METEOROLOGY. 291 



Stations) the tbermoraeter is bung in the open air against the wall of 

 some house, entirely unprotected from rain and radiation and without 

 any uniform altitude above the earth's surface. Even the most careliii 

 observations (made in the large thermometer shelter of the Magdeburg- 

 observatory, constructed according to all the rules of the art according to 

 Wild's plans) show that many errors a re still apparent which interfere with 

 the general reliability of this method. During the rapid changes at sun- 

 rise and sunset the thermometer in the Wild shelter is very much behind 

 the true temperature, especially in calm weather. The wooden louvre 

 work and the quiescent air within require an appreciable time to change 

 their temperature; moreover, we do not wish the temperature of amass 

 of air thus inclosed, but that of the free air as affected b^ radiation. 

 It is necessary that the thermometer should give the temperature of the 

 air with the greatest accuracy, and yet follow the temperature changes 

 with great rapidity ; it must be protected from radiation and rain. The 

 temi)eratue must be independent of incidental local surroundings, the 

 method must be available with the cheapest possible apparatus, and he 

 concludes that the sling-thermometer, hitherto but little used in Ger- 

 many, is the best and most appropriate for general use. It is to a high 

 degree sensitive, the injurious influence of calms is overcome by its 

 rapid movements through the air; the radiation has almost no influence 

 upon it; even in the full sunlight it can be easily protected from rain; 

 the observer can choose a shady place, aud the apparatus is the cheapest 

 imaginable. Still more is all this true of the psychrometer, which as it 

 is now generally used is acknowledged to be the most uncertain instru- 

 ment in meteorology. It is easy for the observer to give the sling psy- 

 chrometer such a linear velocity that it will be brought down to the 

 proper temperature in a lew seconds; some experiments by Assmann 

 show that it required only one-ninth of the time given to a stationary 

 l)sychrometer, the velocity used by him being 6 meters per second. The 

 form of sling-psychrometer recommended by Assmann is manufactured 

 by Fuess of Berlin; it consists of two delicate mercurial thermometers 

 divided to one half degrees; these are mounted u])on a fork-shaped 

 wooden stand through which a silk thread is drawn. The price complete 

 is 13 marks. {Z. 0. G. Jlf., xrx, p. 154.) 



130. [The sling i)sychrometer, in a well-contrived shape, was issued 

 by the late Dr. T. Craig, of the Surgeon-General's Ofiice, in 18G8, to all 

 the observers at Army posts, where it is probably now in occasional use. 

 Numerous experiments with various forms of this instrument have been 

 made by Prof. II. A. Hazen at the Army Signal Office during the i)ast 

 few years, and it is evident that either it or the whirled thermometer or 

 the ventilation psychrometer are the only ones that can be recommemled 

 for accurate ob!^ervations, as it is thoroughly essential that a nearly 

 uniform rapid current of air should flow i)ast the thcrnuuiu^tt r.] 



131. Prof. H. Wild makes a lengthy reply to Dr. Assmann's coinnuuii- 

 catiou and gives in full his own views as to the present condition of our 



