BRITISH TANK INCUBATORS. 



heat enters from the lower end L of the chimney, 

 and the whole traverses the flue L L to the exit 

 \v ; but when the damper F is raised, more or 

 less heat escapes, and if fully raised nearly all 

 the heat escapes at F, none going to heat the 

 tank. This arrangement is worked by the 

 thermostatic capsule S, the main subject of 

 Mr. Hearson's patent, which depends for its 

 action iipon the boiling-point of a volatile liquid. 

 As water boils at 212" F., so sulphuric ether 

 boils at 94°, and by dilution with alcohol can be 

 made to boil at 98°, or any other temperature. 

 The capsule regulator consists of two plates of 

 brass fastened and soldered all round their 

 edges. Between them a little of the modified 

 liquid, which may conveniently be termed com- 

 pensated ether, is introduced, and the capsule 

 sealed. Then directly the supposed heat of 98° 

 is exceeded, at atmospheric pressure, the flattish 

 capsule expands or bulges under the pressure of 

 the vapour which is formed ; but the boiling 

 temperature is easily increased and graduated 

 by pressure upon the top plate of the capsule ; 

 hence we have an easily adjustable regulator. 

 The capsule lies on a little shelf rigidly braced 

 in a position over the eggs and below the tank, 

 and a rod O pressing upon its top plate transmits 

 the expansive motion to P, near the pivot end 

 of the lever G, where there is an adjusting screw 

 P ; there is also on the lever a sliding weight H. 

 By this screw and weight the temperature which 

 lifts the valve F is adjusted. In practice the 

 valve should " play " a little above the top of the 

 chimney. The body of the incubator is packed 

 around with non-conducting material M. The 

 movable egg-tray K K has a bottom of per- 

 forated zinc, and is concave, so as to bring the 

 outer eggs, which are in a cooler part of the 

 chamber, nearer the top tank. The strips B B 

 supporting the tray, are also movable, and 

 being wider one way than the other, by chang- 

 ing their position the tray can be somewhat 

 raised or lowered in the chamber. The air 

 enters through the aperture D, passing through 

 a coarse fabric kept wet by dipping into the 

 water-tray C C, and passes out through the upper 

 ventilating holes E E. 



The incubator here figured is still a deserved 

 favourite, and may well be allowed to stand 

 as the type of the many excellent British tank 

 machines, such as those of Messrs. Tamlin, 

 Phipps, and other well-known makers. An 

 interesting variation is that of Mr. William 

 Lea, in whose " Triumph " incubator the heat 

 is supplied by circulating pipes from a small 

 outside boiler, the inside tank being dispensed 

 with. The principle in all is much the 

 same ; some makers employ larger hot-air 



flues, in which one of them introduces baffles or 

 partial stops, in order to check the rate of 

 draught and economise oil. Some employ 

 deeper tanks to steady the tempera- 

 Other ture, and make the bottom of the 

 Tank tank concave towards the egg-tray. 

 Machines. in order to diminish the inequality 

 between the temperature at the centre 

 and the outer margins. To equalise the heat over 

 all the eggs is one chief difficulty in all machines, 

 heat being lost by radiation from the margins of 

 the chamber, however carefully packed. The 

 amount thus radiated, and consequent inequality, 

 will vary with the arrangements, and the ventila- 

 tion of the machine, and the temperature outside ; 

 and provision against it can only be averaged. 

 One inventor carries his hot-air flue round the 

 edges of the tank ; another brings the hot-water 

 down a few inches below the main body of the 

 tank, all round the egg-chamber. It will be seen 

 that alteration in one detail may alter other con- 

 ditions not intended, and that some good point 

 may be gained at the cost of others. This is 

 illustrated by such a simple matter as the cubic 

 contents of the tank. The effect of increasing it 

 has already been mentioned, in greater steadiness 

 of temperature, thus masking or diminishing the 

 effect of inefficient regulators. On the other 

 hand the Hearson machine uses smaller tanki 

 than at first, the effect of which is that the 

 regulator responds more quickly to sudden 

 changes in outside temperature, which a large 

 tank cannot readily do. Hence, while a small 

 tank may be regulated by shutting out or other- 

 wise diminishing the supply of heat, a large-tank 

 machine may require, in addition, to let out hot 

 air from the egg-chamber through a valve. 

 Attempt has been made to improve the capsule 

 regulator itself in various ways, chiefly in regard 

 to the small amount of the original motion, 

 which has to be multiplied by the long lever. 

 Various ingenious but misguided theorists have 

 supposed that by introducing additional levers, 

 or eccentric cams, into the mechanical part of 

 the apparatus, they improve this state of things. 

 Any person acquainted with mechanics will see 

 that any such complications are the reverse of 

 improvements, introducing needless friction at 

 all the additional parts, with absolutely no gain, 

 and that one simple lever working on a knife- 

 edge is the best possible arrangement that can 

 be adopted. 



There can be no doubt that the success of 

 Hearson's machine and others on similar lines 

 was greatly due to the simple and 

 Ee'^atora. effective action of the capsule regu- 

 lator, and it may be well to describe 

 here, before going farther, some of the prin- 



