Intelligence and Miscellaneous Articles. 159 



duces at the moment when the pressure is removed. As these 

 apparatus are in part constructed of glass, great inconvenience 

 results owing to the constant danger of serious explosions. After 

 several accidents had occurred during these experiments, my assist- 

 ants and myself, in order to obviate the danger, never worked 

 without masks. But the greatest difficulty to contend with is the 

 short duration of the ebullition of the oxygen, and consequently 

 the too short duration of the cold. 



I have attempted to measure the temperature which the boiling 

 oxygen produces. For this purpose I have adopted a method of 

 thermo-electric measurement which, in addition to being highly 

 sensitive, allows us to record all the sudden changes of temperature 

 of the medium. The indications of the apparatus employed have 

 been compared with those of a hydrogen-thermometer between 

 + 100° C. and —130° C. The nature of the function connecting 

 these indications admits of an extrapolation being made. Reserving 

 the description of my method for a future communication, I give 

 here one hundred and eighty-six degrees beloiv zero ( — 186° C.) as 

 the first approximation to the temperature which is produced on 

 the liberation from pressure of liquefied oxygen. 



I have submitted nitrogen successfully to the action of this cold. 

 This gas compressed, cooled in boiling oxygen, and then slightly 

 released from pressure, solidifies and falls like snow in crystals of 

 remarkable size. — Comptes Bendus, December 31, 1883. 



A NEW DEVICE FOR MEASURING POWER. BY C. F. BRACKETT, 

 PHYSICAL LABORATORY OF THE COLLEGE OF NEW JERSEY. 



The following account of a method of measuring the energy 

 expended on or rendered by a dynamo or a magneto-machine will 

 be of interest to those who have to do with the production of elec- 

 tricity in the large way in which it is now employed in the enterprises 

 of the day. 



The machine is so supported, on uprights, that it can freely turn 

 through a small arc of a circle whose centre lies in the geometrical 

 axis of the armature. The support may be effected by means of 

 knife-edges or by means of smooth cylindrical bearings, attached 

 directly to the machine or to a cradle on which the machine rests. 

 In the latter case the cradle is made adjustable so that the bottom 

 or floor can be raised or lowered, thus permitting machines of 

 different construction, when placed thereon, to be brought into 

 proper positions as regards axis of revolution and points of support. 

 When the machine, thus mounted, is set in rotation, with closed 

 circuit, the mechanical couple set up between the armature and field 

 magnets tends to make the latter revolve in the same direction with 

 the armature. The value of the couple, thus operative, and which 

 we desire to know, will be known if we know the value of the couple, 



