520 bulletin: museum of comparative zoology. 



meter proper is contained in a glass-tube which serves as a protection 

 against water pressure, the thermal contact between the bulb and the 

 outer tube being made through mercury. This apparatus, mounted 

 in a frame, is lowered on a wire to the required depth, and is then 

 inverted cither by dropping an iron ring down the wire, or by means 

 of a small propeller which spins if the wire is pulled up a short distance. 

 The temperature is thereby registered, and can be read oflf at any time 

 after the instrument has been hauled up. 



Measurements at small depths are not hard to make, but at great 

 depths they are more difficult and require a much longer time. The 

 weight of the wire presents a difficulty, and the pressures which the 

 apparatus must withstand are very great. Besides, if the results at 

 great depths are to be of value, the readings must be more precise, 

 because the variations of temperature are so much smaller there than 

 near the surface. Of the methods mentioned above, that involving 

 the reversing mercurial thermometer is the only one which has been 

 used at great depths with notable success, and even with this the 

 results are not uniformly accurate and reliable. The problem of 

 designing and constructing a recording thermometer for deep-sea 

 work, embodying some of the main principles described in this paper, 

 was suggested to the wTiter in October, 1915 by Prof. Reginald A. 

 Dal\'. To aid in defraying the cost of designing and manufacture 

 two liberal grants were made from the Bache Fund of the National 

 Academy of Sciences, at the request of Professor Daly, who supple- 

 mented the amount by his own gift and obtained generous subscrip- 

 tions from Messrs. R. L. Agassiz, Livingston Davis, and G. B. Leighton. 



General Plan of the Thermograph. 



The problem of constructing a satisfactory deep-sea thermograph 

 consists essentially in devising a mechanism which will adjust itself 

 quickly to temperature changes, is sufficiently compact, of such a 

 shape that it can be protected from the action of water and great 

 hydrostatic pressure, and more accurate in recorfling than an ordinary 

 mechanical thermograph. It need not be particularly light, for in 

 any case its weight will be small compared to that of the great length 

 of wire by which it will be lowered. The mercury thermometer in 

 glass was chosen as the most reliable means of measuring temperature, 

 and photography as the best means of recording its readings. Errors 

 in measurement are thus reduced to a minimum. If any photograph 

 at all is obtained it is a truthful record of the thermometer reading. 



