378 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
tube iuclosed within a shield of brass, in which appropriate slits expose portions of 
the tube longitudinally. The upper end of the tube is air and water tight, and at 
the lower end is a valve which turned in one direction admits water and ijrevents its 
escape; thus the top of the column of water indicates the depth in fathoms, read from 
a scale attached to the body of the sounder. A reverse movement of the valve 
allows the water to escape, and it is only necessary to replace it in its former position 
to prepare the apparatus for another cast. 
The action of the sounder depends upon the valve being tight and this is assured 
only by constant and intelligent care. It should be rinsed in fresh water, dried and 
oiled after using, and requires frequent examination and adjustment to guard against 
oxidation, and to see that the valve can be moved by hand, while it is sufficiently 
tight to prevent leakage. 
Sir William Thomson's tubes have their interior surfaces coated with a chemical 
preparation that becomes discolored upon contact with sea water, and the compres- 
sion of the column of air or the penetration of the column of water is measured on 
the tube by a scale showing fathoms of depth. It works on the same principle as the 
Bassnett tube, and is sometimes preferred because it has no valve to be cared for. 
The disadvantage is that the number of soundings are limited to the supply of tubes, 
which can not always be renewed when most needed. 
No vessel should be considered seaworthy unless she carries some reliable appa- 
ratus for ascertaining the depth to 50 fathoms at least without slackening her speed. 
The Rogers portable micrometer telescope is a very reliable and useful instrument. 
A description and method of using it is to be found in the revised edition of Bowditch’s 
Navigator, page 177. A modification of that method was adopted on board the Alba- 
tross, which required less computation and avoided the necessity of picking out each 
time the log. cotangent of such a small angle. Lieutenant Schroeder describes it as 
follows : 
The greatest angle this instrument can measure is 1,750 micrometer divisions, or about 1° 45', 
and it is seldom that an angle of over one-half or three-quarters of that is observed with it. In such 
small angles the functions may be considered as proportional to the arcs, that is, the cotangent of the 
angle measured is equal to the cotangent of one micrometer division divided by the number of those 
divisions. The log. cotangent of one division being accurately determined once for all, the rule for 
finding the distance is simply to add that function to the logarithm of the height, and from the sum 
subtract the logarithm of the number of divisions. 
Example : A bgbt-bouse 200 feet bigli is found to subtend an angle of 1,700 microm- 
eter divisions. The value of one division of the instrument on board the Albatross is 
3".6o5, of which the log. cotangent is 4.7615377, or in practice 4.75154. 
SHOKT METHOD. RIGOROUS METHOD. 
200 feet log. 2.30103 3."655 X 1.700 = 6.213".5=1° 43' 33".5. 
1 M. D log. cot. 4. 75154 200 feet log. 2. 30103 
1° 43' 33".5. log. cot. 1.52097 
7. 05257 
1. 700 M. D log. 3. 23045 6,637.4 feet log. 3, 82200 
6,639.3 feet log. 3. 82212 
The smaller the angle, the smaller of course will be the discrepancy. 
For rapid work in a hydrographic survey or reconnaissance, 10 feet is found to 
be a convenient length of staff to handle, and the logarithm of 10 being 1.00000 makes 
the computation all the easier. A board 10 inches broad, painted white, with a 2-iuch 
black stripe down the middle, will be found to be an easily distinguished target. 
