30 Prof. Guthrie on a new Thermostat. 



And, reasoning as in the last case, at the end of the time 



t -f t x each particle will have acquired the velocity gt x , and will 



at 2 

 have described the space -k~, results which, it is obvious, are 



not true as regards the particles at the base of the cylinder. 



In this case also, therefore, it is evident that the adoption of 

 the received theory of pressure has led us to untrue results*. 



6 New Square, Lincoln's Inn, 

 June 10, 1868. 



VI. Description of a new Thermostat. By Frederick Guthrie, 

 Ph.D., F.R.S.E., Professor of Physics and Chemistry, Royal 

 College, Mauritius f. 



IN physical and chemical research, and also in the arts, the 

 problem of maintaining a constant temperature frequently 

 presents itself. To this end Bunsen constructed his ingenious 

 thermostat, whose action depends upon the expansion of mercury. 

 For many purposes Bunsen's thermostat answers admirably ; but 

 in cases where a perfectly constant temperature is required, I 

 have not found it sufficiently nimble in its action of adjustment. 



Requiring, for some experiments on heat, to maintain water for 

 many hours at a temperature approaching to absolute constancy, 

 I constructed the thermostat here figured, and have found it to 

 answer very satisfactorily. 



As others may have experienced the same want, and as I am 

 not able to find any description of a similar instrument, I beg to 

 submit the following description to the readers of the Philoso- 

 phical Magazine. 



I may state at once that, with ordinary care and by using the 

 " distributor " afterwards described, I have no difficulty in keep- 

 ing a temperature constant to 0°*2 Fahrenheit for six hours, in- 

 cluding the time when, towards evening, an additional pressure 

 is applied through the service-pipe. The gas from the service- 

 pipe enters at g ]y passes through g q , and is conducted by the 

 flexible tube t to the lamp which heats the reservoir of water, u. 

 The tubes g x and g 2 join to form the tube b. The lower extre- 



* In the above case we must have -J- = at the base of the cylinder at the 



time t when the second force begins to act, the value of -^ taken without 



ax 

 regard to sign increasing as we ascend. The action of the second force 

 will also cause an instantaneous increase of finite amount in the pressure 

 on the base of the cylinder. 



t Communicated by the Author. 



