274 Methods of determining and calculating 



great an elevation, and then we shall have not only the radiation and 

 conduction of the vessel but the tension of vapor at the surface of 

 the water, and the latter will be greater or less according to its great- 

 er or less distance from the dew-point. The actual absolute loss 

 may be found by a separate experiment on exposing the vessel and 

 water for some hours to the same temperature as that at which the 

 trial took place and in an atmosphere having the same hygrometric 

 tension. The weight lost during the longer exposure compared with 

 its length of time ought to be proportionate to the loss and time in 

 the other case. The number of grains of vapor would then be 

 multiplied by its latent heat at the generating temperature, to obtain 

 the absolute effect in cooling the mass from which it rose. This er- 

 ror like that occasioned by the escape of air and that by the evapo- 

 ration of dew from the surface of the vessel will be in defect. 



7. The nature of the vessel containing the water, its surface, spe- 

 cific heat and the space it leaves open to the air. It should be of 

 such dimensions as to be completely filled when the thermometer 

 and the body under trial are immersed in the water. If of metal, its 

 perfect homogeneity is to be attained, and if of glass the specific heat 

 sl^ould be separately ascertained. 



8. To guard the hot body from loss of heat in passing from the 

 source of heat to the cold water I make use of a thick sheet-iron 

 cylindrical shield which is kept constantly immersed in the melted 

 metal with the piece under trial and conveys it to the very mouth of 

 the water vessel into which it is lowered by a fine wire or thread ena- 

 bling the operator to move it from one part of the vessel to the other. 



9. The vessel and its contents must be weighed with the greatest 

 attainable accuracy at every trial. No reliance should be placed on 

 the apparent levels of the fluid. Graduated measures are entirely 

 out of the question in trials of this kind. To adjust the weight with 

 readiness I employ a dropping tube with a fine point and instead of 

 a piston use a species of micrometer screw, to force out the liquid or 

 draw it in at pleasure. Drops weighing one third of a grain may be 

 easily obtained by this instrument. The method of substitution is 

 adopted in weighing to avoid all inaccuracy in the beam of the bal- 

 ance. 



10. A result is not to be taken as estabhshed until it can be re- 

 produced, at least, within the limits of the errors of observation. I 

 feel assured that much of the erroneous matter which has been pub- 

 lished on this subject has arisen from a want of due care and patience 



