i8o On Underground Temperature, [April, 
sumed convention currents still existing behind the tubing, 
the observations in the upper part of the column were not 
considered trustworthy. But this circumstance does not 
militate against the conclusion at which we have arrived, 
because whatever effect convebtive currents might have 
upon the temperature would be pro tanto produced by these 
concealed currents, so that all they could do would be to 
lessen the effedt of shutting off the currents in the main 
channel. And we may reasonably suppose that the change 
of temperature arising from that proceeding would have 
been still greater than recorded, if the tubing had been in 
close contadt with the rock. 
In the discussions of these observations hitherto published* 
the observations made with the geo-thermometer have been 
looked upon as giving a near approximation to the tempera- 
ture of the earth’s crust at this locality; and, consequently, 
it is these which we propose to consider further. Although 
the sixth column shows that the rate of increase of temper- 
ature was far from equable when comparatively short inter- 
vals are taken into account, yet, when we pass to the eighth 
and ninth columns, we observe that on the whole there is a 
decided diminution of the rate of increase in the lower 
depths. It was probably this circumstance which induced 
M. Dunker to assume that empirical formula for the law of 
increase which led Prof. Mohr to believe that, at the depth 
of 5170 feet, the increase would be nil, and thence to con- 
clude that the source of the heat of the crust must be 
situated within the crust itself, instead of — as is usually 
supposed— coming up from the profound depths below. 
The question which presents itself therefore (and it is a 
most important question) is — -Can this diminution of the 
rate of increase in the indications of the geo-thermometer 
be consistent with an equable rate of increase in descending 
as deep as the observations went into the earth’s crust ? 
The answer, that it can, seems to follow from the consider- 
ations already made on the effedt of convention currents 
upon the temperature, not of the water only, but of the 
rock itself. It is obvious that the continued contadt of 
water at a different temperature from that of the rock must 
alter the temperature of the rock itself where it is in con- 
tact with the water. And it has been remarked that, almost 
beyond dispute, it had actually that effedt ; consequently 
the rockin contadt with the water must havebeen cooled in the 
lower part of the bore-hole. Moreover, the principal cooling 
* Since this article was sent in, the writer has met with a course of ledtures 
entitled “Vortrage iiber Geoiogie, von F. Henrich, Wiesbaden, 1877,” in 
which the Sperenberg observations are discussed with much acumen. 
