182 



Dr. J. S. Lombard. On the Propagation of 



of the stand. The wet surface of the bone or box is next carefully 

 dried with cotton-wool, and protected from external disturbing in- 

 fluences by an enclosure of thick pasteboard placed near the vessel 

 containing the water. 



If now, at a given moment, we wish to commence an experiment, 

 we have merely to raise the whole arrangement, as was done when we 

 removed the bone or box, bring the latter over the water, and then set 

 the stand down again. As the distance between the water and the 

 substances to be brought in contact with it was previously accurately 

 determined, and the necessary adjustment made with the sliding arm, 

 we may be certain that the proper degree of immersion is ensured. 

 Moreover, as in experiments of this kind a second's time is of import- 

 ance, and as the above procedure can be timed so as to bring the 

 surface of the bone or box in contact with the water at a given second 

 (and that, too, without the necessity of the observer taking, for a 

 moment, his eyes off the scale of the galvanometer or the timepiece, 

 as the movements necessary can be performed without looking when 

 once their direction and extent are appreciated), it fulfils another 

 important requisite in this portion of the work.* Before adopting 

 this simple procedure the writer made many experiments with more 

 or less complicated apparatus ; but all these appliances were, one 

 after another, thrown aside as introducing troublesome, and often 

 dangerous, complications. It must be remembered that the exposed 

 surface of bone, or copper bottom of the box, must be protected, when 

 not immersed, from radiation, possible currents of air, &c, otherwise, 

 thermal exchanges will take place through the exposed surfaces, and 

 — using such delicate means of investigation as we are now treating of 

 — the index of the galvanometer will not be steady for a moment ; 

 this being the case, the bone or box cannot be simply suspended, in free 

 air, over the water, to be lowered upon the latter when the appointed 

 time comes ; and all attempts to protect them properly, while thus sus- 

 pended, have led to difficulties, bringing with them, among other evils, 

 delays in the removal of the protections, and, therefore, errors of time. 



It remains now to describe the manner in which the observations 

 were made. 



In the first place, the deflections of the galvanometer were noted 

 regularly every fifteen seconds, commencing from the second at which 

 contact between the bone or copper plate and the water took place, up 

 to six minutes. If, however, as sometimes happened, the first sign of 

 the thermal movement showed itself before the first fifteen seconds 

 had elapsed, of course that particular movement was also noted. After 

 the sixth minute the deflections were noted every half minute or every 



* It is hardly necessary to say that the possibility of currents caused by vibration 

 of the conducting wires of the pile in the movements in question, was fully appre- 

 ciated, and negatived by direct experiment. 



