368 



stead of being connected by a fast screw-joint. This contrivance 

 allows of the descent of the great length of several hundred feet of 

 the auger-rod, without expending its whole momentum upon the drill, 

 which is thus always allowed to fall with the uniform force of its own 

 weight alone, there being sufficient scope in the length of the slit to 

 allow the weight of the auger-rod to become arrested by the elastic 

 rope employed to lift and drop it, to produce the desired churning 

 movement. 



Professor Bache left the chair, which was taken by Dr. Pat- 

 terson. 



Professor Bache then communicated a description of a base 

 apparatus, planned by him and executed by Mr. Wm. Warde- 

 man, mechanician of the coast survey. 



The base apparatus presented some novel feature in construction, 

 the adaptation of others not hitherto used in field work, and a choice of 

 parts previously used by others. The general plan was devised by me, 

 and the details by Mr. William Wardeman, mechanician of the coast 

 survey, by whom they were executed under my direction. The fol- 

 lowing are the general features of the apparatus. 1. The measur- 

 ing bars were upon the compensating system first used, I believe, by 

 Col. Colby in Great Britain, and by Mr. Borden in the United States, 

 but the mode of obtaining the compensation differed entirely from that 

 used by either of these gentlemen. 2. A principle was introduced in 

 reference to the dimensions of the bars which, if at all recognised, 

 has not been hitherto applied. A bar of brass and a bar of iron of 

 the same dimensions, exposed to the same source of heat, will not 

 heat equally in equal times ; this is well known to depend upon the 

 different conducting powers of the two metals, their different specific 

 heats, and the different powers of their surfaces to absorb heat. The 

 bars then, if of equal sections, when the temperature is rising or fall- 

 ing, have not the same temperature, and the system is not compen- 

 sating. The surfaces are easily made to absorb equally by the same 

 coating, and the sections must be so proportioned to each other that 

 the bars will have the same temperature when exposed to variable 

 temperatures of the atmosphere and of the case containing them. 

 Having arranged the sections approximately, using numbers taken 

 from the books, the change, in length, during increase or decrease of 

 temperature, were not perceived when microscopes were used sup- 

 ported upon wooden stands, or even upon stone blocks of small size ; 

 the means of measurement were not sufficiently delicate to perceive 



