METEOROLOGY. 291 



stations) tlie tbermometer is bung in tlie open air against tlie wall of 

 some house, entirely unprotected from rain and radiation and without 

 any uniform altitude above the earth's surf;ice. Even the most careful 

 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 tlwit many errors are still api)arent which interfere with 

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

 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 tbe quiescent air within require an appreciable time to change 

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

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

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

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

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

 temperatue must be independent of incidental local surroundings, the 

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

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

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

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

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

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

 tbe observer can choose a shady place, and tbe 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 tbe most uncertain instru- 

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

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

 proper temperature in a few seconds; some experiments by Assmann 

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

 psychrometer, tbe velocity used by him being G 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 upon a fork-shaped 

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

 is 13 marks. {Z. 0. 0. M., xix, p. 154.) 



130. [Tbe sling-psychrometer, in a well-contrived shape, was issued 

 by the late Dr. T. Craig, of the Surgeon-General's Office, 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 H. A. Hazen at the Army Signal Oftice during tbe past 

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

 tbe ventilation psychrometer are the only ones that can be recomtnended 

 for accurate observations, as it is thoroughly essential that a nearly 

 uniform rapid current of air should flow past the thermometer.] 



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

 cation and gives in full his own views asS to the preseTit Qondition ofQUp 



