

 ing, and the graduations carried across the tube itself, are essential 

 to correct marking and reading. Makers of whom I can by expe- 

 rience speak as very good, are Casella, Negretti and Zambra, of 

 London; Acre, of Edinburgh; and Green, of New York. The wet 

 and dry bulbs, and maximum and minimum registering Thermomc" 

 ters should be placed about five feet above ground, sheltered on all 

 sides from direct, reflected, or refracted sun's rays ; under a screen 

 constructed with slats, or louvre boards on three sides, and to the 

 south of two layers of boards or fences, at intervals of several 

 inches. 



Having merely glanced at the principal instruments now in use 

 by meteorologists, I pass on to discuss for a short time the efiPects 

 so far obtained in this country from the systematic observations 

 now established. It is evident that the primary objects of continued 

 series of meteoroloorical observations is a settlement of the normal 

 climate of the country — especially in its chief elements of Tempera- 

 ture, Pressure, Wind and Precipitation. Having learned the mean 

 condition of the place and the extremes to which it is liable, the 

 organic life, the animal and the vegetable customary to the climate, 

 and possible under such conditions, may be more readily ascertain- 

 ed. But it has been proved that very frequent observations of 

 Pressure and Temperature — say hourly, or even tri-hourly, are not 

 absolutely required at more than one station within a large extent 



