56 



NA TURE 



\_Alay 15, 1884 



confirm the values derived from former results. The " Hour- 

 gla - or " Kaiser Sea." which is admittedly the most prominent 

 mark on the planet, is a very suitable one for comparisons to find 

 the intervals of rotation. Early in 1869 I saw it with a 4}-inch 

 refractor as it passed the central part of the disk. On February 

 2, 1869, it was central at loll., on February 4 at Ilh., and on 

 February 5 at nil. 30m. 



I observed the same object in February of the present year 

 with a 10-inch reflector (power 252), and noted it crossing the 

 planet's central region at the following times : — 



February 14 

 15 

 19 



5 55 



6 35 

 9 5 



11 4 



I have combined my observation of February 4, 1869, with that 

 of February 14, 18S4 (as I regard this pair as the best obtained), 

 to a in the rotation period. The interval includes 5487d. 



l8h. 55m. = 474. 144,900 seconds. Correcting this for the'differ- 

 ence in longitude between Mars and the earth at the two epochs 

 and for defect of illumination (there is no necessity to apply any 

 correction for equation of light, as the apparent diameter of the 

 planet on the dates selected for comparison was about 16", and 

 hence the distances were nearly the same), I find the time of 

 rotation resulting from the discussion of these observations to be 



h. m. s. 



24 37 22-34 (5349 rotations), 

 which is in satisfactory agreement with the periods computed by 

 Kaisei . Schmidt, and Proctor from a much longer series of ob- 

 servations. In order to exhibit the small differences between the 

 period now computed and those resulting from some of the best 

 modern determinations, 1 give the following summary : — 



J. H. Madler 

 1864, F. Kaiser ... 

 1866, R. Wolf ... 

 1869, R. A. Proctor 



1873. F. Kaiser ... 



24 37 23-8 Ast. Nach. 349. 

 24 37 22-62 Ast. Nach. 1468. 

 24 37 22-9 Ast. Nach. 1623. 

 24 37 22735 Mon. Not. vol. xxix. 



p. 232. 

 24 37 22*591 Annalen der Leidcncr 



Sternwarle, vol. iii. 



22 -626s. for the rotation of Mars. This corresponds to a daily 

 rate of 350° '8922, and forms the basis of his computations in 

 his " Ephemerides for Physical Observations of Mars," annually 

 published in the Monthly Notices. 



W. F. Dennini 



1S73, T- F. J. Schmidt 24 37 22-57 Ast. Nach. 1965. 

 18S4, W. F. Denning 24 37 22-34 

 It is obvious that Madler's period of 24I1. 37m. 23'8s. is about 

 one second too great. If we take a mean of the other six values 

 (all within o-6s. of each other) we get 

 h. m. s. 

 24 37 22-626 



which may be fairly regarded as a very near approximation to 

 the true sidereal rotation period of Mars. 



The computations of Kaiser, Schmidt, and Proctor are severally 

 based on very long periods, the comparisons being modern ob- 

 servations with those of either Huyghens or Hooke during the 

 last half of the seventeenth century. It is unfortunate, however, 

 that there is some question as to the correct identification of the 

 spots depicted in some of the ancient drawings. The representa- 

 tions by Hooke on March 2, 1666 (old style), at I2h. 20m. and 

 12I1. 30m., also those by Huyghens in 1659, 1672, and 1683 give 

 a large irregular spot, extending in a north and south direction, 

 which can only be identified as the " Hourglass " or " Kaiser 

 Sea." It would appear, however, that this interpretation is in- 

 correct in certain cases, for the several drawings do not only 

 show disagreements with each other but also when compared 

 with modern observations originate discordances of period, small 

 it is true, but still too large to be attributed to simple errors of 

 observation. No doubt the period which approaches nearest to 

 the truth will become apparent from future observations, though 

 it can hardly admit of definite settlement for many years, inas- 

 much as the differences between the several times of rotation as 

 above deduced are very insignificant, and must so closely accord 

 with the real period of the planet that the errors such as exist 

 must be allowed to accumulate over a lengthened interval before 

 they will become distinctly manifested. A comparison extending 

 over fifteen years is insufficient for the purpose, for a computed 

 time of rotation, erroneous to the extent of one-tenth of a second, 

 will still, at the termination of such a period, answer to the 

 positions of the markings to within 9 minutes of time. It is to 

 be remarked that Mr. Marth, whose opinion is entitled to great 

 weight, has, for iuine time, adopted the period of 24I1. 37m. 



" The Electrical Resistance of the Human Body" 



Will you kindly publish the inclosed from Prof. Dolbear? It 

 furnishes a complete explanation of the discrepancy between his 

 measurements of the resistance of the human body and those 

 which I have recently made. At the same time, as I have 

 pointed out to him, the fact that this resistance may sink below 

 500 ohms with "soaked skin," even if that be "abnormal," is 

 of the highest physiological importance, and goes far to explain 

 the hitherto mysterious deaths from accidental passage of a cur- 

 rent through the body. Most of these, as Prof. Forbes remarked 

 to me, have taken place with alternate, not continuous, current 

 machines. W. H. Stone 



Wandsworth, May n 



College Hill, Mass., April 23, 1884 

 Dear Sir,— I have to acknowledge the receipt of your 

 pamphlet "On the Resistance of the Human Body," for which 

 I am obliged. I am glad to know that physiology has some one 

 in its ranks who is interested in that line of work, and who 

 knows what to do in order to settle such vexed questions. 



I have also seen in the last Electrical Review that has reached 

 me an article on the same matter, in which you refer to me and 

 what has been published concerning some of my work, that needs 

 a little elucidation. In the early days of telephony the experi- 

 ment was often tried of making the human body part of the 

 circuit in order to see how speech could be transmitted through 

 the body, in the language of those days. Bell wanted to know 

 what the resistance of the body was when in such circumstances, 

 and I measured it from hand to hand when thumbs and fingers 

 grasped the terminals of a wire and found it to vary between 

 6000 and 15,000 ohms, and wrote to him to that effect, and from 

 that grew out the statement to which you have referred. Now 

 under such conditions that work is right, as I have frequently since 

 proved. . 



It seems to me that when we speak of the resistance ot the 

 body or of any body, and do not define what is meant by body, it 

 is Ian to assume that the body is the ordinary body under ordinary 

 conditions. If the resistance (the actual) of the wire is found to 

 be a thousand ohms by one party and another one files oft the 

 rust from the contacts and then finds the resistance less, both 

 parties may be right. Now the skin of individuals is more or 

 less horny in texture, and so has high resistance which soaking 

 may reduce, and the question then properly arises, is the hard 

 skin properly a part of the body? The resistance of a farmer s 

 hand i, often twice as great as that of a child's or of a man of 

 sedentary habits, but solely, as I think, because of the thick- 

 ness and density of his skin. Does not the question resolve itself 

 into this— What is the resistance of a dry hand and the resistance 

 of a soaked hand ? What i- the resistance of a good conductor 

 and the resistance of a poor conductor ? If the poor one is made 

 better in any way, its resistance is correspondingly increased. 



If the condition of the body is abnormal, its resistance may 

 also be abnormal. I should call a soaked skin abnormal. 



Still it is of the utmost importance that we should know what 

 the resistance is under all conditions, as being the only way to 

 advance in knowledge of the physiological effects of known 

 currents, and I would again express my gratification at your per- 

 sistent work in this field, and if I can in any way be of service 

 to you I shall be pleased to be employed. 



Yours very truly, 

 To Dr. IV. //. Stone A. E. Dolbear 



Instinct in Birds 



Mr. Graves, who writes on this subject (Nature, vol. xxix. 

 p. 596), is, I fear, not so accurate an observer as the magpie, for 

 he misquotes the day fixed by the birds for building, and then 

 indicates that the young ' ' mags " are restricted to four in each nest, 

 while the fact is there are often six or seven in a nest. The 

 magpie is too fond of a fresh egg for breakfast to escape the 

 attention of the gamekeeper. I have often seen the greater part 

 of their nest shot down, repaired, and reoccupied by the birds 

 year after year. I know of no bird that begins the work of 

 nidification here early in February, nor any that devotes two 

 months to the work. The rook {Coruus frugilegus) is the first to 



