124 Mr. J. Y. Buchanan. On Rapid Variations of [May 31, 



In this way two sets of curves were obtained, consisting of three 

 curves in still air and three in a fresh breeze. The results are not 

 very concordant, for, although the scale of time is very open 1 min. 

 occupying 5 mm. the temperature scale was very close, 1 occupy- 

 ing only 1 mm. The object, however, of the table is to show what 

 can be expected from an instrument of the kind in the measurement 

 of changes of temperature. The results obtained in the open air 

 would necessarily vary somewhat, because, although a fresh breeze 

 was blowing all the time, a fresh breeze varies in velocity. 



In order to obtain the best results from a thermometer it should 

 be exposed to uniform ventilation. This can only be effected by 

 artificial means, and they necessarily tend to efface sharp variations 

 of temperature. The arrangement adopted by Professor Assmann in 

 his psychrometer for ventilating and exposing his thermometers 

 ought to be suitable for this purpose. The current of air produced 

 must be uniform, and the behaviour of the thermometer as regards 

 rate of change of temperatures in the current produced must be 

 accurately determined. 



In Assmann's arrangement the thermometer is enclosed in a metal 

 tube, consequently the diameter of the bulb, on which the sensitive- 

 ness depends, can be made smaller and its length greater than would 

 be safe with a a unprotected instrument. A mercurial thermometer, 

 therefore, ventilated on Assmann's system, ought to be efficient for 

 the measurement of temperatures changing with considerable 

 rapidity. 



Departing from the mercurial thermometer I have found the simple 

 air thermometer very good for indicating and measuring quick varia- 

 tions of temperature. It has the advantage of lightness and cheap- 

 ness. The form which I use is a glass bulb, of about 3 cm. diameter 

 on a straight stem of about 10 cm. length. This can be attached to 

 a (J-tube of greater or less diameter, according as the differences of 

 temperature to be observed are great or small. The \J -tube has some 

 coloured Avater as indicator, and the indications of the instrument 

 are compared with those of a thermometer. As the instrument 

 is only put together when it is wanted, the variations of barometric 

 pressure do not affect it. It has the great advantage that it can be 

 connected with a tambour, and thus be made to record. The sensitive- 

 ness of the glass air thermometer is about the same as that of a very 

 fine mercurial thermometer made for me by Messrs. Hicks. The air 

 thermometer, however, would be very much more sensitive if the ball 

 were made of thin metal instead of glass. 



There is a limit to the sensitiveness of all thermometers depending 

 on the dilatation of a fluid, and I do not think that any such thermo- 

 meter can be constructed which would give directly the true tem- 

 perature of the air in the puffs of Fohn wind which we have been 



