Prof. Challis on the received principles of Hydrodynamics. 309 



For liquids, however, which are violently decomposed by the 

 air, and especially those in which thick clouds are also formed 

 (e. g. the zinc compounds of the lower members of the alcohol 

 radicals), this plan, from the practical impossibility of adjusting 

 with accuracy to the given mark and the required temperature, 

 is not so satisfactory as might be desired. 



To suit liquids of this class I have devised the following im- 

 provement upon Regnault's bottle : — The neck is of as even bore 

 as possible, and divided into as many equal parts as can becon- 

 veniently read. The bottle* actually 

 employed, which answered perfectly, has 

 a capacity of 2*6 cubic centims., the neck 

 being about 3 millims. internal diame- 

 ter and 13 long, divided into half mil- 

 limetres. Just beyond the graduations 

 the neck is widened somewhat for the 

 stopper and for pouring in the liquid. 



When once the water-values have been 

 determined for each division on the neck, 

 it will be seen that it is only necessary to 

 fill the bottle so that the surface of the 

 liquid shall fall within the range of the 

 graduations. Another advantage is that 

 the contents can be raised or lowered to the 

 normal temperature, and the volume read off without addition or 

 subtraction of liquid. 



The half-millimetre divisions of water weighed 5 milligrms. 

 As it is easy to read to a half of one of these divisions, and with 

 care to a quarter, the error need not be more than the weight of 

 liquid equal to one or two milligrammes of water. 



A pipette with a capillary tube will be found convenient for 

 introducing the liquid; and of course the operation of filling 

 with liquids of the character of zinc-ethyl should be done in the 

 absence, as far as possible, of free oxygen. 



XXXVIII. On the received principles of Hydrodynamics, in reply 

 to Mr. Moon. By Professor Challis, M.A., LL.D., F.R.S., 

 F.R.A.S. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



ON reconsidering my argument in the August Number rela- 

 tive to Mr. Moon's objections to the received principles 

 of Hydrodynamics, I found that it might be more clearly exhi- 

 bited by being generalized h\ the following manner. 

 * Made by Cetti. 



